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Trail: Learning the Java Language
Lesson: Classes and Objects
Classes and Objects
Classes
Declaring Classes
Declaring Member Variables
Defining Methods
Providing Constructors for Your Classes
Passing Information to a Method or a Constructor
Objects
Creating Objects
Using Objects
More on Classes
Returning a Value from a Method
Using the this Keyword
Controlling Access to Members of a Class
Understanding Instance and Class Members
Initializing Fields
Summary of Creating and Using Classes and Objects
Questions and Exercises
Questions and Exercises
Nested Classes
Inner Class Example
Summary of Nested Classes
Questions and Exercises
Enum Types
Questions and Exercises
Annotations
Questions and Exercises
Home Page > Learning the Java Language > Classes and Objects
Objects

> A typical Java program creates many objects, which as you know, interact by invoking methods Through these object interactions, a program can carry out various tasks, such as implementing a GUI, running an animation, or sending and receiving information over a network. Once an object has completed the work for which it was created, its resources are recycled for use by other objects.

> Here's a small program, called `CreateObjectDemo`, that creates three objects: one `Point` object and two `Rectangle` objects. You will need all three source files to compile this program.

> >     public class CreateObjectDemo {

> >         public static void main(String[] args) {

> >             //Declare and create a point object
> >             //and two rectangle objects.
> >             Point originOne = new Point(23, 94);
> >             Rectangle rectOne = new Rectangle(originOne, 100, 200);
> >             Rectangle rectTwo = new Rectangle(50, 100);

> >             //display rectOne's width, height, and area
> >             System.out.println("Width of rectOne: " +
> >                     rectOne.width);
> >             System.out.println("Height of rectOne: " +
> >                     rectOne.height);
> >             System.out.println("Area of rectOne: " + rectOne.getArea());

> >             //set rectTwo's position
> >             rectTwo.origin = originOne;

> >             //display rectTwo's position
> >             System.out.println("X Position of rectTwo: "
> >                     + rectTwo.origin.x);
> >             System.out.println("Y Position of rectTwo: "
> >                     + rectTwo.origin.y);

> >             //move rectTwo and display its new position
> >             rectTwo.move(40, 72);
> >             System.out.println("X Position of rectTwo: "
> >                     + rectTwo.origin.x);
> >             System.out.println("Y Position of rectTwo: "
> >                     + rectTwo.origin.y);
> >         }
> >     }

> This program creates, manipulates, and displays information about various objects. Here's the output:

> >     Width of rectOne: 100
> >     Height of rectOne: 200
> >     Area of rectOne: 20000
> >     X Position of rectTwo: 23
> >     Y Position of rectTwo: 94
> >     X Position of rectTwo: 40
> >     Y Position of rectTwo: 72

> The following three sections use the above example to describe the life cycle of an object within a program. From them, you will learn how to write code that creates and uses objects in your own programs. You will also learn how the system cleans up after an object when its life has ended.

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