What Is Polymorphism and How Does Method Overriding Work?

Polymorphism allows one superclass reference to refer to objects of different subclasses, with each object responding differently to the same method call. Method overriding enables this by giving a subclass its own implementation of a method inherited from its superclass.

How the mechanism works

Consider this Java-style example:

class Animal {
    public void speak() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    @Override
    public void speak() {
        System.out.println("Bark");
    }
}

class Cat extends Animal {
    @Override
    public void speak() {
        System.out.println("Meow");
    }
}

A superclass reference can store either subclass object:

Animal first = new Dog();
Animal second = new Cat();

first.speak();   // Bark
second.speak();  // Meow

The variables have the declared type Animal, but their objects have the runtime types Dog and Cat. During execution, dynamic method dispatch selects the overridden method belonging to the actual object. Therefore, first.speak() calls Dog.speak(), not Animal.speak().

For overriding to occur, the subclass method must have the same method name and parameter list as the inherited method. The @Override annotation is not the mechanism itself, but it helps the compiler detect mistakes.