Java Help, Java Tutorials, Java Programming, Java Tricks
Java Help, Java Tutorials, Java Programming, Java Trickslesson four
Working with Objects
CONTENTS
Creating New Objects
Using new
What new Does
A Note on Memory Management
Accessing and Setting Class and Instance Variables
Getting Values
Changing Values
Class Variables
Calling Methods
Class Methods
References to Objects
Casting and Converting Objects and Primitive Types
Casting Primitive Types
Casting Objects
Converting Primitive Types to Objects and Vice Versa
Odds and Ends
Comparing Objects
Determining the Class of an Object
Class and Object Reflection (Java 1.1)
The Java Class Library
Summary
Q&A
Let’s start today’s lesson with an obvious statement: Because Java is an
object-oriented language, you’re
going to be dealing with a lot of objects. You’ll create them, modify them, move
them around, change
their variables, call their methods, combine them with other objects-and, of
course, develop classes and
use your own objects in the mix.
Today, therefore, you’ll learn all about the Java object in its natural habitat.
Today’s topics include
Creating instances of classes
Testing and modifying class and instance variables in your new instance
Calling methods in that object
Casting (converting) objects and other data types from one class to another
Other odds and ends about working with objects
An overview of the Java class libraries
Creating New Objects
When you write a Java program, you define a set of classes. As you learned on
Day 2, “Object-Oriented
Programming and Java,” classes are templates for objects; for the most part, you
merely use the class to
create instances and then work with those instances. In this section, therefore,
you’ll learn how to create a
new object from any given class.
Remember strings from yesterday? You learned that using a string literal-a
series of characters enclosed in
double-quotes-creates a new instance of the class String with the value of that
string.
The String class is unusual in that respect-although it’s a class, there’s an
easy way to create instances of
that class using a literal. The other classes don’t have that shortcut; to
create instances of those classes you
have to do so explicitly by using the new operator.
Note
What about the literals for numbers and characters? Don’t they create
objects, too? Actually, they don’t. The primitive data types for
numbers and characters create numbers and characters, but for
efficiency, they aren’t actually objects. You can put object wrappers
around them if you need to treat them like objects (you’ll learn how to
do this in “Casting and `Converting Objects and Primitive Types”).
Using new
To create a new object, you use the new operator with the name of the class you
want to create an instance
of, then parentheses after that. The following examples create new instances of
the classes String,
Random, and Motorcycle, and store those new instances in variables of the
appropriate types:
String str = new String();
Random r = new Random();
Motorcycle m2 = new Motorcycle();
The parentheses are important; don’t leave them off. The parentheses can be
empty (as in these examples),
in which case the most simple, basic object is created; or the parentheses can
contain arguments that
determine the initial values of instance variables or other initial qualities of
that object:
Date dt = new Date(90, 4, 1, 4, 30);
Point pt = new Point(0,0);
The number and type of arguments you can use inside the parentheses with new are
defined by the class
itself using a special method called a constructor (you’ll learn more about
constructors later today). If you
try and create a new instance of a class with the wrong number or type of
arguments (or if you give it no
arguments and it needs some), then you’ll get an error when you try to compile
your Java program.
Here’s an example of creating several different types of objects using different
numbers and types of
arguments. The Date class, part of the java.util package, creates objects that
represent the current
date. Listing 4.1 is a Java program that shows three different ways of creating
a Date object using new.
Listing 4.1. Laura’s Date program.
1: import java.util.Date;
2:
3: class CreateDates {
4:
5: public static void main(String args[]) {
6: Date d1, d2, d3;
7:
8: d1 = new Date();
9: System.out.println(“Date 1: ” + d1);
10:
11: d2 = new Date(71, 7, 1, 7, 30);
12: System.out.println(“Date 2: ” + d2);
13:
14: d3 = new Date(“April 3 1993 3:24 PM”);
15: System.out.println(“Date 3: ” + d3);
16: }
17: }
Date 1: Tue Feb 13 09:36:56 PST 1996
Date 2: Sun Aug 01 07:30:00 PDT 1971
Date 3: Sat Apr 03 15:24:00 PST 1993
Analysis
In this example, three different date objects are created using different
arguments to the class listed after new. The first instance (line 8) uses
new Date() with no arguments, which creates a Date object for
today’s date (the first line of the output shows a sample; your output
will, of course, read the current date and time for you).
The second Date object you create in this example has five integer arguments.
The arguments represent a
date: year, month, day, hours, and minutes. And, as the output shows, this
creates a Date object for that
particular date: Sunday, August 1, 1971, at 7:30 a.m.
Note
Java numbers months starting from 0. So although you might expect
the seventh month to be July, month 7 in Java is indeed August.
The third version of Date takes one argument, a string, representing the date as
a text string. When the
Date object is created, that string is parsed, and a Date object with that date
and time is created (see the
third line of output). The date string can take many different formats; see the
API documentation for the
Date class (part of the java.util package) for information about what strings
you can use.
What new Does
When you use the new operator, the new instance of the given class is created,
and memory is allocated
for it. In addition (and most importantly), a special method defined in the
given class is called to initialize
the object and set up any initial values it needs. This special method is called
a constructor. Constructors
are special methods, defined in classes, that create and initialize new
instances of classes.
New Term
Constructors are special methods that initialize a new object, set its
variables, create any other objects that object needs, and generally
perform any other operations the object needs to initialize itself.
Multiple constructor definitions in a class can each have a different number or
type of arguments-then,
when you use new, you can specify different arguments in the argument list, and
the right constructor for
those arguments will be called. That’s how each of those different versions of
new that you used in the
CreateDates class can create different Date objects.
When you create your own classes, you can define as many constructors as you
need to implement that
class’s behavior. You’ll learn how to create constructors on Day 7, “More About
Methods.”
A Note on Memory Management
Memory management in Java is dynamic and automatic. When you create a new object
in Java, Java
automatically allocates the right amount of memory for that object in the heap.
You don’t have to allocate
any memory for any objects explicitly; Java does it for you.
What happens when you’re finished with that object? How do you de-allocate the
memory that object
uses? The answer, again, is that memory management is automatic. Once you’re
done with an object, you
reassign all the variables that might hold that object and remove it from any
arrays, thereby making the
object unusable. Java has a “garbage collector” that looks for unused objects
and reclaims the memory that
those objects are using. You don’t have to do any explicit freeing of memory;
you just have to make sure
you’re not still holding onto an object you want to get rid of. You’ll learn
more specific details about the
Java garbage collector and how it works on Day 21, “Under the Hood.”
New Term
A garbage collector is a special thing built into the Java environment
that looks for unused objects. If it finds any, it automatically removes
those objects and frees the memory those objects were using.
Accessing and Setting Class and Instance Variables
Now you have your very own object, and that object may have class or instance
variables defined in it.
How do you work with those variables? Easy! Class and instance variables behave
in exactly the same
ways as the local variables you learned about yesterday; you just refer to them
slightly differently than you
do regular variables in your code.
Getting Values
To get to the value of an instance variable, you use an expression in what’s
called dot notation. With dot
notation, the reference to an instance or class variable has two parts: the
object on the left side of the dot
and the variable on the right side of the dot.
New Term
Dot notation is an expression used to get at instance variables and
methods inside a given object.
For example, if you have an object assigned to the variable myObject, and that
object has a variable
called var, you refer to that variable’s value like this:
myObject.var;
This form for accessing variables is an expression (it returns a value), and
both sides of the dot can also be
expressions. This means that you can nest instance variable access. If that var
instance variable itself
holds an object and that object has its own instance variable called state, you
could refer to it like this:
myObject.var.state;
Dot expressions are evaluated left to right, so you start with myObject’s
variable var, which points to
another object with the variable state. You end up with the value of that state
variable after the entire
expression is done evaluating.
Changing Values
Assigning a value to that variable is equally easy-just tack an assignment
operator on the right side of the
expression:
myObject.var.state = true;
Listing 4.2 is an example of a program that tests and modifies the instance
variables in a Point object.
Point is part of the java.awt package and refers to a coordinate point with an x
and a y value.
Listing 4.2. The TestPoint Class.
1: import java.awt.Point;
2:
3: class TestPoint {
4: public static void main(String args[]) {
5: Point thePoint = new Point(10,10);
6:
7: System.out.println(“X is ” + thePoint.x);
8: System.out.println(“Y is ” + thePoint.y);
9:
10: System.out.println(“Setting X to 5.”);
11: thePoint.x = 5;
12: System.out.println(“Setting Y to 15.”);
13: thePoint.y = 15;
14:
15: System.out.println(“X is ” + thePoint.x);
16: System.out.println(“Y is ” + thePoint.y);
17:
18: }
19:}
X is 10
Y is 10
Setting X to 5.
Setting Y to 15.
X is 5
Y is 15
Analysis
In this example, you first create an instance of Point where X and Y
are both 10 (line 6). Lines 8 and 9 print out those individual values,
and you can see dot notation at work there. Lines 11 through 14
change the values of those variables to 5 and 15, respectively.
Finally, lines 16 and 17 print out the values of X and Y again to show
how they’ve changed.
Class Variables
Class variables, as you’ve already learned, are variables that are defined and
stored in the class itself. Their
values, therefore, apply to the class and to all its instances.
With instance variables, each new instance of the class gets a new copy of the
instance variables that class
defines. Each instance can then change the values of those instance variables
without affecting any other
instances. With class variables, there is only one copy of that variable. Every
instance of the class has
access to that variable, but there is only one value. Changing the value of that
variable changes it for all
the instances of that class.
You define class variables by including the static keyword before the variable
itself. You’ll learn more
about this on Day 6, “Creating Classes and Applications in Java.” For example,
take the following partial
class definition:
class FamilyMember {
static String surname = “Johnson”;
String name;
int age;
…
}
Instances of the class FamilyMember each have their own values for name and age.
But the class
variable surname has only one value for all family members. Change surname, and
all the instances of
FamilyMember are affected.
To access class variables, you use the same dot notation as you do with instance
variables. To get or
change the value of the class variable, you can use either the instance or the
name of the class on the left
side of the dot. Both of the lines of output in this example print the same
value:
FamilyMember dad = new FamilyMember();
System.out.println(“Family’s surname is: ” + dad.surname);
System.out.println(“Family’s surname is: ” + FamilyMember.surname);
Because you can use an instance to change the value of a class variable, it’s
easy to become confused
about class variables and where their values are coming from (remember that the
value of a class variable
affects all the instances). For this reason, it’s a good idea to use the name of
the class when you refer to a
class variable-it makes your code easier to read and strange results easier to
debug.
Calling Methods
Calling a method is similar to referring to an object’s instance variables:
Method calls to objects also use
dot notation. The object itself whose method you’re calling is on the left side
of the dot; the name of the
method and its arguments are on the right side of the dot:
myObject.methodOne(arg1, arg2, arg3);
Note that all calls to methods must have parentheses after them, even if that
method takes no arguments:
myObject.methodNoArgs();
If the method you’ve called returns an object that itself has methods, you can
nest methods as you would
variables. This next example calls the getName() method, which is defined in the
object returned by the
getClass() method, which was defined in myObject. Got it?
myObject.getClass().getName();
You can combine nested method calls and instance variable references as well (in
this case you’re calling
the methodTwo() method, which is defined in the object stored by the var
instance variable, which in
turn is part of the myObject object):
myObject.var.methodTwo(arg1, arg2);
System.out.println(), the method you’ve been using through the book this far to
print out bits of
text, is a great example of nesting variables and methods. The System class
(part of the java.lang
package) describes system-specific behavior. System.out is a class variable that
contains an instance of
the class PrintStream that points to the standard output of the system.
PrintStream instances have
a println() method that prints a string to that output stream.
Listing 4.3 shows an example of calling some methods defined in the String
class. Strings include
methods for string tests and modification, similar to what you would expect in a
string library in other
languages.
Listing 4.3. Several uses of String methods.
1: class TestString {
2:
3: public static void main(String args[]) {
4: String str = “Now is the winter of our discontent”;
5:
6: System.out.println(“The string is: ” + str);
7: System.out.println(“Length of this string: “
8: + str.length());
9: System.out.println(“The character at position 5: “
10: + str.charAt(5));
11: System.out.println(“The substring from 11 to 17: “
12: + str.substring(11, 17));
13: System.out.println(“The index of the character d: “
14: + str.indexOf(‘d’));
15: System.out.print(“The index of the beginning of the “);
16: System.out.println(“substring \”winter\”: “
17: + str.indexOf(“winter”));
18: System.out.println(“The string in upper case: “
19: + str.toUpperCase());
20: }
21: }
The string is: Now is the winter of our discontent
Length of this string: 35
The character at position 5: s
The substring from positions 11 to 17: winter
The index of the character d: 25
The index of the beginning of the substring “winter”: 11
The string in upper case: NOW IS THE WINTER OF OUR DISCONTENT
Analysis
In line 4, you create a new instance of String by using a string
literal (it’s easier that way than using new and then putting the
characters in individually). The remainder of the program simply calls
different string methods to do different operations on that string:
Line 6 prints the value of the string we created in line 4: “Now is the winter
of our
discontent”.
l
Line 7 calls the length() l method in the new String object. This string has 35
characters.
Line 9 calls the charAt() method, which returns the character at the given
position in the string.
Note that string positions start at 0, so the character at position 5 is s.
l
Line 11 calls the substring() method, which takes two integers indicating a
range and returns
the substring at those starting and ending points. The substring() method can
also be called
with only one argument, which returns the substring from that position to the
end of the string.
l
Line 13 calls the indexOf() method, which returns the position of the first
instance of the given
character (here, ‘d’).
l
Line 15 shows a different use of the indexOf() method, which takes a string
argument and
returns the index of the beginning of that string.
l
l Finally, line 19 uses the toUpperCase() method to return a copy of the string
in all uppercase.
Class Methods
Class methods, like class variables, apply to the class as a whole and not to
its instances. Class methods
are commonly used for general utility methods that may not operate directly on
an instance of that class,
but fit with that class conceptually. For example, the String class contains a
class method called
valueOf(), which can take one of many different types of arguments (integers,
booleans, other objects,
and so on). The valueOf() method then returns a new instance of String
containing the string value
of the argument it was given. This method doesn’t operate directly on an
existing instance of String, but
getting a string from another object or data type is definitely a String-like
operation, and it makes sense
to define it in the String class.
Class methods can also be useful for gathering general methods together in one
place (the class). For
example, the Math class, defined in the java.lang package, contains a large set
of mathematical
operations as class methods-there are no instances of the class Math, but you
can still use its methods
with numeric or boolean arguments. For example, the class method Math.max()
takes two arguments
and returns the larger of the two. You don’t need to create a new instance of
Math; just call the method
anywhere you need it, like this:
in biggerOne = Math.max(x, y);
To call a class method, you use dot notation as you do with instance methods. As
with class variables, you
can use either an instance of the class or the class itself on the left site of
the dot. However, for the same
reasons noted in the discussion on class variables, using the name of the class
for class methods makes
your code easier to read. The last two lines in this example produce the same
result (the string “5″):
String s, s2;
s = “foo”;
s2 = s.valueOf(5);
s2 = String.valueOf(5);
References to Objects
As you work with objects, one important thing going on behind the scenes is the
use of references to those
objects. When you assign objects to variables, or pass objects as arguments to
methods, you are passing
references to those objects, not the objects themselves or copies of those
objects.
An example should make this clearer. Examine Listing 4.4, which shows a simple
example of how
references work.
Listing 4.4. A references example.
1: import java.awt.Point;
2:
3: class ReferencesTest {
4: public static void main (String args[]) {
5: Point pt1, pt2;
6: pt1 = new Point(100, 100);
7: pt2 = pt1;
8:
9: pt1.x = 200;
10: pt1.y = 200;
11: System.out.println(“Point1: ” + pt1.x + “, ” + pt1.y);
12: System.out.println(“Point2: ” + pt2.x + “, ” + pt2.y);
13: }
14: }
Point1: 200, 200
Point2: 200, 200
Analysis
In the first part of this program, you declare two variables of type
Point (line 5), create a new Point object to pt1 (line 6), and
finally, assign the value of pt1 to pt2 (line 7).
Now, here’s the challenge. After changing pt1′s x and y instance variables in
lines 9 and 10, what will
pt2 look like?
As you can see, pt2′s x and y instance variables were also changed, even though
you never explicitly
changed them. When you assign the value of pt1 to pt2, you actually create a
reference from pt2 to the
same object to which pt1 refers (see Figure 4.1). Change the object that pt2
refers to, and you also
change the object that pt1 points to, because both are references to the same
object.
Figure 4.1 : References to objects.
Note
If you actually do want pt1 and pt2 to point to separate objects, you
should use new Point() for both lines to create separate objects.
The fact that Java uses references becomes particularly important when you pass
arguments to methods.
You’ll learn more about this later today, but keep these references in mind.
Technical Note
There are no explicit pointers or pointer arithmetic in Java as there are
in C-like languages-just references. However, with these references,
and with Java arrays, you have most of the capabilities that you have
with pointers without the confusion and lurking bugs that explicit
pointers can create.
Casting and Converting Objects and Primitive
Types
Sometimes in your Java programs you may have a value stored somewhere that is
the wrong type for what
you want to do with it. Maybe it’s an instance of the wrong class, or perhaps
it’s a float and you want it
to be an int. To convert the value of one type to another, you use casting.
Casting is a programming term
that means, effectively, converting a value or an object from one type to
another. The result of a cast is a
new value or object; casting does not change the original object or value.
New Time
Casting converts the value of an object or primitive type into another
type.
Although the concept of casting is a simple one, the rules for what types in
Java can be converted to what
other types are complicated by the fact that Java has both primitive types (int,
float, boolean), and
object types (String, Point, Window, and so on). There are three forms of casts
and conversions to
talk about in this section:
Casting between primitive types: int to float or float l to double
l Casting between object types: an instance of a class to an instance of another
class
l Converting primitive types to objects and then extracting primitive values
back out of those objects
Casting Primitive Types
Casting between primitive types allows you to “convert” the value of one type to
another primitive
type-for example, to assign a number of one type to a variable of another type.
