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Objects and Classes<br />

Objects and classes are central concepts for Java programming. It will take you some<br />

time to master these concepts fully, but since every Java program uses at least a couple of<br />

objects and classes, it is a good idea to have a basic understanding of these concepts right<br />

away.<br />

An object is an entity that you can manipulate in your program, generally by calling<br />

methods. For example, System.out refers to an object, and you saw how to manipulate it<br />

by calling the println method. (Actually, several different methods are available: all<br />

called println, one for printing strings, one for printing integers, one for printing floatingpoint<br />

numbers, and so on.) When you call the println method, some activities occur<br />

inside the object, and the ultimate effect is that the object causes text to appear in the<br />

console window. For now, you should think of objects as a “black box” with a public<br />

interface – the methods you can call – and a hidden implementation – the code and data<br />

that are necessary to make these methods work.<br />

Different objects support different sets of methods. For example, the println method can<br />

be applied to the System.out object, but cannot be applied to the string object “Hello,<br />

World!”. That is, it would be an error to call<br />

“Hello, World!”.println(); // This method call is an error<br />

The reason is simple. The System.out and “Hello World!”<br />

objects belong to different classes. The System.out object is an object of the class<br />

PrintStream, and the “Hello World!” object is an object of the class String. You can apply<br />

the println method to any object of the PrintStream class, but the String class does not<br />

support the println method. The String class supports a good number of other methods;<br />

you will see many of them soon. For example, the length method counts the number of<br />

characters in a string. You can apply that method of type String. Thus,<br />

“Hello, World!”.length(); // This method call is OK<br />

is a correct call – it computes the number 13, the number of characters in the string object<br />

“Hello World!”. (The quotation marks are not counted.)<br />

A class has four purposes:<br />

1. A class specifies the methods that you can use for objects that belong to the class.<br />

2. A class is a factory for objects.<br />

3. A class is a holding place for static methods and objects.<br />

4. A class defines implementation details: the data layout of the objects and the code for<br />

the methods.<br />

In out first program, you saw the third (and least important) purpose. The Hello class<br />

holds the static main method. The System class holds the static out object.

To see how a class can be an object factory, let us turn to another class: the Rectangle<br />

class in the Java class library. Objects of type Rectangle describe rectangular shapes.<br />

Note that a Rectangle object isn’t a rectangular shape – it is a set of numbers that describe<br />

the rectangle. Each rectangle is described by the x- and y-coordinates of its top left<br />

corner, its width, and its height. To make a new rectangle, you need to specify these four<br />

values. For example, you can make a new rectangle with the top left corner at (5,10),<br />

width 20 and height 30 as follows:<br />

new Rectangle(5, 10, 20, 30)<br />

The new operator causes the creation of an object of type Rectangle. The process of<br />

creating a new object is called construction. The four values 5, 10, 20, 30 are called the<br />

construction parameters. Different classes will require different construction parameters.<br />

For example, to construct a Rectangle object, you supply four numbers that describe the<br />

position and the size of the rectangle. To construct a Car object, you might supply the<br />

model name and year.<br />

Actually, some classes let you construct objects in multiple ways. For example, you can<br />

also obtain a rectangle object by supplying no construction parameters at all (but you still<br />

must supply the parentheses):<br />

new Rectangle()<br />

This constructs a (rather useless) rectangle with the top left corner at the origin (0,0),<br />

width 0 and height 0. Construction without parameters is called default construction.<br />

To construct any object you do the following:<br />

1. Use the new operator<br />

2. Give the name of the class<br />

3. Supply construction parameters (if any) inside the parentheses<br />

What can you do with a Rectangle object? Not much, for now. Later will see how to<br />

display rectangles and other shapes in a window. You already know how to print a<br />

description of the rectangle object onto the console window – simply call the<br />

System.out.println method:<br />

System.out.println(new Rectangle(5, 10, 20, 30));<br />

The code prints the line<br />


Or more specifically, this code creates an object of type Rectangle, then passes that object<br />

to the println method, and finally forgets the object.<br />

Of course, usually you want to do something more to an object than just create it, print it<br />

and forget it. To remember an object, you need to hold it in an object variable. An<br />

object variable is a storage location that stores not the actual object, but information<br />

about the object’s location.<br />

You can create an object variable by giving the name of the class, followed by a name for<br />

the variable. For example,<br />

Rectangle cerealBox;<br />

This statement defines an object variable, cerealBox. The type of this variable is<br />

Rectangle. In Java, every object variable has a particular type. For example, after the<br />

cerealBox variable has been defined by the preceding statement, thereafter in the program<br />

it must always refer to an object of type Rectangle, never to an object of type Car or<br />

String.<br />

However, so far, the cerealBox variable doesn’t yet refer to any object at all. It is an<br />

uninitialized variable. To make cerealBox refer to an object, simply set it to another<br />

object reference. How do you get another object reference? The new operator returns a<br />

reference to a newly created object.<br />

Rectangle cerealBox = new Rectangle(5, 10, 20, 30);<br />

It is very important that you remember that the cerealBox variable does not contain the<br />

object. It refers to the object. You can have two object variables refer to the same<br />

object:<br />

Rectangle crispyCrunchyStuff = cerealBox;<br />

Now you can access the same Rectangle object both as cerealBox and as<br />

crispyCrunchyStuff.<br />

The Rectangle class has over 50 methods, some useful, some less so. To give you a<br />

flavor of manipulating Rectangle objects, let us look at a method of the Rectangle class.<br />

The translate method moves a rectangle by a certain distance in the x- and y-direction.<br />

For example,<br />

cerealBox.translate(15, 25);<br />

moves the rectangle by 15 units in the x-direction and 25 units in the direction and 25<br />

units in the y-direction. Moving a rectangle doesn’t change its width or height, but it<br />

changes the top left corner. For example, the code fragment<br />

Rectangle cerealBox = new Rectangle(5, 10, 20, 30);

prints<br />

cerealBox.translate(15, 25);<br />

System.out.println(cerealBox);<br />

java.awt.Retangle[x=20,y=35,width=20,height=30]<br />

Let’s turn this code fragment into a complete program. As with the Hello program, you<br />

need to carry out three steps:<br />

1. Invent a new class, say MoveRectangle<br />

2. Supply a main method<br />

3. Place instructions inside the main method<br />

However, for this program, there is an additional step that you need to carry out: You<br />

need to import Rectangle class from a package, which is a collection of classes with a<br />

related purpose. All classes in the standard library are contained in packages. For<br />

example, the System class and the String class are in a package called java.lang, and the<br />

Rectangle class belongs to the package java.awt. (The abbreviation awt stands for<br />

“Abstract Windowing Toolkit”). The java.awt package contains many classes for<br />

drawing windows and graphical shapes.<br />

To use the Rectangle class from the java.awt package, simply place the following line at<br />

the top of your program:<br />

import java.awt.Rectangle;<br />

You never need to import classes from the java.lang package. All classes from this<br />

package are automatically imported. For example, you can use the System and String<br />

classes without importing them.<br />

Thus the complete program is:<br />

Program: MoveRectangle.java<br />

import java.awt.Rectangle;<br />

public class MoveRectangle<br />

{<br />

public static void main(String[] args)<br />

{<br />

Rectangle cerealBox = new Rectangle(5, 10, 20, 30);<br />

cerealBox.translate(15, 25);<br />

System.out.println(cerealBox);<br />

}<br />


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