OBJECT ORIENTED PROGRAMMING THROUGH JAVA : Unit - 2 : Topic - 2 : Declaring Objects

Unit - 2

Topic 2 : Declaring and Creating Objects in Java

Declaring objects of a class is a two-step process in Java:

Step 1: Declare a reference variable

Before we can use an object, we must declare a reference variable of the class type.
This variable doesn’t create the object — it just stores the reference (memory address) of an object once it is created.

Example:

Box mybox; // declares a reference variable of type Box

Here, mybox is null by default (it doesn’t refer to any object yet).

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Step 2: Create the object using new

To actually create the object in memory, we use the new operator. The new operator:

1. Allocates memory for the object.

2. Returns a reference to the object (similar to a memory address).

Example:

mybox = new Box(); // creates a Box object and assigns its reference to mybox

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Combining both steps

In most cases, we combine the declaration and creation in one line:

Box mybox = new Box();

This means:

1. Declare mybox as a reference to a Box object.

2. Create a Box object in memory and assign its reference to mybox.

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Example Program: Declaring and Creating Objects

class Box {
    double width;
    double height;
    double depth;

    void volume() {
        double vol = width * height * depth;
        System.out.println("Volume is " + vol);
    }
}

public class ObjectDemo {
    public static void main(String[] args) {
        // Step-by-step declaration and object creation
        Box box1;         // declare reference variable
        box1 = new Box(); // create object

        // Combined declaration and creation
        Box box2 = new Box();

        // Assign values to box1
        box1.width = 10;
        box1.height = 20;
        box1.depth = 15;

        // Assign values to box2
        box2.width = 5;
        box2.height = 6;
        box2.depth = 7;

        // Display volumes
        box1.volume();
        box2.volume();
    }
}

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Sample Output

Volume is 3000.0
Volume is 210.0

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Key Points to Remember

• A class variable is just a reference; the actual object is created using new.

• If you declare a reference but don’t assign an object, it will be null. Accessing it will cause a NullPointerException.

• You can create multiple objects from the same class, each with its own data.

Example:

Box b1 = new Box();
Box b2 = new Box(); 

Here, b1 and b2 are two separate objects in memory.