Casting between primitive
types most commonly occurs with the numeric types; boolean values cannot be cast
to any other primitive
type.
Often, if the type you are casting to is “larger” than the type of the value
you’re converting, you may not
have to use an explicit cast. You can often automatically treat a byte or a
character as an int, for
example, or an int as a long, an int as a float, or anything as a double
automatically. In most
cases, because the larger type provides more precision than the smaller, no loss
of information occurs
when the value is cast. The exception is casting integers to floating-point
values; casting an int or a
long to a float or a long to a double may cause some loss of precision.
To convert a large value to smaller type, you must use an explicit cast, because
converting that value may
result in a loss of precision. Explicit casts look like this:
(typename)value
In this form, typename is the name of the type you’re converting to (for
example: short, int, float,
boolean), and value is an expression that results in the value you want to
convert. So, for example, in
this expression the value of x is divided by the value of y and the result is
cast to an int:
(int) (x / y);
Note that because the precedence of casting is higher than that of arithmetic,
you have to use parentheses
here; otherwise, the value of x would be cast first and then divided by y (which
might very well be a very
different result).
Casting Objects
Instances of classes can also be cast to instances of other classes, with one
restriction: The class of the
object you’re casting and the class you’re casting it to must be related by
inheritance; that is, you can cast
an object only to an instance of its class’s sub- or superclass-not to any
random class.
Analogous to converting a primitive value to a larger type, some objects may not
need to be cast
explicitly. In particular, because subclasses contain all the same information
as their superclass, you can
use an instance of a subclass anywhere a superclass is expected. (Did you just
have to read that sentence
four times before you understood it? I had to rewrite it a whole lot of times
before it became even that
simple. Bear with me, its not that bad. Let’s try an example.) Suppose you have
a method that takes two
arguments: one of type Object, and one of type Number. You don’t have to pass
instances of those
particular classes to that method. For the Object argument, you can pass any
subclass of Object (any
object, in other words), and for the Number argument you can pass in any
instance of any subclass of
Number (Integer, Boolean, Float, and so on); you don’t have to explicitly
convert them first.
Casting downward in the class hierarchy is automatic, but casting upward is not.
Converting an instance of
a subclass to an instance of a superclass loses the information the original
subclass provided and requires
an explicit cast. To cast an object to another class, you use the same casting
operation that you used for
base types:
(classname)object
In this case, classname is the name of the class you want to cast the object to,
and object is a
reference to the object you’re casting. Note that casting creates a reference to
the old object of the type
classname; the old object still continues to exist as it did before.
Here’s a (fictitious) example of a cast of an instance of the class GreenApple
to an instance of the class
Apple (where GreenApple is theoretically a subclass of Apple with more
information to define the
apple as green):
GreenApple a;
Apple a2;
a = new GreenApple();
a2 = (Apple) a;
In addition to casting objects to classes, you can also cast objects to
interfaces-but only if that object’s
class or one of its superclasses actually implements that interface. Casting an
object to an interface means
that you can call one of that interface’s methods even if that object’s class
does not actually implement that
interface. You’ll learn more about interfaces in Week 3.
Converting Primitive Types to Objects and Vice Versa
Now you know how to cast a primitive type to another primitive type and how to
cast between classes.
How can you cast one to the other?
You can’t! Primitive types and objects are very different things in Java and you
can’t automatically cast or
convert between the two. However, the java.lang package includes several special
classes that
correspond to each primitive data type: Integer for ints, Float for floats,
Boolean for
booleans, and so on. Note that the class names have an initial capital letter,
and the primitive types are
lowercase. Java treats these names very differently, so don’t confuse them, or
your methods and variables
won’t behave the way you expect.
Using class methods defined in these classes, you can create an
object-equivalent for all the primitive
types using new. The following line of code creates an instance of the Integer
class with the value 35:
Integer intObject = new Integer(35);
Once you have actual objects, you can treat those values as objects. Then, when
you want the primitive
values back again, there are methods for that as well-for example, the intValue()
method extracts an
int primitive value from an Integer object:
int theInt = intObject.intValue(); // returns 35
See the Java API documentation for these special classes for specifics on the
methods for converting
primitives to and from objects.
Note
In Java 1.0 there are special type classes for Boolean,
Character, Double, Float, Integer, and Long. Java 1.1
adds classes for Byte and Short, as well as a special wrapper class
for Void. The latter classes are used primarily for object reflection.
Odds and Ends
This section is a catchall for other information about working with objects,
particularly the following:
l Comparing objects
l Finding out the class of any given object
l Testing to see whether an object is an instance of a given class
Comparing Objects
Yesterday you learned about operators for comparing values: equals, not equals,
less than, and so on. Most
of these operators work only on primitive types, not on objects. If you try to
use other values as operands,
the Java compiler produces errors.
The exception to this rule is with the operators for equality: == (equal) and !=
(not equal). These
operators, when used with objects, test whether the two operands refer to
exactly the same object in
memory.
What should you do if you want to be able to compare instances of your class and
have meaningful
results? You have to implement special methods in your class, and you have to
call those methods using
those method names.
Technical Note
Java does not have the concept of operator overloading-that is, the
ability to redefine the behavior of the built-in operators using methods
in your own classes. The built-in operators remain defined only for
numbers.
A good example of this is the String class. It is possible to have two strings,
two independent objects in
memory with the same values-that is, the same characters in the same order.
According to the == operator,
however, those two String objects will not be equal, because, although their
contents are the same, they
are not the same object.
The String class, therefore, defines a method called equals() that tests each
character in the string
and returns true if the two strings have the same values. Listing 4.5
illustrates this.
Listing 4.5. A test of string equality.
1: class EqualsTest {
2: public static void main(String args[]) {
3: String str1, str2;
4: str1 = “she sells sea shells by the sea shore.”;
5: str2 = str1;
6:
7: System.out.println(“String1: ” + str1);
8: System.out.println(“String2: ” + str2);
9: System.out.println(“Same object? ” + (str1 == str2));
10:
11: str2 = new String(str1);
12:
13: System.out.println(“String1: ” + str1);
14: System.out.println(“String2: ” + str2);
15: System.out.println(“Same object? ” + (str1 == str2));
16: System.out.println(“Same value? ” + str1.equals(str2));
17: }
18: }
String1: she sells sea shells by the sea shore.
String2: she sells sea shells by the sea shore.
Same object? true
String1: she sells sea shells by the sea shore.
String2: she sells sea shells by the sea shore.
Same object? false
Same value? true
Analysis
The first part of this program (lines 4 through 6) declares two
variables (str1 and str2) assigns the literal she sells sea
shells by the sea shore. to str1, and then assigns that
value to str2. As you learned earlier when we talked about object
references, now str1 and str2 point to the same object, and the
equality test at line 10 proves that.
In the second part, you create a new string object with the same value as str1
and assign str2 to that
new string object. Now you have two different string objects in str1 and str2,
both with the same
value. Testing them to see whether they’re the same object by using the ==
operator (line 16) returns the
expected answer (false-they are not the same object in memory), as does testing
them using the
equals() method (line 17) (true-they have the same values).
Technical Note
Why can’t you just use another literal when you change str2, rather
than using new? String literals are optimized in Java-if you create a
string using a literal, and then use another literal with the same
characters, Java knows enough to give you the first String object
back. Both strings are the same objects-to create two separate objects
you have to go out of your way.
Determining the Class of an Object
Want to find out the class of an object? Here’s the way to do it for an object
assigned to the variable obj:
String name = obj.getClass().getName();
What does this do? The getClass() method is defined in the Object class, and as
such is available
for all objects. The result of that method is a Class object (where Class is
itself a class), which has a
method called getName(). getName() returns a string representing the name of the
class.
Another test that might be useful to you is the instanceof operator. instanceof
has two operands:
an object on the left and the name of a class on the right. The expression
returns true or false based on
whether the object is an instance of the named class or any of that class’s
subclasses:
“foo” instanceof String // true
Point pt = new Point(10, 10);
pt instanceof String // false
The instanceof operator can also be used for interfaces; if an object implements
an interface, the
instanceof operator with that interface name on the right side returns true.
You’ll learn all about
interfaces in Week 3.
Class and Object Reflection (Java 1.1)
Reflection, also known as introspection, is a somewhat lofty term to describe
the ability to “look inside” a
class or an object and get information about that object’s variables and methods
as well as actually set and
get the values of those variables and to call methods. Object reflection is
useful for tools such as class
browsers or debuggers, where getting at the information of an object on-the-fly
allows you to explore what
that object can do, or for component-based programs such as Java Beans, where
the ability for one object
to query another object about what it can do (and then ask it to do something)
is useful to building larger
applications.
The classes that support reflection of Java classes and objects will be part of
the core Java 1.1 API (they
are not available in the 1.0.2 version of the JDK). A new package,
java.lang.reflect, will contain
new classes to support reflection, which include the following:
l Field, for managing and finding out information about class and instance
variables
l Method, for managing class and instance methods
Constructor, for managing the special methods for creating new instances of
classes (you’ll
learn more about constructors on Day 7)
l
l Array, for managing arrays
Modifier, for decoding modifier information about classes, variables and methods
(more about
modifiers on Day 15, “Modifiers, Access Control, and Class Design”)
l
In addition, there will be a number of new methods available in the Class class
to help tie together the
various reflection classes.
You can find out more about the new reflection classes and methods from
http://java.sun.com/products/JDK/1.1/designspecs/reflection/.
The Java Class Library
To finish up today, let’s look at the Java class library. Actually, you’ve had
some experience with some of
the Java classes already, so they shouldn’t seem that strange.
The Java class library provides the set of classes that are guaranteed to be
available in any commercial
Java environment (for example, in any Java development environment or in
browsers such as Netscape).
Those classes are in the java package and include all the classes you’ve seen so
far in this book, plus a
whole lot more classes you’ll learn about later on in this book (and more you
may not learn about at all).
The Java Developer’s Kit comes with documentation for all of the Java class
library, which includes
descriptions of each class’s instance variables, methods, constructors,
interfaces, and so on. You can get to
this documentation (called the Java Application Programmer’s Interface, or API)
via the Web at
http://java.sun.com:80/products/JDK/CurrentRelease/api/packages.html. A
shorter summary of the Java API is in appendix C as well. Exploring the Java
class library and its methods
and instance variables is a great way to figure out what Java can and cannot do,
as well as how it can
become a starting point for your own development.
Here are the class packages that are part of the Java class library:
java.lang-Classes that apply to the language itself, including the Object class,
the String
class, and the System class. It also contains the special classes for the
primitive types (Integer,
Character, Float, and so on). You’ll get at least a glance at most of the
classes in this package
in this first week.
l
java.util-Utility classes, such as Date, as well as simple collection classes,
such as Vector
and Hashtable. You’ll learn more about these classes in the Bonus Week.
l
java.io-Input and output classes for writing to and reading from streams (such
as standard input
and output) and for handling files. Day 19, “Streams and I/O,” describes the
classes in this package.
l
java.net-Classes for networking support, including Socket and URL (a class to
represent
references to documents on the World Wide Web). You’ll learn a little about
networking on Day 14,
“Windows, Networking, and Other Tidbits,” and then on Day 26, “Client/Server
Networking in
Java.”
l
java.awt-This is the Abstract Windowing Toolkit. It contains classes to
implement graphical
user interface features, including classes for Window, Menu, Button, Font,
CheckBox, and so
on. It also includes mechanisms for managing system events and for processing
images (in the
java.awt.Image package). You’ll learn all about the awt in Week 2.
l
l java.applet-Classes to implement Java applets.
In addition to the Java classes, your development environment may also include
additional classes that
provide other utilities or functionality. Although these classes may be useful,
because they are not part of
the standard Java library, they may not be available to other people trying to
run your Java program unless
you explicitly include those classes with your program. This is particularly
important for applets, because
applets are expected to be able to run on any platform, using any Java-enabled
browser. Only classes
inside the java package are guaranteed to be available on all browsers and Java
environments.
Summary
Objects, objects everywhere. Today, you’ve learned all about how to deal with
objects: how to create
them, how to find out and change the values of their variables, and how to call
their methods. You have
also learned how to copy and compare them and how to convert them into other
objects. Finally, you have
learned a bit about the Java class libraries-which give you a whole slew of
classes to play with in your
own programs.
You now have the fundamentals of how to deal with most simple things in the Java
language. All you
have left are arrays, conditionals, and loops, which you’ll learn about
tomorrow. Then you’ll learn how to
define and use classes in Java applications on Day 6, and launch directly into
applets next week. With just
about everything you do in your Java programs, you’ll always come back to
objects.
Q&A
Q: I’m confused about the differences between objects and the primitive data
types, such as
int and boolean.
A: The primitive types in the language (byte, short, int, long, float, double,
boolean,
and char) represent the smallest things in the language. They are not objects,
although in many
ways they can be handled like objects-they can be assigned to variables and
passed in and out of
methods. Most of the operations that work exclusively on objects, however, will
not work with
primitive types.
Objects are instances of classes and, as such, are usually much more complex
data types than
simple numbers and characters, often containing numbers and characters as
instance or class
variables.
Q: No pointers in Java? If you don’t have pointers, how are you supposed to do
something like
linked lists, where you have a pointer from one nose to another so you can
traverse them?
A: Java doesn’t have no pointers at all; it has no explicit pointers. Object
references are, effectively,
pointers. So to create something like a linked list, you would create a class
called Node, which
would have an instance variable also of type Node. Then to link together node
objects all you
need to do is assign a node object to the instance variable of the object just
before it in the list.
Because object references are pointers, linked lists set up this way will behave
as you would
expect them to.
Q: In the section on calling methods, you had examples of calling a method with
a different
number of arguments each time-and it gave a different kind of result. How is
that possible?
A: That’s called method overloading. Overloading means that the same method can
have different
behavior based on the arguments it’s called with-and the number and type of
arguments can vary.
When you define methods in your own classes, you define separate method
signatures with
different sets of arguments and different definitions. When a method is called,
Java figures out
which definition to execute based on the number and type of arguments with which
you called it.
You’ll learn all about this on Day 6.
Q: No operator overloading in Java? Why not? I thought Java was based on C++,
and C++ has
operator overloading.
A: Java was indeed based on C++, but it was also designed to be simple, so many
of C++’s features
have been removed. The argument against operator overloading is that because the
operator can
be defined to mean anything; it makes it very difficult to figure out what any
given operator is
doing at any one time. This can result in entirely unreadable code. When you use
a method, you
know it can mean many things to many classes, but when you use an operator you
would like to
know that it always means the same thing. Given the potential for abuse, the
designers of Java felt
it was one of the C++ features that was best left out.
Java Help, Java Tutorials, Java Programming, Java Tricks
Java Help, Java Tutorials, Java Programming, Java Tricks
Lesson Three
Java Basics
CONTENTS
Statements and Expressions
Variables and Data Types
Declaring Variables
Notes on Variable Names
Variable Types
Assigning Values to Variables
Comments
Literals
Number Literals
Boolean Literals
Character Literals
String Literals
Expressions and Operators
Arithmetic
More About Assignment
Incrementing and Decrementing
Comparisons
Logical Operators
Bitwise Operators
Operator Precedence
String Arithmetic
Summary
Q&A
Already this week you’ve learned about Java programming in very broad terms-what
a Java program andan executable look like, and how to create simple classes. For
the remainder of this week, you’re going to get down to details and deal with
the specifics of what the Java language looks like.
Today you won’t define any classes or objects or worry about how any of them
communicate inside a
Java program. Rather, you’ll draw closer and examine simple Java statements-the
basic things you can do
in Java within a method definition such as main().
Today you’ll learn about the following:
Java statements and expressions
Variables and data types
Comments
Literals
Arithmetic
Comparisons
Logical operators
Technical Note
Java looks a lot like C++, and-by extension-like C. Much of the
syntax will be very familiar to you if you are used to working in these
languages. If you are an experienced C or C++ programmer, you may
want to pay special attention to the technical notes (such as this one),
because they provide information about the specific differences
between these and other traditional languages and Java.
Statements and Expressions
A statement indicates the simplest tasks you can accomplish in Java; a statement
forms a single Java
operation. All the following are simple Java statements:
int i = 1;
import java.awt.Font;
System.out.println(“This motorcycle is a “
+ color + ” ” + make);
m.engineState = true;
Statements sometimes return values-for example, when you add two numbers
together or test to see
whether one value is equal to another. These kind of statements are called
expressions. You’ll learn about
these later today.
White space in Java statements, as with C, is unimportant. A statement can be
contained on a single line
or on multiple lines, and the Java compiler will be able to read it just fine.
The most important thing to
remember about Java statements is that each one ends with a semicolon (;).
Forget the semicolon, and
your Java program won’t compile.
Java also has compound statements, or blocks, which can be placed wherever a
single statement can.
Block statements are surrounded by braces ({}). You’ll learn more about blocks
on Day 5, “Arrays,
Conditionals, and Loops.”
Variables and Data Types
Variables are locations in memory in which values can be stored. Each one has a
name, a type, and a
value. Before you can use a variable, you have to declare it. After it is
declared, you can then assign
values to it (you can also declare and assign a value to a variable at the same
time, as you’ll learn in this
section).
Java actually has three kinds of variables: instance variables, class variables,
and local variables.
Instance variables, as you learned yesterday, are used to define the attributes
of a particular object. Class
variables are similar to instance variables, except their values apply to all
that class’s instances (and to the
class itself) rather than having different values for each object.
Local variables are declared and used inside method definitions, for example,
for index counters in loops,
as temporary variables, or to hold values that you need only inside the method
definition itself. They can
also be used inside blocks, which you’ll learn about on Day 5. Once the method
(or block) finishes
executing, the variable definition and its value cease to exist. Use local
variables to store information
needed by a single method and instance variables to store information needed by
multiple methods in the
object.
