OBJECT ORIENTED PROGRAMMING THROUGH JAVA : Unit - 2 : Topic - 3 : Assigning One Object to Another

 Unit - 2

Topic 3 : Assigning One Object to Another in Java

In Java, when you assign one object reference to another, you are not creating a new object. Instead, both references will point to the same object in memory.

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How It Works

Example:

Box b1 = new Box(); // Create a new Box object
Box b2 = b1;        // Assign b1 to b2

At this point:

• No new object is created when assigning b1 to b2.

• Both b1 and b2 refer to the same memory location.

• Any changes made through b1 will also be visible when accessing through b2 (and vice versa).

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Visualization in Memory

Before Assignment:
b1 → [Box Object #1]
b2 → null

After: b2 = b1;
b1 → [Box Object #1] ← b2

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Example Program

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

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

public class ObjectReferenceDemo {
    public static void main(String[] args) {
        Box b1 = new Box();
        b1.width = 10;
        b1.height = 20;
        b1.depth = 15;

        Box b2 = b1; // b2 now refers to the same object as b1

        System.out.println("Volume using b1:");
        b1.volume();

        System.out.println("Volume using b2:");
        b2.volume();

        // Change values using b2
        b2.width = 5;
        b2.height = 6;
        b2.depth = 7;

        System.out.println("After modifying via b2:");

        // Check values through b1
        b1.volume();

        // Check values through b2
        b2.volume();
    }
}

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Sample Output

Volume using b1:
Volume is 3000.0
Volume using b2:
Volume is 3000.0
After modifying via b2:
Volume is 210.0
Volume is 210.0

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Key Takeaways

• Object variables in Java hold references, not actual object data.

• Assigning one reference to another makes them point to the same object.

• Any changes via one reference affect the same underlying object.

• To make an actual copy, you need to create a new object and copy the values manually (or use cloning).