Down Casting in Java: Definition, Rules, and Examples

Down Casting in Java

Down Casting is the process of converting a superclass reference (that actually refers to a subclass object) into a subclass reference.

Key Points

• Down casting is also known as explicit casting.
• It must be performed by the programmer using a cast operator.
• The compiler does not perform down casting automatically.
• Up casting must occur before down casting.
• If down casting is done without a valid up cast, it results in a ClassCastException at runtime.

Example

class Parent {

    void show() {

        System.out.println("Parent method");

    }

}

class Child extends Parent {

    void display() {

        System.out.println("Child method");

    }

}

Parent p = new Child(); // Up casting

Child c = (Child) p; // Down casting

c.show(); // Allowed

c.display(); // Allowed

Invalid Down Casting

Parent p = new Parent();

Child c = (Child) p; // ClassCastException

Why Down Casting Is Risky

• Performed at runtime, not compile time.
• Can throw ClassCastException if the object is not actually a subclass object.
• Should be used carefully.

Safe Down Casting (Using instanceof)

if (p instanceof Child) {

    Child c = (Child) p;

    c.display();

}

Summary

• Down casting = Superclass reference → Subclass reference
• Explicitly done by programmer
• Requires prior up casting
• Can cause runtime exception if done incorrectly