Although all three kinds of variables are declared in much the same ways, class
and instance variables
are accessed and assigned in slightly different ways from local variables. Today
you’ll focus on variables
as used within method definitions; tomorrow you’ll learn how to deal with
instance and class variables.
Note Unlike other languages, Java does not have global variables-that is,
variables that are global to all parts of a program. Instance and class
variables can be used to communicate global information between
and among objects. Remember that Java is an object-oriented
language, so you should think in terms of objects and how they
interact, rather than in terms of programs.
Declaring Variables
To use any variable in a Java program, you must first declare it. Variable
declarations consist of a type
and a variable name:
int myAge;
String myName;
boolean isTired;
Variable definitions can go anywhere in a method definition (that is, anywhere a
regular Java statement
can go), although they are most commonly declared at the beginning of the
definition before they are
used:
public static void main (String args[]) {
int count;
String title;
boolean isAsleep;
…
}
You can string together variable names with the same type on one line:
int x, y, z;
String firstName, LastName;
You can also give each variable an initial value when you declare it:
int myAge, mySize, numShoes = 28;
String myName = “Laura”;
boolean isTired = true;
int a = 4, b = 5, c = 6;
If there are multiple variables on the same line with only one initializer (as
in the first of the previous
examples), the initial value applies to only the last variable in a declaration.
You can also group
individual variables and initializers on the same line using commas, as with the
last example.
Local variables must be given values before they can be used (your Java program
will not compile if you
try to use an unassigned local variable). For this reason, it’s a good idea
always to give local variables
initial values. Instance and class variable definitions do not have this
restriction. (Their initial value
depends on the type of the variable: null for instances of classes, 0 for
numeric variables, ” for
characters, and false for booleans.)
Notes on Variable Names
Variable names in Java can start with a letter, an underscore (_), or a dollar
sign ($). They cannot start
with a number. After the first character, your variable names can include any
letter or number. Symbols,
such as %, *, @, and so on, are often reserved for operators in Java, so be
careful when using symbols in
variable names.
In addition, the Java language uses the Unicode character set. Unicode is a
character set definition that
not only offers characters in the standard ASCII character set, but also
includes several thousand other
characters for representing most international alphabets. This means that you
can use accented characters
and other glyphs as legal characters in variable names, as long as they have a
Unicode character number
above 00C0.
Warning
The Unicode specification is a two-volume set of lists of thousands of
characters. If you don’t understand Unicode, or don’t think you have a
use for it, it’s safest just to use plain numbers and letters in your
variable names. You’ll learn a little more about Unicode later.
Finally, note that the Java language is case sensitive, which means that
uppercase letters are different
from lowercase letters. This means that the variable X is different from the
variable x, and a rose is not
a Rose is not a ROSE. Keep this in mind as you write your own Java programs and
as you read Java
code other people have written.
By convention, Java variables have meaningful names, often made up of several
words combined. The
first word is lowercase, but all following words have an initial uppercase
letter:
Button theButton;
long reallyBigNumber;
boolean currentWeatherStateOfPlanetXShortVersion;
Variable Types
In addition to the variable name, each variable declaration must have a type,
which defines what values
that variable can hold. The variable type can be one of three things:
One of the eight primitive data types
The name of a class or interface
An array
You’ll learn about how to declare and use array variables on Day 5; this lesson
focuses on the primitive
and class types.
Primitive Types
The eight primitive data types handle common types for integers, floating-point
numbers, characters, and
boolean values (true or false). They’re called primitive because they’re built
into the system and are
not actual objects, which makes them more efficient to use. Note that these data
types are
machine-independent, which means that you can rely on their sizes and
characteristics to be consistent
across your Java programs.
There are four Java integer types, each with a different range of values (as
listed in Table 3.1). All are
signed, which means they can hold either positive or negative numbers. Which
type you choose for your
variables depends on the range of values you expect that variable to hold; if a
value becomes too big for
the variable type, it is silently truncated.
Table 3.1. Integer types.
Type Size Range
byte 8 bits -128 to 127
short 16 bits -32,768 to 32,767
int 32 bits -2,147,483,648 to 2,147,483,647
long 64 bits -9,223,372,036,854,775,808 to
9,223,372,036,854,775,807
Floating-point numbers are used for numbers with a decimal part. Java
floating-point numbers are
compliant with IEEE 754 (an international standard for defining floating-point
numbers and arithmetic).
There are two floating-point types: float (32 bits, single precision) and double
(64 bits, double
precision).
The char type is used for individual characters. Because Java uses the Unicode
character set, the char
type has 16 bits of precision, unsigned.
Finally, the boolean type can have one of two values, true or false. Note that
unlike in other C-like
languages, boolean is not a number, nor can it be treated as one. All tests of
boolean variables should
test for true or false.
Note that all the primitive types are in lowercase. Be careful when you use them
in your programs that
you do use the lowercase, because there are also classes with the same names
(and an initial capital
letter) that have different behavior-so, for example, the primitive type boolean
is different from the
Boolean class. You’ll learn more about these special classes and what they’re
used for on Day 4,
“Working with Objects.”
Class Types
In addition to the eight primitive data types, variables in Java can also be
declared to hold an instance of
a particular class:
String LastName;
Font basicFont;
OvalShape myOval;
Each of these variables can hold instances of the named class or of any of its
subclasses. The latter is
useful when you want a variable to be able to hold different instances of
related classes. For example,
let’s say you had a set of fruit classes-Apple, Pear, Strawberry, and so on- all
of which inherited
from the general class Fruit. By declaring a variable of type Fruit, that
variable can then hold
instances of any of the Fruit classes. Declaring a variable of type Object means
that variable can
hold any object.
Technical Note
Java does not have a typedef statement (as in C and C++). To
declare new types in Java, you declare a new class; then variables can
be declared to be of that class’s type.
Assigning Values to Variables
Once a variable has been declared, you can assign a value to that variable by
using the assignment
operator =, like this:
size = 14;
tooMuchCaffiene = true;
Comments
Java has three kinds of comments: two for regular comments in source code and
one for the special
documentation system javadoc.
The symbols /* and */ surround multiline comments, as in C or C++. All text
between the two
delimiters is ignored:
/* I don’t know how I wrote this next part; I was working
really late one night and it just sort of appeared. I
suspect the code elves did it for me. It might be wise
not to try and change it.
*/
These comments cannot be nested; that is, you cannot have a comment inside a
comment.
Double-slashes (//) can be used for a single line of comment. All the text up to
the end of the line is
ignored:
int vices = 7; // are there really only 7 vices?
The final type of comment begins with /** and ends with */. The contents of
these special comments
are used by the javadoc system, but are otherwise used identically to the first
type of comment.
javadoc is used to generate API documentation from the code. You’ll learn more
about javadoc on
Day 22, “Java Programming Tools.”
Literals
Literal is a programming language term that essentially means that what you type
is what you get. For
example, if you type 4 in a Java program, you automatically get an integer with
the value 4. If you type
‘a’, you get a character with the value a. Literals are used to indicate simple
values in your Java
programs.
New Term
A literal is a simple value where “what you type is what you get.”
Numbers, characters, and strings are all examples of literals.
Literals may seem intuitive most of the time, but there are some special cases
of literals in Java for
different kinds of numbers, characters, strings, and boolean values.
Number Literals
There are several integer literals. 4, for example, is a decimal integer literal
of type int (although you
can assign it to a variable of type byte or short because it’s small enough to
fit into those types). A
decimal integer literal larger than an int is automatically of type long. You
also can force a smaller
number to a long by appending an L or l to that number (for example, 4L is a
long integer of value
4). Negative integers are preceded by a minus sign-for example, -45.
Integers can also be expressed as octal or hexadecimal: A leading 0 indicates
that a number is octal-for
example, 0777 or 0004. A leading 0x (or 0X) means that it is in hex (0xFF,
0XAf45). Hexadecimal
numbers can contain regular digits (0-9) or upper- or lowercase hex digits (a-f
or A-F).
Floating-point literals usually have two parts, the integer part and the decimal
part-for example,
5.77777. A floating-point literal results in a floating-point number of type
double, regardless of the
precision of the number. You can force the number to the type float by appending
the letter f (or F) to
that number-for example, 2.56F.
You can use exponents in floating-point literals using the letter e or E
followed by the exponent (which
can be a negative number): 10e45 or .36E-2.
Boolean Literals
Boolean literals consist of the keywords true and false. These keywords can be
used anywhere you
need a test or as the only possible values for boolean variables.
Character Literals
Character literals are expressed by a single character surrounded by single
quotes: ‘a’, ‘#’, ’3′, and
so on. Characters are stored as 16-bit Unicode characters. Table 3.2 lists the
special codes that can
represent nonprintable characters, as well as characters from the Unicode
character set. The letter d in the
octal, hex, and Unicode escapes represents a number or a hexadecimal digit (a-f
or A-F).
Table 3.2. Character escape codes.
Escape Meaning
\n Newline
\t Tab
\b Backspace
\r Carriage return
\f Formfeed
\\ Backslash
\’ Single quote
\” Double quote
\ddd Octal
\xdd Hexadecimal
\udddd Unicode character
Technical Note
C and C++ programmers should note that Java does not include
character codes for \a (bell) or \v (vertical tab).
String Literals
A combination of characters is a string. Strings in Java are instances of the
class String. Strings are not
simply arrays of characters as they are in C or C++, although they do have many
array-like
characteristics (for example, you can test their length, and access and change
individual characters).
Because string objects are real objects in Java, they have methods that enable
you to combine, test, and
modify strings very easily.
String literals consist of a series of characters inside double quotes:
“Hi, I’m a string literal.”
“” //an empty string
Strings can contain character constants such as newline, tab, and Unicode
characters:
“A string with a \t tab in it”
“Nested strings are \”strings inside of\” other strings”
“This string brought to you by Java\u2122″
In the last example, the Unicode code sequence for \u2122 produces a trademark
symbol ( ).
Note
Just because you can represent a character using a Unicode escape
does not mean your computer can display that character-the computer
or operating system you are running may not support Unicode, or the
font you’re using may not have a glyph (picture) for that character.
All that Unicode escapes in Java provide is a way to encode Unicode
characters for systems that support Unicode.
Java 1.1 will provide better capabilities for the display of Unicode
characters and for handling international character sets.
When you use a string literal in your Java program, Java automatically creates
an instance of the class
String for you with the value you give it. Strings are unusual in this respect;
the other literals do not
behave in this way (none of the primitive data types are actual objects), and
usually creating a new object
involves explicitly creating a new instance of a class. You’ll learn more about
strings, the String class,
and the things you can do with strings later today and tomorrow.
Expressions and Operators
Expressions are the simplest form of statement in Java that actually
accomplishes something: All
expressions, when evaluated, return a value (other statements don’t necessarily
do so). Arithmetic and
tests for equality and magnitude are common examples of expressions. Because
they return a value, you
can assign that result to a variable or test that value in other Java
statements.
Most of the expressions in Java use operators. Operators are special symbols for
things like arithmetic,
various forms of assignment, increment and decrement, and logical operations.
New Term
Expressions are statements that return a value.
New Term
Operators are special symbols that are commonly used in expressions.
Arithmetic
Java has five operators for basic arithmetic (see Table 3.3).
Table 3.3. Arithmetic operators.
Operator Meaning Example
+ Addition 3 + 4
- Subtraction 5 – 7
* Multiplication 5 * 5
/ Division 14 / 7
% Modulus 20 % 7
Each operator takes two operands, one on either side of the operator. The
subtraction operator (-) can
also be used to negate a single operand.
Integer division results in an integer. Because integers don’t have decimal
fractions, any remainder is
ignored. The expression 31 / 9, for example, results in 3 (9 goes into 31 only 3
times).
Modulus (%) gives the remainder once the operands have been evenly divided. For
example, 31 % 9
results in 4 because 9 goes into 31 three times, with 4 left over.
Note that the result type of most arithmetic operations involving integers is an
int regardless of the
original type of the operands (shorts and bytes are both automatically converted
to int). If either or
both operands is of type long, the result is of type long. If one operand is an
integer and another is a
floating-point number, the result is a floating point. (If you’re interested in
the details of how Java
promotes and converts numeric types from one type to another, you may want to
check out the Java
Language Specification on Sun’s official Java Web site at http://java.sun.com/;
that’s more
detail than I want to cover here.)
Listing 3.1 is an example of simple arithmetic in Java.
Listing 3.1. Simple arithmetic.
1: class ArithmeticTest {
2: public static void main (String args[]) {
3: short x = 6;
4: int y = 4;
5: float a = 12.5f;
6: float b = 7f;
7:
8: System.out.println(“x is ” + x + “, y is ” + y);
9: System.out.println(“x + y = ” + (x + y));
10: System.out.println(“x – y = ” + (x – y));
11: System.out.println(“x / y = ” + (x / y));
12: System.out.println(“x % y = ” + (x % y));
13:
14: System.out.println(“a is ” + a + “, b is ” + b);
15: System.out.println(“a / b = ” + (a / b));
16: }
17: }
x is 6, y is 4
x + y = 10
x – y = 2
x / y = 1
x % y = 2
a is 12.5, b is 7
a / b = 1.78571
Analysis
In this simple Java application (note the main() method), you
initially define four variables in lines 3 through 6: x and y, which are
integers (type int), and a and b, which are floating-point numbers
(type float). Keep in mind that the default type for floating-point
literals (such as 12.5) is double, so to make sure these are
numbers of type float, you have to use an f after each one (lines 5
and 6).
The remainder of the program merely does some math with integers and
floating-point numbers and
prints out the results.
There is one other thing to mention about this program: the method
System.out.println().
You’ve seen this method on previous days, but you haven’t really learned exactly
what it does. The
System.out.println() method merely prints a message to the standard output of
your system-to
the screen, to a special window, or maybe just to a special log file, depending
on your system and the
development environment you’re running. The System.out.println() method takes a
single
argument-a string-but you can use + to concatenate multiple values into a single
string, as you’ll learn
later today.
More About Assignment
Variable assignment is a form of expression; in fact, because one assignment
expression results in a
value, you can string them together like this:
x = y = z = 0;
In this example, all three variables now have the value 0.
The right side of an assignment expression is always evaluated before the
assignment takes place. This
means that expressions such as x = x + 2 do the right thing; 2 is added to the
value of x, and then
that new value is reassigned to x. In fact, this sort of operation is so common
that Java has several
operators to do a shorthand version of this, borrowed from C and C++. Table 3.4
shows these shorthand
assignment operators.
Table 3.4. Assignment operators.
Expression Meaning
x += y x = x + y
x -= y x = x – y
x *= y x = x * y
x /= y x = x / y
Technical Note
Technically, the shorthand assignment and longhand expressions are
not exactly equivalent, particularly in cases where x or y may
themselves be complicated expressions and your code relies on side
effects of those expressions. In most instances, however, they are
functionally equivalent. For more information about very complicated
expressions, evaluation order, and side effects, you may want to
consult the Java Language Specification.
Incrementing and Decrementing
As in C and C++, the ++ and — operators are used to increment or decrement a
variable’s value by 1.
For example, x++ increments the value of x by 1 just as if you had used the
expression x = x + 1.
Similarly x– decrements the value of x by 1. (Unlike C and C++, Java allows x
to be floating point.)
These increment and decrement operators can be prefixed or postfixed; that is,
the ++ or — can appear
before or after the value it increments or decrements. For simple increment or
decrement expressions,
which one you use isn’t overly important. In complex assignments, where you are
assigning the result of
an increment or decrement expression, which one you use makes a difference.
Take, for example, the following two expressions:
y = x++;
y = ++x;
These two expressions yield very different results because of the difference
between prefix and postfix.
When you use postfix operators (x++ or x–), y gets the value of x before x is
changed; using prefix,
the value of x is assigned to y after the change has occurred. Listing 3.2 is a
Java example of how all this
works.
Listing 3.2. Test of prefix and postfix increment operators.
1: class PrePostFixTest {
2:
3: public static void main (String args[]) {
4: int x = 0;
5: int y = 0;
6:
7: System.out.println(“x and y are ” + x + ” and ” + y );
8: x++;
9: System.out.println(“x++ results in ” + x);
10: ++x;
11: System.out.println(“++x results in ” + x);
12: System.out.println(“Resetting x back to 0.”);
13: x = 0;
14: System.out.println(“————”);
15: y = x++;
16: System.out.println(“y = x++ (postfix) results in:”);
17: System.out.println(“x is ” + x);
18: System.out.println(“y is ” + y);
19: System.out.println(“————”);
20:
21: y = ++x;
22: System.out.println(“y = ++x (prefix) results in:”);
23: System.out.println(“x is ” + x);
24: System.out.println(“y is ” + y);
25: System.out.println(“————”);
26:
27: }
28: }
x and y are 0 and 0
x++ results in 1
++x results in 2
Resetting x back to 0.
————
y = x++ (postfix) results in:
x is 1
y is 0
————
y = ++x (prefix) results in:
x is 2
y is 2
————
In the first part of this example, you increment x alone using both prefix and
postfix increment
operators. In each, x is incremented by 1 each time. In this simple form, using
either prefix or
postfix works the same way.
In the second part of this example, you use the expression y = x++, in which the
postfix increment
operator is used. In this result, the value of x is incremented after that value
is assigned to y. Hence the
result: y is assigned the original value of x (0), and then x is incremented by
1.
In the third part, you use the prefix expression y = ++x. Here, the reverse
occurs: x is incremented
before its value is assigned to y. Because x is 1 from the previous step, its
value is incremented (to 2),
and then that value is assigned to y. Both x and y end up being 2.
Technical Note
Technically, this description is not entirely correct. In reality, Java
always completely evaluates all expressions on the right of an
expression before assigning that value to a variable, so the concept of
“assigning x to y before x is incremented” isn’t precisely right.
