Up Casting in Java: Definition, Rules, and Examples

Up Casting in Java

Up Casting is the process of assigning a subclass object to a superclass reference or a super-interface reference.

Key Points

• Up casting is also known as implicit casting.
• It is performed automatically by the compiler.
• In up casting, the compiler treats the subclass object as a superclass object.
• No explicit cast is required.

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

Accessing Members During Up Casting

• When up casting occurs, the programmer can access only the members of the superclass using the superclass reference.
• Subclass-specific members cannot be accessed directly.

class Parent {

    void show() {

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

    }

}

class Child extends Parent {

    void display() {

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

    }

}

Parent p = new Child();

p.show(); // Allowed

// p.display(); // Not allowed

Up Casting with Method Overriding

• If method overriding is present:

             ✓The overridden method of the subclass is executed.

             ✓This is due to runtime polymorphism (dynamic method dispatch).

class Parent {

    void run() {

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

    }

}

class Child extends Parent {

    void run() {

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

    }

}

Parent p = new Child();

p.run(); // Output: Child running

👉 Here, the method call depends on the object, not the reference.

Why Up Casting Is Important

• Required to achieve runtime polymorphism.
• Helps in writing flexible and loosely coupled code.
• Widely used in method arguments, collections, and frameworks.

Important Correction

❌ Up casting is required to achieve runtime error → Incorrect

✅ Up casting is required to achieve runtime polymorphism → Correct