A Simple Class

Here is a class called Box that defines three member variables: width, height, and depth.

 
class Box { 
    int width; 
    int height; 
    int depth; 
}

A class defines a new type of data. In this case, the new type is called Box.

To create a Box object, you will use a statement like the following:

Box myBox = new Box(); // create a Box object called mybox

myBox is an instance of Box.

mybox.width = 100;

This statement assigns the width from mybox object to 100. Here is a complete program that uses the Box class:

 
class Box {
  int width;
  int height;
  int depth;
}
public class Main {
  public static void main(String args[]) {
    Box myBox = new Box();
    myBox.width = 10;

    System.out.println("myBox.width:"+myBox.width);
  }
}

The output:

myBox.width:10

Declaring Objects

Object creation is a two-step process.

1. declare a variable of the class type.
2. acquire a physical copy of the object and assign it to that variable using the new operator.

The new operator dynamically allocates memory for an object and returns a reference to it. The following lines are used to declare an object of type Box:

Box mybox; // declare reference to object 
mybox = new Box(); // allocate a Box object

The first line declares mybox as a reference to an object of type Box.

Box mybox = new Box();

Any attempt to use a null mybox will result in a compile-time error.

class Box {
  int width;
  int height;
  int depth;
}

public class Main {
  public static void main(String args[]) {
    Box myBox;
    myBox.width = 10;
  }
}

If you try to compile the code above, you will get the following error message from the Java compiler.

D:\>javac Main.java
Main.java:10: variable myBox might not have been initialized
    myBox.width = 10;
    ^
1 error

Object instance

The following program declares two Box objects:

 
class Box {
  int width;
  int height;
  int depth;
}

public class Main {
  public static void main(String args[]) {
    Box myBox1 = new Box();
    Box myBox2 = new Box();

    myBox1.width = 10;

    myBox2.width = 20;

    System.out.println("myBox1.width:" + myBox1.width);
    System.out.println("myBox2.width:" + myBox2.width);
  }
}

The output produced by this program is shown here:

myBox1.width:10
myBox2.width:20