Instead, Java takes the value of x and “remembers” it, evaluates
(increments) x, and then assigns the original value of x to y. Although
in most simple cases this distinction may not be important, for more
complex expressions with side effects, it may change the behavior of
the expression overall. See the Language Specification for many more
details about expression evaluation in Java.
Comparisons
Java has several expressions for testing equality and magnitude. All of these
expressions return a boolean
value (that is, true or false). Table 3.5 shows the comparison operators.
Table 3.5. Comparison operators.
Operator Meaning Example
== Equal x == 3
!= Not equal x != 3
< Less than x < 3
> Greater than x > 3
<= Less than or equal to x <= 3
>= Greater than or equal to x >= 3
Logical Operators
Expressions that result in boolean values (for example, the comparison
operators) can be combined by
using logical operators that represent the logical combinations AND, OR, XOR,
and logical NOT.
For AND combinations, use either the & or && operators. The entire expression
will be true only if both
expressions on either side of the operator are also true; if either expression
is false, the entire expression
is false. The difference between the two operators is in expression evaluation.
Using &, both sides of the
expression are evaluated regardless of the outcome. Using &&, if the left side
of the expression is false,
the entire expression is assumed to be false (the value of the right side
doesn’t matter), so the expression
returns false, and the right side of the expression is never evaluated. (This is
often called a
“short-circuited” expression.)
For OR expressions, use either | or ||. OR expressions result in true if either
or both of the expressions
on either side is also true; if both expression operands are false, the
expression is false. As with & and
&&, the single | evaluates both sides of the expression regardless of the
outcome; and || is
short-circuited: If the left expression is true, the expression returns true and
the right side is never
evaluated.
In addition, there is the XOR operator ^, which returns true only if its
operands are different (one true
and one false, or vice versa) and false otherwise (even if both are true).
In general, only the && and || are commonly used as actual logical combinations.
&, |, and ^ are more
commonly used for bitwise logical operations.
For NOT, use the ! operator with a single expression argument. The value of the
NOT expression is the
negation of the expression; if x is true, !x is false.
Bitwise Operators
Finally, here’s a short summary of the bitwise operators in Java. Most of these
expressions are inherited
from C and C++ and are used to perform operations on individual bits in
integers. This book does not go
into bitwise operations; it’s an advanced topic covered better in books on C or
C++. Table 3.6
summarizes the bitwise operators.
Table 3.6. Bitwise operators.
Operator Meaning
& Bitwise AND
| Bitwise OR
^ Bitwise XOR
<< Left shift
>> Right shift
>>> Zero fill right shift
~ Bitwise complement
<<= Left shift assignment (x = x << y)
>>= Right shift assignment (x = x >> y)
>>>= Zero fill right shift assignment (x = x
>>> y)
x&=y AND assignment (x = x & y)
x|=y OR assignment (x = x | y)
x^=y XOR assignment (x = x ^ y)
Operator Precedence
Operator precedence determines the order in which expressions are evaluated.
This, in some cases, can
determine the overall value of the expression. For example, take the following
expression:
y = 6 + 4 / 2
Depending on whether the 6 + 4 expression or the 4 / 2 expression is evaluated
first, the value of y
can end up being 5 or 8. Operator precedence determines the order in which
expressions are evaluated,
so you can predict the outcome of an expression. In general, increment and
decrement are evaluated
before arithmetic, arithmetic expressions are evaluated before comparisons, and
comparisons are
evaluated before logical expressions. Assignment expressions are evaluated last.
Table 3.7 shows the specific precedence of the various operators in Java.
Operators further up in the table
are evaluated first; operators on the same line have the same precedence and are
evaluated left to right
based on how they appear in the expression itself. For example, given that same
expression y = 6 + 4
/ 2, you now know, according to this table, that division is evaluated before
addition, so the value of y
will be 8.
Table 3.7. Operator precedence.
Operator Notes
. [] () Parentheses (()) are used to group expressions; dot
(.) is used for access to methods and variables
within objects and classes (discussed tomorrow);
square brackets ([]) are used for arrays (this is
discussed later on in the week)
++ — ! ~
instanceof
The instanceof operator returns true or
false based on whether the object is an instance
of the named class or any of that class’s subclasses
(discussed tomorrow)
new (type)expression The new operator is used for creating new instances
of classes; () in this case is for casting a value to
another type (you’ll learn about both of these
tomorrow)
* / % Multiplication, division, modulus
+ – Addition, subtraction
<< >> >>> Bitwise left and right shift
< > <= >= Relational comparison tests
== != Equality
& AND
^ XOR
| OR
&& Logical AND
|| Logical OR
? : Shorthand for if…then…else (discussed on
Day 5)
= += -= *= /= %= ^= Various assignments
&= |= <<= >>= >>>= More assignments
You can always change the order in which expressions are evaluated by using
parentheses around the
expressions you want to evaluate first. You can nest parentheses to make sure
expressions evaluate in the
order you want them to (the innermost parenthetic expression is evaluated
first). The following
expression results in a value of 5, because the 6 + 4 expression is evaluated
first, and then the result of
that expression (10) is divided by 2:
y = (6 + 4) / 2
Parentheses also can be useful in cases where the precedence of an expression
isn’t immediately clear-in
other words, they can make your code easier to read. Adding parentheses doesn’t
hurt, so if they help you
figure out how expressions are evaluated, go ahead and use them.
String Arithmetic
One special expression in Java is the use of the addition operator (+) to create
and concatenate strings. In
most of the examples shown today and in earlier lessons, you’ve seen lots of
lines that looked something
like this:
System.out.println(name + ” is a ” + color + ” beetle”);
The output of that line (to the standard output) is a single string, with the
values of the variables (name
and color), inserted in the appropriate spots in the string. So what’s going on
here?
The + operator, when used with strings and other objects, creates a single
string that contains the
concatenation of all its operands. If any of the operands in string
concatenation is not a string, it is
automatically converted to a string, making it easy to create these sorts of
output lines.
Technical Note
An object or type can be converted to a string if you implement the
method toString(). All objects have a default string
representation, but most classes override toString() to provide a
more meaningful printable representation.
String concatenation makes lines such as the previous one especially easy to
construct. To create a string,
just add all the parts together-the descriptions plus the variables-and print it
to the standard output, to the
screen, to an applet, or anywhere.
The += operator, which you learned about earlier, also works for strings. For
example, take the following
expression:
myName += ” Jr.”;
This expression is equivalent to this:
myName = myName + ” Jr.”;
just as it would be for numbers. In this case, it changes the value of myName,
which might be something
like John Smith to have a Jr. at the end (John Smith Jr.).
Summary
As you have learned in the last two lessons, a Java program is made up primarily
of classes and objects.
Classes and objects, in turn, are made up of methods and variables, and methods
are made up of
statements and expressions. It is those last two things that you’ve learned
about today; the basic building
blocks that enable you to create classes and methods and build them up to a
full-fledged Java program.
Today, you have learned about variables, how to declare them and assign values
to them; literals for
easily creating numbers, characters, and strings; and operators for arithmetic,
tests, and other simple
operations. With this basic syntax, you can move on tomorrow to learning about
working with objects
and building simple, useful Java programs.
To finish up this summary, Table 3.8 is a list of all the operators you have
learned about today so that
you can refer back to them.
Table 3.8. Operator summary.
Operator Meaning
+ Addition
- Subtraction
* Multiplication
/ Division
% Modulus
< Less than
> Greater than
<= Less than or equal to
>= Greater than or equal to
== Equal
!= Not equal
&& Logical AND
|| Logical OR
! Logical NOT
& AND
| OR
^ XOR
<< Left shift
>> Right shift
>>> Zero fill right shift
~ Complement
= Assignment
++ Increment
—- Decrement
+= Add and assign
-= Subtract and assign
*= Multiply and assign
/= Divide and assign
%= Modulus and assign
&= AND and assign
|= OR and assign
<<= Left shift and assign
^= XOR and assign
>>= Right shift and assign
>>>= Zero fill right shift and assign
Q&A
Q: I didn’t see any way to define constants.
A: You can’t create local constants in Java; you can create only constant
instance and class
variables. You’ll learn how to do this tomorrow.
Q: What happens if you assign an integer value to a variable that is too large
for that variable
to hold?
A: Logically, you would think that the variable is just converted to the next
larger type, but this isn’t
what happens. What does happen is called overflow. This means that if a number
becomes too
big for its variable, that number wraps around to the smallest possible negative
number for that
type and starts counting upward toward zero again.
Because this can result in some very confusing (and wrong) results, make sure
that you declare
the right integer type for all your numbers. If there’s a chance a number will
overflow its type,
use the next larger type instead.
Q: How can you find out the type of a given variable?
A: If you’re using any of the primitive types (int, float, boolean), and so on,
you can’t. If you
care about the type, you can convert the value to some other type by using
casting. (You’ll learn
about this tomorrow.)
If you’re using class types, you can use the instanceof operator, which you’ll
learn more
about tomorrow.
Q: Why does Java have all these shorthand operators for arithmetic and
assignment? It’s
really hard to read that way.
A: The syntax of Java is based on C++, and therefore on C. One of C’s implicit
goals is the
capability of doing very powerful things with a minimum of typing. Because of
this, shorthand
operators, such as the wide array of assignments, are common.
There’s no rule that says you have to use these operators in your own programs,
however. If you
find your code to be more readable using the long form, no one will come to your
house and
make you change it.
Q: You covered simple math in this section using operators. I’m assuming that
Java has ways
of doing more complex math operations?
A: You assume correctly. A special class in the java.lang package, called
java.lang.Math,
has a number of methods for exponential, trigonometric, and other basic math
operations. In fact,
because you call these methods using the Math class itself, these are prime
examples of class
methods. You’ll learn more about this tomorrow.
Java Help, Java Tutorials, Java Programming, Java Tricks
Lesson Two
Thinking in Objects: An Analogy
CONTENTS
Objects and Classes
Behavior and Attributes
Attributes
Behavior
Creating a Class
Inheritance, Interfaces, and Packages
Inheritance
Creating a Class Hierarchy
How Inheritance Works
Single and Multiple Inheritance
Interfaces and Packages
Creating a Subclass
Summary
Q&A
Object-oriented programming (OOP) is one of the biggest programming ideas of
recent years, and you might worry that you must spend ears learning all about
object-oriented programming methodologies and how they can make your life easier
than The Old Way of programming. It all comes down to organizing your programs
in ways that echo how things are put together in the real world.
Today you’ll get an overview of object-oriented programming concepts in Java and
how they relate to how you structure your own
programs:
What classes and objects are and how they relate to each other
The two main parts of a class or object: its behaviors and its attributes
Class inheritance and how inheritance affects the way you design your programs
Some information about packages and interfaces
If you’re already familiar with object-oriented programming, much of today’s
lesson will be old hat to you.
You may want to skim it and go to a movie today instead. Tomorrow, you’ll get
into more specific details.
Thinking in Objects: An Analogy
Consider, if you will, Legos. Legos, for those who do not spend much time with
children, are small plastic building blocks in various colors and sizes. They
have small round bits on one side that fit into small round holes on other Legos
so that they fit together snugly to create larger shapes. With different Lego
parts (Lego wheels, Lego engines, Lego hinges, Lego pulleys), you can put
together castles, automobiles, giant robots that swallow cities, or just about
anything else you can imagine. Each Lego part is a small object that fits
together with other small objects in predefined ways to create other larger
objects. That is roughly how object-oriented
programming works: putting together smaller elements to build larger ones.
Here’s another example. You can walk into a computer store and, with a little
background and often some help, assemble an entire pc computer system from
various components: a motherboard, a CPU chip, a video card, a hard disk, a
keyboard, and so on. Ideally, when you finish assembling all the various
self-contained units, you have a system in which all the units work together to
create a larger system with which you can solve the problems you bought the
computer for in the first place.
Internally, each of those components may be vastly complicated and engineered by
different companies with different methods of design. But you don’t need to know
how the component works, what every chip on the board does, or how, when you
press the A key, an A gets sent to your computer. As the assembler of the
overall system, each component you use is a self-contained unit, and all you are
interested in is how the units interact with each other. Will this video card
fit into the slots on the motherboard, and will this monitor work with this
video card? Will each particular component speak the right commands to the other
components it interacts with so that each part of the computer is understood by
every other part? Once you know what the interactions are between the components
and can match the interactions, putting together the overall system is easy.
What does this have to do with programming? Everything. Object-oriented
programming works in exactly this same way. Using object-oriented programming,
your overall program is made up of lots of different self-contained components
(objects), each of which has a specific role in the program and all of which can
talk to each other in predefined ways.
Objects and Classes
Object-oriented programming is modeled on how, in the real world, objects are
often ade up of many kindsof smaller objects. This capability of combining
objects, owever, is only one very general aspect ofobject-oriented programming.
Object-oriented rogramming provides several other concepts and features to make
creating and using objects easier and more flexible, and the most important of
these features is classes.
When you write a program in an object-oriented language, you don’t define actual
objects. You define classes of objects, where a class is a template for multiple
objects with similar features. Classes embody all the features of a particular
set of objects. For example, you might have a Tree class that describes the
features of all trees (has leaves and roots, grows, creates chlorophyll). The
Tree class serves as an abstract model for the concept of a tree-to reach out
and grab, or interact with, or cut down a tree you have to have a concrete
instance of that tree. Of course, once you have a tree class, you can create
lots of different instances of that tree, and each different tree instance can
have different features (short, tall, bushy, drops leaves in autumn), while
still behaving like and being immediately recognizable as a tree (see Figure
2.1).
Figure 2.1 : The Tree class and several Tree instances.
New Term
A class is a generic template for a set of objects with similar features.
An instance of a class is another word for an actual object. If class is the
general (generic) representation of an object, an instance is its concrete
representation. So what, precisely, is the difference between an instance and an
object? Nothing, really. Object is the more general term, but both instances and
objects are the concrete representation of a class. In fact, the terms instance
and object are often used interchangeably in OOP lingo.
An instance of a tree and a tree object are both the same thing.
New Term
An instance is the specific concrete representation of a class.
Instances and objects are the same thing.
What about an example closer to the sort of things you might want to do in Java
programming? You might create a class for the user interface element called a
button. The Button class defines the features of a button (its label, its size,
its appearance) and how it behaves. (Does it need a single-click or a
double-click to activate it? Does it change color when it’s clicked? What does
it do when it’s activated?) After you define the Button class, you can then
easily create instances of that button-that is, button objects-that all take on
the basic features of the button as defined by the class, but may have different
appearances and behavior based on what you want that particular button to do. By
creating a Button class, you don’t have to keep rewriting the code for each
individual button you want to use in your program, and you can reuse the Button
class to create different kinds of buttons as you need them in this program and
in other programs.
Tip
If you’re used to programming in C, you can think of a class as sort of creating
a new composite data type by using struct and typedef. Classes, however, can
provide much more than just a collection of data, as you’ll discover in the rest
of today’s lesson. When you write a Java program, you design and construct a set
of classes. Then when your program runs, instances of those classes are created
and discarded as needed. Your task, as a Java programmer, is to create the right
set of classes to accomplish what your program needs to accomplish. Fortunately,
you don’t have to start from the very beginning: The Java environment comes with
a standard set
of classes (called a class library) that implement a lot of the basic behavior
you need-not only for basic programming tasks (classes to provide basic math
functions, arrays, strings, and so on), but also for graphics and networking
behavior. In many cases, the Java class libraries may be enough so that all you
have to do in your Java program is create a single class that uses the standard
class libraries. For complicated Java programs, you may have to create a whole
set of classes with defined interactions between them.
New Term
A class library is a collection of classes intended to be reused repeatedly in
different programs. The standard Java class libraries contain quite a few
classes for accomplishing basic programming tasks in Java.
Behavior and Attributes
Every class you write in Java has two basic features: attributes and behavior.
In this section you’ll learn about each one as it applies to a theoretical
simple class called Motorcycle. To finish up this section, you’ll create the
Java code to implement a representation of a motorcycle.
Attributes
Attributes are the individual things that differentiate one object from another
and determine the appearance, state, or other qualities of that object. Let’s
create a theoretical class called Motorcycle. A motorcycle class might include
the following attributes and have these typical values:
l Color: red, green, silver, brown
l Style: cruiser, sport bike, standard
l Make: Honda, BMW, Bultaco
Attributes of an object can also include information about its state; for
example, you could have features for engine condition (off or on) or current
gear selected.
Attributes are defined in classes by variables. Those variables’ types and names
are defined in the class, and each object can have its own values for those
variables. Because each instance of a class can have different values for its
variables, these variables are often called instance variables.
New Term
An instance variable defines the attributes of the object. Instance variables’
types and names are defined in the class, but their values
are set and changed in the object. Instance variables may be initially set when
an object is created and stay constant throughout the life of the object, or
they may be able to change at will as the program runs. Change the value of the
variable, and you
change an object’s attributes. In addition to instance variables, there are also
class variables, which apply to the class itself and to all its
instances. Unlike instance variables, whose values are stored in the instance,
class variables’ values are stored in the class itself. You’ll learn about class
variables later on this week and more specifics about instance variables
tomorrow.
Behavior
A class’s behavior determines how an instance of that class operates; for
example, how it will “react” if asked to do something by another class or object
or if its internal state changes. Behavior is the only way objects can do
anything to themselves or have anything done to them. For example, to go back to
the theoretical Motorcycle class, here are some behaviors that the Motorcycle
class might have:
l Start the engine
l Stop the engine
l Speed up
l Change gear
l Stall
To define an object’s behavior, you create methods, a set of Java statements
that accomplish some task. Methods look and behave just like functions in other
languages but are defined and accessible solely inside a class. Java does not
have functions defined outside classes (as C++ does).
New Term
Methods are functions defined inside classes that operate on instances
of those classes.
While methods can be used solely to operate on an individual object, methods are
also used between objects to communicate with each other. A class or an object
can call methods in another class or object to communicate changes in the
environment or to ask that object to change its state. Just as there are
instance and class variables, there are also instance and class methods.
