Polymorphism

A variable declared as a parent type can refer to any subtype object, and calling an overridden method runs the subtype's version — decided at runtime, not at compile time.

Animal[] animals = { new Dog("Rex"), new Cat("Tom") };

for (Animal a : animals) {
    a.makeSound(); // runs Dog's or Cat's version, whichever it is
}

Rex barks
Tom meows

This is what makes it possible to write a method that accepts Animal and have it correctly handle any current or future subclass, without an if/instanceof chain for every kind.

Common mistakes

• Writing if (a instanceof Dog) { ... } else if (a instanceof Cat) chains to work around not overriding a method properly — usually a sign the method should be overridden on Animal instead.