Upcasting and Downcasting · Polymorphism in Java | Java

Casting Between Parent and Child

	Upcasting	Downcasting
Direction	child → parent	parent → child
Syntax	`Animal a = new Dog();` (automatic)	`Dog d = (Dog) a;` (explicit)
Safe?	always	only if the object really is a Dog
Can call	only parent methods	child methods too

Safe Downcasting with instanceof

Example

Check with instanceof before casting. Java 16+ lets you bind a variable in the check itself (a instanceof Dog d).

public class Main {
    public static void main(String[] args) {
        Animal[] animals = { new Dog(), new Cat(), new Dog() };
        for (Animal a : animals) {
            a.eat();
            if (a instanceof Dog d) {      // check + cast in one step
                d.fetch();
            }
        }
    }
}

class Animal {
    void eat() { System.out.println(getClass().getSimpleName() + " eats"); }
}

class Dog extends Animal {
    void fetch() { System.out.println("  Dog fetches the ball"); }
}

class Cat extends Animal { }

Output

Dog eats
  Dog fetches the ball
Cat eats
Dog eats
  Dog fetches the ball

ClassCastException

Example

Casting to the wrong type compiles but fails at runtime.

public class Main {
    public static void main(String[] args) {
        Object o = "I am a String";
        System.out.println("Length: " + ((String) o).length());
        Integer n = (Integer) o;   // wrong type
        System.out.println(n);
    }
}

Output

Length: 13
Exception: java.lang.ClassCastException: class java.lang.String cannot be cast to class java.lang.Integer (java.lang.String and java.lang.Integer are in module java.base of loader 'bootstrap')