Instance methods (which are so common that they’re usually just called methods)
apply and operate on an instance of a class; class methods apply and operate on
the class itself. You’ll learn more about class methods later on this week.
Creating a Class
Up to this point, today’s lesson has been pretty theoretical. In this section,
you’ll create a working example of the Motorcycle class so that you can see how
instance variables and methods are defined in a class in Java. You’ll also
create a Java application that creates a new instance of the Motorcycle class
and shows its instance variables.
Note
I’m not going to go into a lot of detail about the actual syntax of this example
here. Don’t worry too much about it if you’re not really sure
what’s going on; it will become clear to you later on this week. All you really
need to worry about in this example is understanding the
basic parts of this class definition. Ready? Let’s start with a basic class
definition. Open the text editor you’ve been using to create Java source code
and enter the following (remember, upper- and lowercase matters):
class Motorcycle {
}
Congratulations! You’ve now created a class. Of course, it doesn’t do very much
at the moment, but that’s a
Java class at its very simplest.
First, let’s create some instance variables for this class-three of them, to be
specific. Just below the first line,
add the following three lines:
String make;
String color;
boolean engineState = false;
Here you’ve created three instance variables: Two, make and color, can contain
String objects (a string is the generic term for a series of characters; String,
with a capital S, is part of that standard class library mentioned earlier). The
third, engineState, is a boolean variable that refers to whether the engine is
off or on; a value of false means that the engine is off, and true means that
the engine is on. Note that boolean is lowercase b.
New Term
A boolean is a value of either true or false.
Technical Note
boolean in Java is a real data type that can have the values true or false.
Unlike in C, booleans are not numbers. You’ll hear about this again tomorrow so
that you won’t forget. Now let’s add some behavior (methods) to the class. There
are all kinds of things a motorcycle can do, but to keep things short, let’s add
just one method-a method that starts the engine. Add the following lines below
the
instance variables in your class definition:
void startEngine() {
if (engineState == true)
System.out.println(“The engine is already on.”);
else {
engineState = true;
System.out.println(“The engine is now on.”);
}
}
The startEngine() method tests to see whether the engine is already running (in
the part engineState == true) and, if it is, merely prints a message to that
effect. If the engine isn’t already running, it changes the state of the engine
to true (turning the engine on) and then prints a message. Finally, because the
startEngine() method doesn’t return a value, its definition includes the word
void at the beginning. (You can also define methods to return values; you’ll
learn more about method definitions on Day 6, “Creating Classes and Applications
in Java.”)
Tip
Here and throughout this book, whenever I refer to the name of a method, I’ll
add empty parentheses to the end of the name (for
example, as I did in the first sentence of the previous paragraph: “The
startEngine() method…” This is a convention used in the
programming community at large to indicate that a particular name is a method
and not a variable. The parentheses are silent.
With your methods and variables in place, save the program to a file called
Motorcycle.java (remember that you should always name your Java source files the
same names as the class they define). Listing 2.1 shows what your program should
look like so far.
Listing 2.1. The Motorcycle.java file.
1:class Motorcycle {
2:
3: String make;
4: String color;
5: boolean engineState = false;
6:
7: void startEngine() {
8: if (engineState == true)
9: System.out.println(“The engine is already on.”);
10: else {
11: engineState = true;
12: System.out.println(“The engine is now on.”);
13: }
14: }
15:}
Tip
The indentation of each part of the class isn’t important to the Java compiler.
Using some form of indentation, however, makes your lass definition easier for
you and other people to read. The indentation used here, with instance variables
and methods indented from the lass definition, is the style used throughout this
book. The Java class libraries use a similar indentation. You can choose any
indentation style hat you like.
Before you compile this class, let’s add one more method just below the
startEngine() method (that is, between lines 14 and 15). The showAtts() method
is used to print the current values of all the instance variables in an instance
of your Motorcycle class. Here’s what it looks like:
void showAtts() {
System.out.println(“This motorcycle is a “
+ color + ” ” + make);
if (engineState == true)
System.out.println(“The engine is on.”);
else System.out.println(“The engine is off.”);
}
The showAtts() method prints two lines to the screen: the make and color of the
motorcycle object and whether the engine is on or off. Now you have a Java class
with three instance variables and two methods defined. Save that file again, and
compile it using one of the following methods:
Note
After this point, I’m going to assume you know how to compile and run Java
programs. I won’t repeat this information after this.
Windows
From inside a DOS shell, CD to the directory containing your Java source file,
and use the javac command to compile it:
javac Motorcycle.java
Macintosh
Drag and drop the Motorcycle.java file onto the Java Compiler icon.
Salaris
From a command line, CD to the directory containing your Java source file, and
use the javac command to compile it:
javac Motorcycle.java
When you run this little program using the java or Java Runner programs, you’ll
get an error. Why? When you run a compiled Java class directly, Java assumes
that the class is an application and looks for a main()
method. Because we haven’t defined a main() method inside the class, the Java
interpreter (java) gives you
an error something like one of these two errors:
In class Motorcycle: void main(String argv[]) is not defined
Exception in thread “main”: java.lang.UnknownError
To do something with the Motorcycle class-for example, to create instances of
that class and play with them-you’re going to need to create a separate Java
applet or application that uses this class or add a main()method to this one.
For simplicity’s sake, let’s do the latter. Listing 2.2 shows the main() method
you’ll add to the Motorcycle class. You’ll want to add this method to your
Motorcycle.java source file just before the last closing brace (}), underneath
the startEngine() and showAtts() methods.
Listing 2.2. The main() method for Motorcycle.java.
1: public static void main (String args[]) {
2: Motorcycle m = new Motorcycle();
3: m.make = “Yamaha RZ350″;
4: m.color = “yellow”;
5: System.out.println(“Calling showAtts…”);
6: m.showAtts();
7: System.out.println(“——–”);
8: System.out.println(“Starting engine…”);
9: m.startEngine();
10: System.out.println(“——–”);
11: System.out.println(“Calling showAtts…”);
12: m.showAtts();
13: System.out.println(“——–”);
14: System.out.println(“Starting engine…”);
15: m.startEngine();
16:}
With the main() method in place, the Motorcycle class is now an official
application, and you can
compile it again and this time it’ll run. Here’s how the output should look:
Calling showAtts…
This motorcycle is a yellow Yamaha RZ350
The engine is off.
——–
Starting engine…
The engine is now on.
——–
Calling showAtts…
This motorcycle is a yellow Yamaha RZ350
The engine is on.
——–
Starting engine…
The engine is already on.
Analysis
The contents of the main() method are all going to look very new to you, so
let’s go through it line by line so that you at least have a basic idea of what
it does (you’ll get details about the specifics of all of this tomorrow and the
day after).
The first line declares the main() method. The first line of the main() method
always looks like this; you’ll learn the specifics of each part later this week.
Line 2, Motorcycle m = new Motorcycle();, creates a new instance of the
Motorcycle class and stores a reference to it in the variable m. Remember, you
don’t usually operate directly on classes in your Java programs; instead, you
create objects from those classes and then call methods in those objects. Lines
3 and 4 set the instance variables for this Motorcycle object: The make is now a
Yamaha RZ350 (a very pretty motorcycle from the mid-1980s), and the color is
yellow.
Lines 5 and 6 call the showAtts() method, defined in your Motorcycle object.
(Actually, only 6 does; 5 just prints a message that you’re about to call this
method.) The new motorcycle object then prints out the values of its instance
variables-the make and color as you set in the previous lines-and shows that the
engine is off.
Line 7 prints a divider line to the screen; this is just for prettier output.
Line 9 calls the startEngine() method in the motorcycle object to start the
engine. The engine should now be on.
Line 11 prints the values of the instance variables again. This time, the report
should say the engine is now on.
Line 15 tries to start the engine again, just for fun. Because the engine is
already on, this should print the message The engine is already on.
Listing 2.3 shows the final Motorcycle class, in case you’ve been having trouble
compiling and running the one you’ve got (and remember, this example and all the
examples in this book are available on the CD that accompanies the book):
Listing 2.3. The final version of Motorcycle.java.
1: class Motorcycle {
2:
3: String make;
4: String color;
5: boolean engineState;
6:
7: void startEngine() {
8: if (engineState == true)
9: System.out.println(“The engine is already on.”);
10: else {
11: engineState = true;
12: System.out.println(“The engine is now on.”);
13: }
14: }
15:
16: void showAtts() {
17: System.out.println(“This motorcycle is a “
18: + color + ” ” + make);
19: if (engineState == true)
20: System.out.println(“The engine is on.”);
21: else System.out.println(“The engine is off.”);
22: }
23:
24: public static void main (String args[]) {
25: Motorcycle m = new Motorcycle();
26: m.make = “Yamaha RZ350″;
27: m.color = “yellow”;
28: System.out.println(“Calling showAtts…”);
29: m.showAtts();
30: System.out.println(“——”);
31: System.out.println(“Starting engine…”);
32: m.startEngine();
33: System.out.println(“——”);
34: System.out.println(“Calling showAtts…”);
35: m.showAtts();
36: System.out.println(“——”);
37: System.out.println(“Starting engine…”);
38: m.startEngine();
39: }
40:}
Inheritance, Interfaces, and Packages
Now that you have a basic grasp of classes, objects, methods, variables, and how
to put them all together in a Java program, it’s time to confuse you again.
Inheritance, interfaces, and packages are all mechanisms for organizing classes
and class behaviors. The Java class libraries use all these concepts, and the
best class libraries you write for your own programs will also use these
concepts.
Inheritance
Inheritance is one of the most crucial concepts in object-oriented programming,
and it has a very direct effect on how you design and write your Java classes.
Inheritance is a powerful mechanism that means when you write a class you only
have to specify how that class is different from some other class; inheritance
will give you automatic access to the information contained in that other class.
With inheritance, all classes-those you write, those from other class libraries
that you use, and those from the standard utility classes as well-are arranged
in a strict hierarchy (see Figure 2.2). Each class has a superclass (the class
above it in the hierarchy), and each class can have one or more subclasses
(classes below that class in the hierarchy). Classes further down in the
hierarchy are said to inherit from classes further up in the hierarchy.
Figure 2.2 : A class hierarchy.
Subclasses inherit all the methods and variables from their superclasses-that
is, in any particular class, if the superclass defines behavior that your class
needs, you don’t have to redefine it or copy that code from some other class.
Your class automatically gets that behavior from its superclass, that superclass
gets behavior from its superclass, and so on all the way up the hierarchy. Your
class becomes a combination of all the features of the classes above it in the
hierarchy.
New Term
Inheritance is a concept in object-oriented programming where all classes are
arranged in a strict hierarchy. Each class in the hierarchy
has superclasses (classes above it in the hierarchy) and any number of
subclasses (classes below it in the hierarchy). Subclasses inherit
attributes and behavior from their superclasses.
At the top of the Java class hierarchy is the class Object; all classes inherit
from this one superclass. Object is the most general class in the hierarchy; it
defines behavior inherited by all the classes in Java. Each class further down
in the hierarchy adds more information and becomes more tailored to a specific
purpose. In this way, you can think of a class hierarchy as defining very
abstract concepts at the top of the hierarchy and those ideas becoming more
concrete the farther down the chain of superclasses you go. Most of the time
when you write new Java classes, you’ll want to create a class that has all the
information some other class has, plus some extra information. For example, you
may want a version of a Button with its own built-in label. To get all the
Button information, all you have to do is define your class to inherit from
Button. Your class will automatically get all the behavior defined in Button
(and in Button’s superclasses), so all you have to worry about are the things
that make your class different from Button itself.
This mechanism for defining new classes as the differences between them and
their superclasses is called subclassing.
Subclassing involves creating a new class that inherits from some other class in
the class hierarchy. Using subclassing, you only need to define the differences
between your class and its parent; the additional behavior is all available to
your class through inheritance.
New Term
Subclassing is the process of creating a new class that inherits from some other
already-existing class.
What if your class defines an entirely new behavior and isn’t really a subclass
of another class? Your class can also inherit directly from Object, which still
allows it to fit neatly into the Java class hierarchy. In fact, if you create a
class definition that doesn’t indicate its superclass in the first line, Java
automatically assumes you’re inheriting from Object. The Motorcycle class you
created in the previous section inherited from
Object.
Creating a Class Hierarchy
If you’re creating a larger set of classes for a very complex program, it makes
sense for your classes not only to inherit from the existing class hierarchy,
but also to make up a hierarchy themselves. This may take some planning
beforehand when you’re trying to figure out how to organize your Java code, but
the advantages are significant once it’s done:
When you develop your classes in a hierarchy, you can factor out information
common to multiple classes in superclasses, and then reuse that superclass’s
information over and over again. Each subclass gets that common information from
its superclass.
l
Changing (or inserting) a class further up in the hierarchy automatically
changes the behavior of its subclasses-no need to change or recompile any of the
lower classes because they get the new information through inheritance and not
by copying any of the code.
l
For example, let’s go back to that Motorcycle class and pre tend you created a
Java program to implement all the features of a motorcycle. It’s done, it works,
and everything is fine. Now, your next task is to create a Java class called
Car.
Car and Motorcycle have many similar features-both are vehicles driven by
engines. Both have transmissions, headlamps, and speedometers. So your first
impulse may be to open your Motorcycle class file and copy over a lot of the
information you already defined into the new class Car.
A far better plan is to factor out the common information for Car and Motorcycle
into a more general class hierarchy. This may be a lot of work just for the
classes Motorcycle and Car, but once you add Bicycle, Scooter, Truck, and so on,
having common behavior in a reusable superclass significantly reduces the amount
of work you have to do overall.
Let’s design a class hierarchy that might serve this purpose. Starting at the
top is the class Object, which is the root of all Java classes. The most general
class to which a motorcycle and a car both belong might be called Vehicle. A
vehicle, generally, is defined as a thing that propels someone from one place to
another. In the Vehicle class, you define only the behavior that enables someone
to be propelled from point a to point b, and nothing more.
Below Vehicle? How about two classes: PersonPoweredVehicle and
EnginePoweredVehicle? EnginePoweredVehicle is different from Vehicle because it
has an engine, and the behaviors might include stopping and starting the engine,
having certain amounts of gasoline and oil, and perhaps the speed or gear in
which the engine is running. Person-powered vehicles have some kind of mechanism
for translating people motion into vehicle motion-pedals, for example. Figure
2.3 shows what you have so far.
Figure 2.3 : The basic vehicle hierarchy.
Now let’s become even more specific. With EnginePoweredVehicle, you might have
several classes:
Motorcycle, Car, Truck, and so on. Or you can factor out still more behavior and
have intermediate
classes for TwoWheeled and FourWheeled vehicles, with different behaviors for
each (see Figure 2.4).
Figure 2.4 : Two-wheeled and four-wheeled vehicles.
Finally, with a subclass for the two-wheeled engine-powered vehicles, you can
have a class for motorcycles. Alternatively, you could additionally define
scooters and mopeds, both of which are two-wheeled engine-powered vehicles but
have different qualities from motorcycles. Where do qualities such as make or
color come in? Wherever you want them to go-or, more usually, where they fit
most naturally in the class hierarchy. You can define the make and color on
Vehicle, and all the subclasses will have those variables as well. The point to
remember is that you have to define a feature or a behavior only once in the
hierarchy; it’s automatically reused by each subclass.
How Inheritance Works
How does inheritance work? How is it that instances of one class can
automatically get variables and methods from the classes further up in the
hierarchy?
For instance variables, when you create a new instance of a class, you get a
“slot” for each variable defined in the current class and for each variable
defined in all its superclasses. In this way, all the classes combine to form a
template for the current object, and then each object fills in the information
appropriate to its situation. Methods operate similarly: New objects have access
to all the method names of its class and its superclasses, but method
definitions are chosen dynamically when a method is called. That is, if you call
a method on a particular object, Java first checks the object’s class for the
definition of that method. If it’s not defined in the object’s class, it looks
in that class’s superclass, and so on up the chain until the method definition
is found (see Figure 2.5).
Figure 2.5 : How methods are located.
Things get complicated when a subclass defines a method that has the same
signature (name, number, and type of arguments) as a method defined in a
superclass. In this case, the method definition that is found first (starting at
the bottom and working upward toward the top of the hierarchy) is the one that
is actually executed. Therefore, you can intentionally define a method in a
subclass that has the same signature as a method in a superclass, which then
“hides” the superclass’s method. This is called overriding a method. You’ll
learn all about methods on Day 7, “More About Methods.”
New Term
Overriding a method is creating a method in a subclass that has the same
signature (name, number, and type of arguments) as a method in a superclass.
That new method then hides the superclass’s method
(see Figure 2.6).
Figure 2.6 : Overriding methods.
Single and Multiple Inheritance
Java’s form of inheritance, as you learned in the previous sections, is called
single inheritance. Single inheritance means that each Java class can have only
one superclass (although any given superclass can have multiple subclasses).
In other object-oriented programming languages, such as C++, classes can have
more than one superclass, and they inherit combined variables and methods from
all those classes. This is called multiple inheritance. Multiple inheritance can
provide enormous power in terms of being able to create classes that factor just
about all imaginable behavior, but it can also significantly complicate class
definitions and the code to produce them.
Java makes inheritance simpler by being only singly inherited.
Interfaces and Packages
There are two remaining concepts to discuss here: packages and interfaces. Both
are advanced topics for implementing and designing groups of classes and class
behavior. You’ll learn about both interfaces and packages on Day 16, “Packages
and Interfaces,” but they are worth at least introducing here.
Recall that each Java class has only a single superclass, and it inherits
variables and methods from that superclass and all its superclasses. Although
single inheritance makes the relationship between classes and the functionality
those classes implement easy to understand and to design, it can also be
somewhat restrictive-in particular, when you have similar behavior that needs to
be duplicated across different “branches” of the class hierarchy. Java solves
this problem of shared behavior by using the concept of interfaces, which
collect
method names into one place and then allow you to add those methods as a group
to the various classes that need them. Note that interfaces contain only method
names and interfaces (arguments, for example), not actual definitions.
Although a single Java class can have only one superclass (due to single
inheritance), that class can also implement any number of interfaces. By
implementing an interface, a class provides method implementations (definitions)
for the method names defined by the interface. If two very disparate classes
implement the same interface, they can both respond to the same method calls (as
defined by that interface), although what each class actually does in response
to those method calls may be very different.
New Term
An interface is a collection of method names, without definitions, that can be
added to classes to provide additional behavior not included
with those methods the class defined itself or inherited from its superclasses.
You don’t need to know very much about interfaces right now. You’ll learn more
as the book progresses, so if all this is very confusing, don’t panic! The final
new Java concept for today is packages. Packages in Java are a way of grouping
together related classes and interfaces in a single library or collection.
Packages enable modular groups of classes to be available only if they are
needed and eliminate potential conflicts between class names in different groups
of classes.
You’ll learn all about packages, including how to create and use them, in Week
3. For now, there are only a
few things you need to know:
l The class libraries in the Java Developer’s Kit are contained in a package
called java. The classes in the java package are guaranteed to be available in
any Java implementation and are the only classes guaranteed to be available
across different implementations. The java package itself contains other
packages for classes that define the language, the input and output classes,
some basic networking, the
window toolkit functions, and classes that define applets. Classes in other
packages (for example, classes in the sun or netscape packages) may be available
only in specific implementations.
By default, your Java classes have access to only the classes in java.lang (the
base language package inside the java package). To use classes from any other
package, you have to either refer to them explicitly by package name or import
them into your source file.
l
To refer to a class within a package, list all the packages that class is
contained in and the class name, all separated by periods (.). For example, take
the Color class, which is contained in the awt package (awt stands for Abstract
Windowing Toolkit). The awt package, in turn, is inside the java package. To
refer to the Color class in your program, you use the notation java.awt.Color.
l
Creating a Subclass
To finish up today, let’s create a class that is a subclass of another class and
override some methods. You’ll also get a basic feel for how packages work in
this example.
Probably the most typical instance of creating a subclass, at least when you
first start programming in Java, is creating an applet. All applets are
subclasses of the class Applet (which is part of the java.applet package). By
creating a subclass of Applet, you automatically get all the behavior from the
window toolkit and the layout classes that enable your applet to be drawn in the
right place on the page and to interact with system operations, such as
keypresses and mouse clicks. In this example, you’ll create an applet similar to
the Hello World applet from yesterday, but one that draws the Hello string in a
larger font and a different color. To start this example, let’s first construct
the class definition itself. Let’s go to your text editor, and enter the
following class definition:
public class HelloAgainApplet extends java.applet.Applet {
}
Here, you’re creating a class called HelloAgainApplet. Note the part that says
extends java.applet.Applet-that’s the part that says your applet class is a
subclass of the Applet class. Note that because the Applet class is contained in
the java.applet package, you don’t have automatic access to that class, and you
have to refer to it explicitly by package and class name.
The other part of this class definition is the public keyword. Public means that
your class is available to the Java system at large once it is loaded. Most of
the time you need to make a class public only if you want it to be visible to
all the other classes in your Java program, but applets, in particular, must be
declared to be public. (You’ll learn more about public classes in Week 3.)
A class definition with nothing in it doesn’t really have much of a point;
without adding or overriding any of its superclasses’ variables or methods,
there’s no reason to create a subclass at all. Let’s add some information to
this class, inside the two enclosing braces, to make it different from its
superclass.
First, add an instance variable to contain a Font object:
Font f = new Font(“TimesRoman”, Font.BOLD, 36);
The f instance variable now contains a new instance of the class Font, part of
the java.awt package. This particular Font object is a Times Roman font,
boldface, 36 points high. In the previous Hello World applet, the font used for
the text was the default font: 12-point Times Roman. Using a Font object, you
can change the font of the text you draw in your applet.
By creating an instance variable to hold this font object, you make it available
to all the methods in your class. Now let’s create a method that uses it.
When you write applets, there are several “standard” methods defined in the
applet superclasses that you will commonly override in your applet class. These
include methods to initialize the applet, to make it start running, to handle
operations such as mouse movements or mouse clicks, or to clean up when the
applet stops running. One of those standard methods is the paint() method, which
actually displays your applet onscreen. The default definition of paint()
doesn’t do anything-it’s an empty method. By overriding paint(), you tell the
applet just what to draw on the screen. Here’s a definition of paint():
public void paint(Graphics g) {
g.setFont(f);
g.setColor(Color.red);
g.drawString(“Hello again!”, 5, 40);
}
There are two things to know about the paint() method. First, note that this
method is declared public, just as the applet itself was. The paint() method is
actually public for a different reason-because the method it’s overriding is
also public. If a superclass’s method is defined as public, your override method
also has to be public, or you’ll get an error when you compile the class.
Second, note that the paint() method takes a single argument: an instance of the
Graphics class. The Graphics class provides platform-independent behavior for
rendering fonts, colors, and behavior for drawing basic lines and shapes. You’ll
learn a lot more about the Graphics class in Week 2, when you create more
extensive applets.
Inside your paint() method, you’ve done three things:
You’ve told the graphics object that the default drawing font will be the one
contained in the instance
variable f.
l
l You’ve told the graphics object that the default color is an instance of the
Color class for the color red. Finally, you’ve drawn your “Hello Again!” string
onto the screen, at the x and y positions of 5 and 25. The string will be
rendered in the new font and color.
l
For an applet this simple, this is all you need to do. Here’s what the applet
looks like so far:
public class HelloAgainApplet extends java.applet.Applet {
Font f = new Font(“TimesRoman”,Font.BOLD,36);
public void paint(Graphics g) {
g.setFont(f);
g.setColor(Color.red);
g.drawString(“Hello again!”, 5, 40);
}
}
If you’ve been paying close attention, you’ll notice that something is wrong
with this example up to this point. If you don’t know what it is, try saving
this file (remember, save it to the same name as the class:
HelloAgainApplet.java) and compiling it. You should get a bunch of errors
similar to this one:
HelloAgainApplet.java:7: Class Graphics not found in type declaration.
Why are you getting these errors? Because the classes you’re referring to in
this class, such as Graphics and Font, are part of a package that isn’t
available by default. Remember that the only package you have access to
automatically in your Java programs is java.lang. You referred to the Applet
class in the first line of the class definition by referring to its full package
name (java.applet.Applet). Further on in the program,
however, you referred to all kinds of other classes as if they were available.
The compiler catches this and tells you that you don’t have access to those
other classes.
There are two ways to solve this problem: Refer to all external classes by full
package name or import the appropriate class or package at the beginning of your
class file. Which one you choose to do is mostly a matter of choice, although if
you find yourself referring to a class in another package lots of times, you may
want to import it to cut down on the amount of typing.
In this example, you’ll import the classes you need. There are three of them:
Graphics, Font, and Color.
All three are part of the java.awt package. Here are the lines to import these
classes. These lines go at the
top of your program, before the actual class definition:
import java.awt.Graphics;
import java.awt.Font;
import java.awt.Color;
Tip
You also can import an entire package of public classes by using an asterisk (*)
in place of a specific class name. For example, to
import all the classes in the awt package, you can use this line:
import java.awt.*;
Now, with the proper classes imported into your program, HelloAgainApplet.java
should compile cleanly to a class file. Listing 2.4 shows the final version to
double-check.
Listing 2.4. The final version of HelloAgainApplet.java.
1:import java.awt.Graphics;
2:import java.awt.Font;
3:import java.awt.Color;
4:
5:public class HelloAgainApplet extends java.applet.Applet {
6:
7: Font f = new Font(“TimesRoman”,Font.BOLD,36);
8:
9: public void paint(Graphics g) {
10: g.setFont(f);
11: g.setColor(Color.red);
12: g.drawString(“Hello again!”, 5, 40);
13: }
14:}
To test it, create an HTML file with the <APPLET> tag as you did yesterday.
Here’s an HTML file to use:
<HTML>
<HEAD>
<TITLE>Another Applet</TITLE>
</HEAD>
<BODY>
<P>My second Java applet says:
<BR><APPLET CODE=”HelloAgainApplet.class” WIDTH=200 HEIGHT=50>
</APPLET>
</BODY>
</HTML>
For this HTML example, your Java class file is in the same directory as this
HTML file. Save the file to HelloAgainApplet.html and fire up your Java-enabled
browser or the Java applet viewer. Figure 2.7 shows the result you should be
getting (the “Hello Again!” string is red).
Figure 2.7 : The HelloAgain applet.
Summary
If this is your first encounter with object-oriented programming, a lot of the
information in this lesson is going to seem really theoretical and overwhelming.
Fear not-the further along in this book you get, and the more Java classes and
applications you create, the easier it is to understand.
One of the biggest hurdles of object-oriented programming is not necessarily the
concepts; it’s their names. OOP has lots of jargon surrounding it. To summarize
today’s material, here’s a glossary of terms and concepts you learned today:
class: A template for an object, which contains variables and methods
representing behavior and
attributes. Classes can inherit variables and methods from other classes.
class method: A method defined in a class, which operates on the class itself
and can be called via the class or any of its instances.
class variable: A variable that is “owned” by the class and all its instances as
a whole and is stored in the class.
instance: The same thing as an object; each object is an instance of some class.
instance method: A method defined in a class, which operates on an instance of
that class. Instance methods are usually called just methods.
instance variable: A variable that is owned by an individual instance and whose
value is stored in the instance.
interface: A collection of abstract behavior specifications that individual
classes can then implement.
object: A concrete instance of some class. Multiple objects that are instances
of the same class have access to the same methods, but often have different
values for their instance variables.
package: A collection of classes and interfaces. Classes from packages other
than java.lang must be explicitly imported or referred to by full package name.
subclass: A class lower in the inheritance hierarchy than its parent, the
superclass. When you create a new class, it’s often called subclassing.
superclass: A class further up in the inheritance hierarchy than its child, the
subclass.
Q&A
Q: Methods are effectively functions that are defined inside classes. If they
look like functions and act like functions, why aren’t they called functions?
A: Some object-oriented programming languages do call them functions (C++ calls
them member functions). Other object-oriented languages differentiate between
functions inside and outside a body of a class or object, where having separate
terms is important to understanding how each works.
Because the difference is relevant in other languages and because the term
method is now in such common use in object-oriented technology, Java uses the
word as well.
Q: I understand instance variables and methods, but not the idea of class
variables and methods.
A: Most everything you do in a Java program will be with objects. Some behaviors
and attributes, however, make more sense if they are stored in the class itself
rather than in the object. For example, to create a new instance of a class, you
need a method that is defined and available in the class itself. (Otherwise, how
can you create an object? You need an object to call the method, but you don’t
have
an object yet.) Class variables, on the other hand, are often used when you have
an attribute whose value you want to share with all the instances of a class.
Most of the time, you’ll use instance variables and methods. You’ll learn more
about class variables and methods later this week.
Java Help, Java Tutorials, Java Programming, Java Tricks
Lesson One
What Is Java?
Java’s Past, Present, and Future
Why Learn Java?
Java Is Platform Independent
Java Is Object Oriented
Java Is Easy to Learn
Getting Started Programming in Java
Getting a Java Development Environment
Installing the JDK and Sample Files
Configuring the JDK
Creating a Java Application
Creating a Java Applet
Troubleshooting
Summary
Q&A
What exactly Java is, and its current status
Why you should learn Java-its various features and advantages over other
programming languages Getting started programming in Java-what you’ll need in
terms of software and background, as well as some basic terminology
How to create your first Java programs-to close this day, you’ll create both a
simple Java application and a simple Java applet!
What Is Java?
Based on the enormous amount of press Java is getting and the amount of
excitement it has generated, you may get the impression that Java will save the
world-or at least solve all the problems of the Internet.
Not so. Java’s hype has run far ahead of its capabilities, and while Java is
indeed new and interesting, it really is another programming language with which
you write programs that run on the Internet. In this respect, Java is closer to
popular programming languages such as C, C++, Visual Basic, or Pascal, than it
is to a page description language such as HTML, or a very simple scripting
language such as JavaScript.
More specifically, Java is an object-oriented programming language developed by
Sun Microsystems, a company best known for its high-end UNIX workstations.
Modeled after C++, the Java language was designed to be small, simple, and
portable across platforms and operating systems, both at the source and at the
binary level, which means that Java programs (applets and applications) can run
on any machine that has the Java virtual machine installed (you’ll learn more
about this later).
Java is usually mentioned in the context of the World Wide Web, where browsers
such as Netscape’s Navigator and Microsoft’s Internet Explorer claim to be “Java
enabled.” Java enabled means that the browser in question can download and play
Java programs, called applets, on the reader’s system. Applets appear in a Web
page much the same way as images do, but unlike images, applets are dynamic and
interactive. Applets can be used to create animation, figures, forms that
immediately respond to inputfrom the reader, games, or other interactive effects
on the same Web pages among the text and graphics.
Figure 1.1 shows an applet running in Netscape 3.0. (This applet, at http://prominence.com/java/poetry/,
is an electronic version of the refrigerator magnets
that you can move around to create poetry or messages.)
New Term
Applets are programs that are downloaded from the World Wide Web by a Web
browser and run inside an HTML Web page. You’ll need a Java-enabled browser such
as Netscape Navigator or Microsoft’s
Internet Explorer to run applets.
To create an applet, you write it in the Java language, compile it using a Java
compiler, and refer to that applet in your HTML Web pages. You put the resulting
HTML and Java files on a Web site in the same
way that you make ordinary HTML and image files available. Then, when someone
using a Java-enabled browser views your page with the embedded applet, that
browser downloads the applet to the local system and executes it, allowing your
reader to view and interact with your applet in all its glory. (Readers using
other browsers may see text, a static graphic, or nothing.) You’ll learn more
about how applets, browsers, and the World Wide Web work together later in this
book.
While applets are probably the most popular use of Java, the important thing to
understand about Java s that you can do so much more with it than create and use
applets. Java was written as a full-fledged general-purpose programming language
in which you can accomplish the same sorts of tasks and solve the same sorts of
problems that you can in other programming languages, such as C or C++.
Java’s Past, Present, and Future
The Java language was developed at Sun Microsystems in 1991 as part of a
research project to develop software for consumer electronics devices-television
sets, VCRs, toasters, and the other sorts of machines
you can buy at any department store. Java’s goals at that time were to be small,
fast, efficient, and easily portable to a wide range of hardware devices. Those
same goals made Java an ideal language for
distributing executable programs via the World Wide Web and also a
general-purpose programming language for developing programs that are easily
usable and portable across different platforms.
The Java language was used in several projects within Sun (under the name Oak),
but did not get very much commercial attention until it was paired with HotJava.
HotJava, an experimental World Wide Web
browser, was written in 1994 in a matter of months, both as a vehicle for
downloading and running applets and also as an example of the sort of complex
application that can be written in Java. Although HotJava got a lot of attention
in the Web community, it wasn’t until Netscape incorporated HotJava’s
ability to play applets into its own browser that Java really took off and
started to generate the excitement that it has both on and off the World Wide
Web. Java has generated so much excitement, in fact, that
inside Sun the Java group spun off into its own subsidiary called JavaSoft.
Versions of Java itself, or, as it’s most commonly called, the Java API,
correspond to versions of Sun’s Java Developer’s Kit, or JDK. As of this
writing, the current version of the JDK is 1.0.2. Previously
released versions of the JDK (alphas and betas) did not have all the features or
had a number of security-related bugs. Most Java tools and browsers conform to
the features in the 1.0.2 JDK, and all the
examples in this book run on that version as well.
The next major release of the JDK and therefore of the Java API will be 1.1,
with a prerelease version available sometime in the later part of 1996. This
release will have few changes to the language, but a
number of additional capabilities and features added to the class library.
Throughout this book, if a feature will change or will be enhanced in 1.1, we’ll
let you know, and in the last two days of this book
you’ll find out more about new Java features for 1.1 and for the future.
Currently, to program in Java, you’ll need a Java development environment of
some sort for your platform. Sun’s JDK works just fine for this purpose and
includes tools for compiling and testing Java
applets and applications. In addition, a wide variety of excellent Java
development environments have been developed, including Sun’s own Java Workshop,
Symantec’s Café, Microsoft’s Visual J++ (which is
indeed a Java tool, despite its name), and Natural Intelligence’s Roaster, with
more development tools appearing all the time.
To run and view Java applets, you’ll need a Java-enabled browser or other tool.
As mentioned before, recent versions of Netscape Navigator (2.0 and higher) and
Internet Explorer (3.0) can both run Java
applets. (Note that for Windows you’ll need the 32-bit version of Netscape, and
for Macintosh you’ll need Netscape 3.0.) You can also use Sun’s own HotJava
browser to view applets, as long as you have the 1.0
prebeta version (older versions are not compatible with newer applets, and vice
versa). Even if you don’t have a Java-enabled browser, many development tools
provide simple viewers with which you can run
your applets. The JDK comes with one of these; it’s called the appletviewer.
Note
If you’re running Windows 3.x as your main system, very few toolsexist for you
to be able to work with Java. As I write this, the only
Java tool available for writing and running Java applets is a version ofthe JDK
from IBM called the ADK. You can write applets using this
tool, and view them using the applet viewer that comes with thatpackage (neither
Netscape nor Internet Explorer will run Java applets
on Windows 3.1). See http://www.alphaWorks.ibm.com/ for more information.
What’s in store for Java in the future? A number of new developments have been
brewing (pardon the
pun):
Sun is developing a number of new features for the Java environment, including a
number of new class libraries for database integration, multimedia, electronic
commerce, and other uses. Sun also
has a Java-based Web server, a Java-based hardware chip (with which you can
write Java-specific systems), and a Java-based operating system. You’ll learn
about all these things later in this book.
The 1.1 release of the JDK will include many of these features; others will be
released as separate
packages.
Sun is also developing a framework called Java Beans, which will allow the
development of component objects in Java, similarly to Microsoft’s ActiveX (OLE)
tech-nology. These different
components can then be easily combined and interact with each other using
standard component assembly tools. You’ll learn more about Java Beans later in
this book.
Java capabilities will be incorporated into a wide variety of operating systems,
including Solaris, Windows 95, and MacOS. This means that Java applications (as
opposed to applets) can run nearly
anywhere without needing additional software to be installed.
Many companies are working on performance enhancements for Java programs,
including the aforementioned Java chip and what are called just-in-time
compilers.
Why Learn Java?
At the moment, probably the most compelling reason to learn Java-and probably
the reason you bought this book-is that applets are written in Java. Even if
that were not the case, Java as a programming
language has significant advantages over other languages and other environments
that make it suitable for just about any programming task. This section
describes some of those advantages.
Java Is Platform Independent Platform independence-that is, the ability of a
program to move easily from one computer system to
another-is one of the most significant advantages that Java has over other
programming languages, particularly if your software needs to run on many
different platforms. If you’re writing software for the
World Wide Web, being able to run the same program on many different systems is
crucial to that program’s success. Java is platform independent at both the
source and the binary level.
New Term
Platform independence means that a program can run on anycomputer system. Java
programs can run on any system for which a Java virtual machine has been
installed.
At the source level, Java’s primitive data types have consistent sizes across
all development platforms. Java’s foundation class libraries make it easy to
write code that can be moved from platform to platform
without the need to rewrite it to work with that platform. When you write a
program in Java, you don’t need to rely on features of that particular operating
system to accomplish basic tasks. Platform
independence at the source level means that you can move Java source files from
system to system and have them compile and run cleanly on any system.
Platform independence in Java doesn’t stop at the source level, however. Java
compiled binary files are also platform independent and can run on multiple
platforms (if they have a Java virtual machine
available) without the need to recompile the source.
Normally, when you compile a program written in C or in most other languages,
the compiler translates your program into machine code or processor
instructions. Those instructions are specific to the
processor your computer is running-so, for example, if you compile your code on
an Intel-based system,the resulting program will run only on other Intel-based
systems. If you want to use the same program on
another system, you have to go back to your original source code, get a compiler
for that system, and recompile your code so that you have a program specific to
that system. Figure 1.2 shows the result of
this system: multiple executable programs for multiple systems.
Things are different when you write code in Java. The Java development
environment actually has two parts: a Java compiler and a Java interpreter. The
Java compiler takes your Java program and, instead of
generating machine codes from your source files, it generates bytecodes.
Bytecodes are instructions that look a lot like machine code, but are not
specific to any one processor.
To execute a Java program, you run a program called a bytecode interpreter,
which in turn reads the bytecodes and executes your Java program (see Figure
1.3). The Java bytecode interpreter is often also
called the Java virtual machine or the Java runtime.
New Term
Java bytecodes are a special set of machine instructions that are notspecific to
any one processor or computer system. A platform-specific bytecode interpreter
executes the Java bytecodes. The bytecodeinterpreter is also called the Java
virtual machine or the Java runtime
interpreter.
Where do you get the bytecode interpreter? For applets, the bytecode interpreter
is built into every Java-enabled browser, so you don’t have to worry about
it-Java applets just automatically run. For more
general Java applications, you’ll need to have the interpreter installed on your
system in order to run that Java program. Right now, you can get the Java
interpreter as part of your development environment, or if
you buy a Java program, you’ll get it with that package. In the future, however,
the Java bytecode interpreter will most likely come with every new operating
system-buy a Windows machine, and you’ll
get Java for free.
Why go through all the trouble of adding this extra layer of the bytecode
interpreter? Having your Java programs in bytecode form means that instead of
being specific to any one system, your programs can be
run on any platform and any operating or window system as long as the Java
interpreter is available. This capability of a single binary file to be
executable across platforms is crucial to what makes applets work
because the World Wide Web itself is also platform independent. Just as HTML
files can be read on any platform, so can applets be executed on any platform
that has a Java-enabled browser.
The disadvantage of using bytecodes is in execution speed. Because
system-specific programs run directly on the hardware for which they are
compiled, they run significantly faster than Java bytecodes,
which must be processed by the interpreter. For many basic Java programs, speed
may not be an issue. If you write programs that require more execution speed
than the Java interpreter can provide, you have
several solutions available to you, including being able to link native code
into your Java program or using special tools (called just-in-time compilers) to
convert your Java bytecodes into native code and
speed up their execution. Note that by using any of these solutions, you lose
the portability that Java bytecodes provide. You’ll learn about each of these
mechanisms on Day 20, “Using Native Methods and
Libraries.”
Java Is Object Oriented
To some, the object-oriented programming (OOP) technique is merely a way of
organizing programs, and it can be accomplished using any language. Working with
a real object-oriented language and
programming environment, however, enables you to take full advantage of
object-oriented methodology and its capabilities for creating flexible, modular
programs and reusing code.
Many of Java’s object-oriented concepts are inherited from C++, the language on
which it is based, but it borrows many concepts from other object-oriented
languages as well. Like most object-oriented
programming languages, Java includes a set of class libraries that provide basic
data types, system input and output capabilities, and other utility functions.
These basic libraries are part of the standard Java
environment, which also includes simple libraries, form networking, common
Internet protocols, and user interface toolkit functions. Because these class
libraries are written in Java, they are portable across
platforms as all Java applications are.
You’ll learn more about object-oriented programming and Java tomorrow.
Java Is Easy to Learn
In addition to its portability and object orientation, one of Java’s initial
design goals was to be small and simple, and therefore easier to write, easier
to compile, easier to debug, and, best of all, easy to learn.
Keeping the language small also makes it more robust because there are fewer
chances for programmers to make mistakes that are difficult to fix. Despite its
size and simple design, however, Java still has a
great deal of power and flexibility.
Java is modeled after C and C++, and much of the syntax and object-oriented
structure is borrowed from the latter. If you are familiar with C++, learning
Java will be particularly easy for you because you have
most of the foundation already. (In fact, you may find yourself skipping through
the first week of this book fairly rapidly. Go ahead; I won’t mind.)Although
Java looks similar to C and C++, most of the more complex parts of those
languages have been
excluded from Java, making the language simpler without sacrificing much of its
power. There are no pointers in Java, nor is there pointer arithmetic. Strings
and arrays are real objects in Java. Memory
management is automatic. To an experienced programmer, these omissions may be
difficult to get used to, but to beginners or programmers who have worked in
other languages, they make the Java language
far easier to learn.
However, while Java’s design makes it easier to learn than other programming
languages, working with a programming language is still a great deal more
complicated than, say, working in HTML. If you have
no programming language background at all, you may find Java difficult to
understand and to grasp. But don’t be discouraged! Learning programming is a
valuable skill for the Web and for computers in
general, and Java is a terrific language to start out with.
Getting Started Programming in JavaEnough background! For the second half of
this day let’s actually dive into simple Java programming and
create two Java programs: a standalone Java application and an applet that you
can view in a Java-enabled browser. Although both these programs are extremely
simple, they will give you an idea of
what a Java program looks like and how to compile and run it.
Getting a Java Development Environment In order to write Java programs, you
will, of course, need a Java development environment. (Although
browsers such as Netscape allow you to play Java applets, they don’t let you
write them. For that you’ll need a separate tool.) Sun’s JDK, which is available
for downloading at the JavaSoft Web site
(http://www.javasoft.com/) and included on the CD for this book, will do just
fine. It runs on Solaris, Windows 95 and NT, and Macintosh. However, despite the
JDK’s popularity, it is not the easiest
development tool to use. If you’re used to using a graphical user
interface-based development tool with an integrated editor and debugger, you’ll
most likely find the JDK’s command-line interfaces rather
primitive. Fortunately, the JDK is not the only tool in town.
As mentioned earlier, a number of third-party development environments (called
integrated development environments, or IDEs) are also available for developing
in Java. These include Sun’s Java Workshop for
Solaris, Windows NT and Windows 95 (you can get more information about it athttp://www.sun.com/developer-products/java/);
Symantec’s Café for Windows 95,
Windows NT, and Macintosh (http://cafe.symantec.com/); Microsoft’s Visual J++
forWindows 95 and Windows NT (http://www.microsoft.com/visualj/); and Natural
Intelligence’s Roaster(http://www.natural.com/pages/products/roaster/index.html).
All three are
commercial programs, but you might be able to download trial or limited versions
of these programs to
try them out. You’ll learn more about the features and capabilities of the
various Java IDEs on Day 22,
“Java Programming Tools.”
Note
I find the graphical development environments far easier to use than
the standard JDK. If you have the money and the time to invest in one
of these tools, I highly recommend you do so. It’ll make your Java
development experience much more pleasant.
Installing the JDK and Sample Files
Sun’s JDK for Solaris, Windows, and Macintosh is included as part of the CD-ROM
that comes with this
book. Also on the CD-ROM are all of the code examples from this book-a great
help if you don’t want to
type them all in again. To install either the JDK or the sample files (or both),
use one of the following
procedures:
Note
If you don’t have access to a CD-ROM drive, you can also get accessto these
files over the World Wide Web. You can download the JDK
itself from http://java.sun.com/products/JDK/1.0.2/and install it per the
instructions on those pages. The sample files
from this book are available on the Web site for this book:http://www.lne.com/Web/JavaProf/.
If you download the JDK and source files, as opposed to getting themoff the
CD-ROM, make sure you read the section “Configuring the
JDK” to make sure everything is set up right.Windows Sun’s JDK runs on Windows
95 and Windows NT. It does not run on
Windows 3.x.
To install the JDK or the sample files on Windows, run the Setup program on the
CD-ROM
(double-clicking the CD icon will do this automatically). By default, the
package will be installed into
C:\Java; you can install it anywhere on your hard disk that you’d like. You’ll
be given options to install
the JDK, the sample files, and various other extra files; choose the options you
want and those files will
be installed.
If you’ve installed the JDK, note that in the directory JDK\lib there is a file
called classes.zip. Do
not unzip this file; it needs to remain in zip form for it to work correctly.
The file JDK\src.zip
contains the source code for many of the JDK libraries; you can unzip this one
if you like. Make sure if
you do that you have a zip program that supports long filenames, or it will not
work correctly!
Macintosh
Sun’s JDK for Macintosh runs on System 7 (MacOS) for 68KB or
Power Mac.
To install the JDK or the sample files on the Macintosh, double-click the
installation program on the
CD-ROM. By default, the package will be installed into the folder Java on your
hard disk; you can
install it anywhere on your disk that you’d like. You’ll be given options to
install the JDK, the sample
files, and various other extra files; choose the options you want and those
files will be installed.
Solaris
Sun’s JDK for Solaris runs on Solaris 2.3, 2.4, and 2.5, as well as the
x86 version of Solaris.
The CD-ROM for this book contains the tarred and zipped JDK in the directory
jdk/solaris/jdk1.02.tgz. Using the utilities gunzip and tar, you can extract the
contents of
that file anywhere on the file system you would like. For example, if you copy
the .tgz file to your
home directory and use the following commands to extract it, you’ll end up with
a java directory that
contains the full JDK:
gunzip ./jdk1.02.tgz
tar xvf ./jdk1.02.tar
Note that in the directory java\lib there is a file called classes.zip. Do not
unzip this file; it
needs to remain in zip form for it to work correctly. The file java\src.zip
contains the source code
for many of the JDK libraries; you can unzip this one if you’re interested in
the source code.
The sample files are also contained on the CD-ROM in authors/authors.tar. Create
a directory
where the sample files will live (for example, a directory called javasamples in
your home directory),
copy the authors.tar file there, and then use the tar command to extract it,
like this:
mkdir ~/javasamples
cp /cdrom/authors/authors.tar
tar xvf authors.tar
Configuring the JDK
If you’ve installed the JDK using the setup programs from the CD-ROM, chances
are good that it has
been correctly configured for you. However, because most common problems with
Java result from
configuration errors, I recommend that you double-check your configuration to
make sure everything is
right. And if you’ve installed the JDK from a source other than the CD-ROM,
you’ll definitely want to
read this section to make sure you’re all set up.
Windows
The JDK needs two important modifications to yourautoexec.bat file in order to
work correctly: The JDK\bin
directory must be in your execution path, and you must have the CLASSPATH
variable set up.
Edit your autoexec.bat file using your favorite editor (Notepad will do just
fine). Look for a line
that looks something like this:
PATH C:\WINDOWS;C:\WINDOWS\COMMAND;C:\DOS; …
Somewhere in that line you should see an entry for the JDK; if you installed the
JDK from CD-ROM, it’ll
look something like this (the dots are there to indicate that there may be other
stuff on this line):
PATH C:\WINDOWS; … C:\TEAchY~1\JDK\BIN; …
If you cannot find any reference to JDK\BIN or JAVA\BIN in your PATH, you’ll
need to add it. Simply
include the full pathname to your JDK installation to the end of that line,
starting with C: and ending
with BIN; for example, C:\JAVA\BIN or C:\Java\JDK\BIN.
Note
The directories Teach Yourself Java and TEAchY~1 are
actually the same thing; the former is how the directory appears in
Windows 95, and the latter is how it appears in DOS. Either one will
work fine; there’s no need to change it if one or the other appears.
Note, however, that if the pathname contains spaces, it must be in
quotes.
The second thing you’ll need to add to the autoexec.bat file (if it isn’t
already there) is a
CLASSPATH variable. Look for a line that looks something like this:
SET CLASSPATH=C:\TEAchY~1\JDK\lib\classes.zip;.;The CLASSPATH variable may also
have other entries in it for Netscape or Internet Explorer, but the one
you’re most interested in is a reference to the classes.zip file in the JDK, and
to the current
directory (.). If your autoexec.bat file does not include either of these
locations, add a line to the
file that contains both these things (the line shown above will work just fine).
After saving your autoexec.bat file, you’ll need to restart Windows for the
changes to take effect.
Macintosh
The JDK for Macintosh should need no further configuration after
installation.
Solaris
To configure the JDK for Solaris, all you need to do is add the
java/bin or jdk/bin directory to your execution path. Usually a
line something like this in your .cshrc, .login, or .profile
files will work:
set path= (~/java/bin/ $path)
This line assumes that you’ve installed the JDK (as the directory java) into
your home directory; if
you’ve installed it somewhere else, you’ll want to substitute that pathname.
Make sure you use the source command with the name of the appropriate file to
make sure the changes
take effect (or log out and log back in again):
source ~/.login
Creating a Java Application
Now let’s actually get to work. We’ll start by creating a simple Java
application: the classic Hello World
example that many programming language books use to begin.
Java applications are different from Java applets. Applets, as you have learned,
are Java programs that
are downloaded over the World Wide Web and executed by a Web browser on the
reader’s machine.
Applets depend on a Java-enabled browser in order to run.
New Term
Java applications, however, are more general programs written in the
Java language. Java applications don’t require a browser to run; in
fact, Java can be used to create all the kinds of applications that you
would normally use a more conventional programming language tocreate.
Java applications are standalone Java programs that do not require a Web browser
to run. Java
applications are more general-purpose programs such as you’d find on any
computer.
A single Java program can be an applet or an application, or both, depending on
how you write that
program and the capabilities that program uses. Throughout this first week as
you learn the Java
language, you’ll be writing mostly applications; then you’ll apply what you’ve
learned to write applets in
Week 2. If you’re eager to get started with applets, be patient. Everything that
you learn while you’re
creating simple Java applications will apply to creating applets, and it’s
easier to start with the basics
before moving onto the hard stuff. You’ll be creating plenty of applets in Week
2.Creating the Source File
As with all programming languages, your Java source files are created in a plain
text editor, or in an
editor that can save files in plain ASCII without any formatting characters. On
UNIX, emacs, pico,
and vi will work; on Windows, Notepad or DOS Edit are both text editors that
will work (although I
prefer to use the shareware TextPad). On the Macintosh, SimpleText (which came
with your Mac) or the
shareware BBedit will work. If you’re using a development environment like Café
or Roaster, it’ll have
its own built-in text editor you can use.
Note
If you’re using Windows to do your Java development, you may have
to make sure Windows understands the .java file extension before
you start; otherwise, your text editor may insist on giving all your
files a .txt extension. The easiest way to do this is to go to any
Windows Explorer window, choose View|Options|File Types, choose
New Type, and add Java Source File and .java to the
Description of Type and Associated Extension boxes, respectively.
Fire up your editor of choice and enter the Java program shown in Listing 1.1.
Type this program, as
shown, in your text editor. Be careful that all the parentheses, braces, and
quotes are there, and that
you’ve used all the correct upper- and lowercase letters.
Note
You can also find the code for these examples on the CD-ROM as
part of the sample code. However, it’s a good idea to actually type
these first few short examples in so that you get a feel for what Java
code actually looks like.
Listing 1.1. Your first Java application.
1: class HelloWorld {
2: public static void main (String args[]) {
3: System.out.println(“Hello World!”);
4: }
5: }
Warning
The number before each line is part of the listing and not part of the
program; the numbers are there so I can refer to specific line numbers
when I explain what’s going on in the program. Do not include them
in your own file.
After you’ve finished typing in the program, save the file somewhere on your
disk with the name
HelloWorld.java. This is very important. Java source files must have the same
name as the class
they define (including the same upper- and lowercase letters), and they must
have the extension .java.
Here, the class definition has the name HelloWorld, so the filename must be
HelloWorld.java. If
you name your file something else (even something like helloworld.java or
Helloworld.java), you won’t be able to compile it. Make absolutely certain the
name isHelloWorld.java.
You can save your Java files anywhere you like on your disk, but I like to have
a central directory or
folder to keep them all in. For the examples in this chapter, I’ve put my files
into a directory called
TYJtests (short for Teach Yourself Java Tests).
Compiling and Running the Source File
Now it’s time to compile the file. If you’re using the JDK, you can use the
instructions for your computer
system contained in the next few pages. If you’re using a graphical development
environment, there will
most likely be a button or option to compile the file (check with the
documentation that came with your
program).
Windows
To compile the Java source file, you’ll use the command-line Java
compiler that comes with the JDK. To run the compiler, you’ll need to
first start up a DOS shell. In Windows 95, the DOS shell is under the
Programs menu (it’s called MS-DOS Prompt).
From inside DOS, change directories to the l
ocation where you’ve saved your HelloWorld.java file.
I put mine into the directory TYJtests, so to change directories I’d use this
command:
CD C:\TYJtests
Once you’ve changed to the right directory, use the javac command as follows,
with the name of the
file as you saved it in Windows (javac stands for Java compiler). Note that you
have to make sure you
type all the same upper- and lowercase here as well:
javac HelloWorld.java
Note
The reason that I’ve emphasized using the original filename is that
once you’re inside the DOS shell, you might notice that your nice
long filenames have been truncated to old-style 8.3 names and that, in
fact, HelloWorld.java actually shows up as HELLOW~1.jav.
Don’t panic; this is simply a side effect of Windows 95 and how it
manages long filenames. Ignore the fact that the file appears to be
HELLOW~1.jav and just use the filename you originally used when you saved the
file.
Figure 1.4 shows what I’ve done in the DOS shell so you can make sure you’re
following along.
Figure 1.4 : Compiling Java in the DOS shell.
If all goes well, you’ll end up with a file called HelloWorld.class (or at least
that’s what it’ll be
called if you look at it outside the DOS shell; from inside DOS its called
HELLOW~1.cla). That’s your
Java bytecode file. If you get any errors, go back to your original source file
and make sure you typed it
exactly as it appears in Listing 1.1 with the same upper- and lowercase. Also
make sure the filename has
exactly the same upper- and lowercase as the name of the class (that is, both
should be HelloWorld).
Once you have a class file, you can run that file using the Java bytecode
interpreter. The Java interpreter
is called simply java, and you run it from the DOS shell as you did javac. Run
your Hello World
program like this from the command line, with all the same upper- and lowercase
(and note that the
argument to the java program does not have a .class extension):
java HelloWorld
If your program was typed and compiled correctly, you should get the phrase
Hello World! printed
to your screen as a response. Figure 1.5 shows how I did it.
Figure 1.5 : Running Java applications in the DOS shell.
Note
Remember, the Java compiler and the Java interpreter are different
things. You use the Java compiler (javac) for your Java source files
to create .class files, and you use the Java interpreter (java) to
actually run your class files.
Macintosh
The JDK for the Mac comes with an application called Java
Compiler. To compile your Java source file, simply drag and drop it
on top of the Java Compiler icon. The program will compile your
Java file and, if there are no errors, create a file called
HelloWorld.class in the same folder as your original source
file.
Tip
Putting an alias for Java Compiler on the desktop makes it easy to
drag and drop Java source files.
If you get any errors, go back to your original source file and make sure you
typed it exactly as it appears
in Listing 1.1, with the same upper- and lowercase. Also make sure the filename
has exactly the same
upper- and lowercase as the name of the class (that is, both should be
HelloWorld).
Once you’ve successfully generated a HelloWorld.class file, simply double-click
it to run it. The
application Java Runner, part of the Mac JDK, will start, and the program will
ask you for command-line
arguments. Leave that screen blank and click OK. A window labeled stdout will
appear with the
message Hello World!. Figure 1.6 shows that window.
Figure 1.6 : Running Java applications on the Mac using Java Runner.
That’s it! Keep in mind as you work that you use the Java Compiler application
to compile your .java
files into .class files, which you can then run using Java Runner.
To compile the Java source file in Solaris, you’ll use the command-line Java
compiler that comes with the
JDK. From a UNIX command line, cd to the directory that contains your Java
source file. I put mine in
the directory TYJtests, so to change directories I’d use this command:
cd ~/TYJtests
Once you’re in the right directory, use the javac command with the name of the
file, like this:
javac HelloWorld.java
If all goes well, you’ll end up with a file called HelloWorld.class in the same
directory as your
source file. That’s your Java bytecode file. If you get any errors, go back to
your original source file and
make sure you typed it exactly as it appears in Listing 1.1, with the same
upper- and lowercase letters.
Also make sure the filename has exactly the same upper- and lowercase letters as
the name of the class
(that is, both should be HelloWorld).
Once you have a class file, you can run that file using the Java bytecode
interpreter. The Java interpreter
is called simply java, and you run it from the command line as you did javac,
like this (and note that
the argument to the java program does not have a .class extension):
java HelloWorld
If your program was typed and compiled correctly, you should get the phrase
Hello World! printed
to your screen as a response. Figure 1.7 shows a listing of all the commands I
used to get to this point
(the part with [desire]~[1] is my system prompt).
Figure 1.7 : Compiling and running a Java application on Solaris.
Note
Remember that the Java compiler and the Java interpreter are
different things. You use the Java compiler (javac) for your Java
source files to create .class files, and you use the Java interpreter
(java) to actually run your class files.
Creating a Java Applet
Creating applets is different from creating a simple application. Java applets
run and are displayed inside
a Web page with other page elements, and therefore have special rules for how
they behave. Because of
these special rules for applets, creating an applet may in many cases be more
complex than creating an
application.
For example, to create a simple Hello World applet, instead of merely being able
to print a message as a
set of characters, you have to make space for your message on the Web pages and
then use special font
and graphics operations to paint the message to the screen.
Note
Actually, you can run a plain Java application as an applet, but the
Hello World message will print to a special window or to a log
file, depending on how the browser has its output set up. You’ll learn
more about this next week.
Creating the Source File
In this example, you’ll create a simple Hello World applet, place it inside a
Web page, and view the
result. As with the Hello World application, you’ll first create the source file
in a plain text editor. Listing
1.2 shows the code for the example.
Listing 1.2. The Hello World applet.
1: import java.awt.Graphics;
2:
3: public class HelloWorldApplet extends java.applet.Applet {
4:
5: public void paint(Graphics g) {
6: g.drawString(“Hello world!”, 5, 25);
7: }
8:}
Save that file just as you did the Hello World application, with the filename
exactly the same as the name
of the class. In this case the class name is HelloWorldApplet, so the filename
you save it to would
be HelloWorldApplet.java. As with the application, I put the file in a directory
called TYJch01,
but you can save it anywhere you like.
Compiling the Source File
The next step is to compile the Java applet file. Despite the fact that this is
an applet, you compile the file
exactly the same way you did the Java application, using one of the following
procedures:
javac HelloWorldApplet.java
javac HelloWorldApplet.java
Windows
From inside a DOS shell, cd to the directory containing your applet
source file, and use the javac command to compile it (watch those
upper- and lowercase letters):
Macintosh
Drag and drop the HelloWorldApplet.java file onto the Java
Compiler icon.
Salaris
From a command line, cd to the directory containing your applet
source file and use the javac command to compile it:
Including the Applet in a Web Page
If you’ve typed the file correctly, you should end up with a file called
HelloWorldApplet.class in
the same directory as your source file. That’s your Java applet file; to have
the applet run inside a Web
page you must refer to that class file inside the HTML code for that page using
the <APPLET> tag.
Listing 1.3 shows a simple HTML file you can use.
Listing 1.3. The HTML with the applet in it.
1: <HTML>
2: <HEAD>
3: <TITLE>Hello to Everyone!</TITLE>
4: </HEAD><BODY>
5: <P>My Java applet says:
6: <APPLET CODE=”HelloWorldApplet.class” WIDTH=150 HEIGHT=25>
7: </APPLET>
8: </BODY>
9: </HTML>
You’ll learn more about <APPLET> later in this book, but here are two things to
note about it:
Use the CODE attribute to indicate the name of the class that contains your
applet, here
HelloWorldApplet.Class.
Use the WIDTH and HEIGHT attributes to indicate the size of the applet on the
page. The browser
uses these values to know how big a chunk of space to leave for the applet on
the page. Here, a
box 150 pixels wide and 25 pixels high is created.
Save the HTML file in the same directory as your class file, with a descriptive
name and an .html
extension (for example, you might name your HTML file the same name as your
applet-HelloWorldApplet.html).
Note
As mentioned earlier with the Java source files, your text editor may
insist on naming your HTML files with a .txt extension if Windows
does not understand what the .html extension is used for. Select
View|Options|File Types from any Windows Explorer window to add
a new file type for HTML files to solve this problem.
Now you’re ready for the final test-actually viewing the result of running your
applet. To view the applet,
you need one of the following:
A browser that supports Java applets, such as Netscape 2.0 or Internet Explorer
3.0. If you’re
running on the Macintosh, you’ll need Netscape 3.0 or later. If you’re running
on Windows 95 or
NT, you’ll need the 32-bit version of Netscape. And if you’re using Internet
Explorer, you’ll need
the 3.0 beta 5 or later (the final version will do just fine).
The appletviewer application, which is part of the JDK. The appletviewer is not
a Web
browser and won’t let you to see the entire Web page, but it’s acceptable for
testing to see how an
applet will look and behave if there is nothing else available.
An applet viewer or runner tool that comes with your development environment.
If you’re using a Java-enabled browser such as Netscape to view your applet
files, you can use the Open
File… item under the File menu to navigate to the HTML file containing the
applet (make sure you open
the HTML file and not the class file). In Internet Explorer, select File|Open
and then Browse to find the
file on your disk. You don’t need to install anything on a Web server yet; all
this works on your local
system. Note that the Java applet may take a while to start up after the page
appears to be done loading;
be patient. Figure 1.8 shows the result of running the applet in Netscape.
Figure 1.8 : The applet running in Netscape.
If you don’t have a Web browser with Java capabilities built into it, you can
use the JDK’s
appletviewer program to view your Java applet.
appletviewer HTML/HelloWorldApplet.html
Windows or Solaris
To run the appletviewer in Windows or Solaris versions of the JDK, cd to the
directory where your HTML and class files are contained and use the appletviewer
command with the name ofthe HTML file you just created:
The appletviewer will show you only the applet itself, not the HTML text around
the applet.
Although the appletviewer is a good way to do simple tests of Java applets, it’s
a better idea to get a
Java-enabled browser so that you can see your applet on its page in its full
glory.
Troubleshooting
If you’ve run into any problems with the previous examples, this section can
help. Here are some of the
most common problems and how to fix them:
Bad command or filename or Command not found
These errors result when you do not have the JDK’s bin directory in your
execution path, or the
path to that directory is wrong. On Windows, double-check your autoexec.bat
file; on UNIX,
check the system file with your path commands in it (.cshrc, .login, .profile,
or some
similar file).
javac: invalid argument
Make sure the name of the file you’re giving to the javac command is exactly the
same name as
the file. In particular, in the DOS shell you want to use the Windows filename
with a .java
extension, not the DOS equivalent (HELLOW~1.jav, for example).
Warning: public class HelloWorldApplet must be defined in a file
called HelloWorldApplet.java
This error most often happens if there is a mismatch between the name of the
class as defined in
the Java file itself (the name following the word class) and the name of the
java source file. Both
the filenames must match, including upper- and lowercase letters (this
particular error implies that
the filename had lowercase letters). Rename either the filename or the class
name, and this error
will go away.
Insufficient-memory errors
The JDK is not the most efficient user of memory. If you’re getting errors about
memory, consider
closing larger programs before running Java compiles, turn on virtual memory, or
install more RAM.
Other code errors
If you’re unable to compile the Java source files because of other errors I
haven’t mentioned here,
be sure that you’ve typed them in exactly as they appear, including all upper-
and lowercase letters.
Java is case sensitive, meaning that upper- and lowercase letters are treated
differently, so you will
need to make sure that everything is capitalized correctly. If all else fails,
try comparing your
source files to the files on the CD-ROM.
Summary
Today you’ve gotten a basic introduction to the Java language and its goals and
features. Java is a
programming language, similar to C or C++, in which you can develop a wide range
of programs. The
most common use of Java at the moment is in creating applets for HotJava, an
advanced World Wide
Web browser also written in Java. Applets are Java programs that are downloaded
and run as part of a
Web page. Applets can create animation, games, interactive programs, and other
multimedia effects on
Web pages.
Java’s strengths lie in its portability-both at the source and at the binary
level, in its object-oriented
design-and in its simplicity. Each of these features helps make applets
possible, but they also make Java
an excellent language for writing more general-purpose programs that do not
require a Java-enabled
browser to run. These general-purpose Java programs are called applications.
To end this day, you experimented with an example of an applet and an example of
an application,
getting a feel for the differences between the two and how to create, compile,
and run Java programs-or,
in the case of applets, how to include them in Web pages. From here, you now
have the foundation to
create more complex applications and applets. Onward to Day 2, “Object-Oriented
Programming and
Java”!
Q&A
Q: I know a lot about HTML, but not much about computer programming. Can I still
write
Java programs?
A: If you have no programming experience whatsoever, you most likely will find
programming
Java significantly more difficult than HTML. However, Java is an excellent
language to learn
programming with, and if you patiently work through the examples and the
exercises in this
book, you should be able to learn enough to get started with Java.
Q: What’s the relationship between JavaScript and Java?
A: They have the same first four letters.
A common misconception in the Web world today is that Java and JavaScript have
more in
common than they actually do. Java is the general-purpose programming language
that you’ll
learn about in this book; you use it to create applets. JavaScript is a
Netscape-invented scripting
language that looks sort of like Java; with it you can do various nifty things
in Web pages. They
are independent languages, used for different purposes. If you’re interested in
JavaScript
programming, you’ll want to pick up another book, such as Teach Yourself
JavaScript in a Week
or Laura Lemay’s Web Workshop: JavaScript, both also available from Sams.net
Publishing.
Q: According to today’s lesson, Java applets are downloaded via a Java-enabled
browser such
as Netscape and run on the reader’s system. Isn’t that an enormous security
hole? What
stops someone from writing an applet that compromises the security of my
system-or
worse, that damages my system?
A: Sun’s Java team has thought a great deal about the security of applets within
Java-enabled
browsers and has implemented several checks to make sure applets cannot do nasty
things:
Java applets cannot read or write to the disk on the local system.
Java applets cannot execute any programs on the local system.
Java applets cannot connect to any machines on the Web except for the server
from which
they are originally downloaded.
Note that some of these restrictions may be allowed in some browsers or may be
turned on in the
browser configuration. However, you cannot expect any of these capabilities to
be available.
In addition, the Java compiler and interpreter check both the Java source code
and the Java
bytecodes to make sure that the Java programmer has not tried any sneaky tricks
(for example,
overrunning buffers or stack frames).
These checks obviously cannot stop every potential security hole (no system can
promise that!),
but they can significantly reduce the potential for hostile applets. You’ll
learn more about
security issues for applets on Day 8, “Java Applet Basics,” and in greater
detail on Day 21,
“Under the Hood.”
Q: I followed all the directions you gave for creating a Java applet. I loaded
it into HotJava,but Hello World didn’t show up. What did I do wrong?
A: Don’t use HotJava to view applets you’ve created in this book; get a more
up-to-date browsersuch as Netscape or Internet Explorer. HotJava was an
experimental browser and has not beenupdated since soon after its original
release. The steps you take to define and write an applethave changed since
then, and the applets you write now will not run on HotJava.
Q: You’ve mentioned Solaris, Windows, and Macintosh in this chapter. What about
otheroperating systems?
A: If you use a flavor of UNIX other than Solaris, chances are good that the JDK
has been ported to your system. Here are some examples:
SGI’s version of the JDK can be found at
http://www.sgi.com/Products/cosmo/cosmo_instructions.html.
Information about Java for Linux can be found at
http://www.blackdown.org/java-linux/.
IBM has ported the JDK to OS/2 and AIX. Find out more from
http://www.ncc.hurley.ibm.com/javainfo/.
OSF is porting the JDK to HP/UX, Unixware, Sony NEWS, and Digital UNIX. See
http://www.osf.org/mall/web/javaport.htm.
(Thanks to Elliote Rusty Harold’s Java FAQ at
http://www.sunsite.unc.edu/javafaq/javafaq/html for this information.)
Q: Why doesn’t Java run on Windows 3.1?
A: Technical limitations in Windows 3.1 make porting Java to Windows 3.1
particularly difficult.Rumor has it that both IBM and Microsoft are working on
ports, but no real information is forthcoming.
Q: I’m using Notepad on Windows to edit my Java files. The program insists on
adding a.txt extension to all my files, regardless of what I name them (so I
always end up with files like HelloWorld.java.txt). Short of renaming them
before I compile them, what else can I do to fix this?
A: Although you can rename the files just before you compile them, that can get
to be a pain, particularly when you have a lot of files. The problem here is
that Windows doesn’t understand the .java extension (you may also have this
problem with HTML’s .html extension as well). To fix this, go into any Windows
Explorer window and select View|Options|File Types. Fromthat panel, select New
Type. Enter Java Source Files in the Description of Type box and .java into the
Associated Extension box. Then click OK. Do the same with HTML files if you need
to, and click OK again. You should now be able to use Notepad (or any other text
editor) to create and save Java and HTML files.
Q: Where can I learn more about Java and find applets and applications to play
with?
A: You can read the rest of this book! Here are some other places to look for
Java information and Java applets:
The Java home page at http://www.java.sun.com/ is the official source for Java
information, including information about the JDK, about the upcoming 1.1
release, and about developer tools such as the Java Workshop, as well as
extensive documentation.
Gamelan, at http://www.gamelan.com/, is a repository of applets and Java
information, organized into categories. If you want to play with applets or
applications, this is the place to look.
For Java discussion, check out the comp.lang.java newsgroups, including
comp.lang.java.programmer, comp.lang.java.tech,
comp.lang.java.advocacy, and so on. (You’ll need a Usenet newsreader to access
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