What is polymorphism and how does Java implement it?

There are two forms. Compile-time polymorphism is overloading: the compiler chooses a method from the static types and argument list. Runtime polymorphism is overriding: the JVM dispatches to the implementation of the actual object, even when the reference is the parent type.

abstract class Animal {
    abstract String sound();
}

class Dog extends Animal {
    @Override
    String sound() {
        return "woof";
    }
}

Animal animal = new Dog();
System.out.println(animal.sound());

Rules that affect dispatch

• An overriding method cannot reduce visibility
• static, private and final methods are not dynamically dispatched
• Return types may be covariant in an override
• Fields are hidden, not overridden, so access depends on the reference type
• Overload selection uses the compile-time types

Polymorphism is what makes interfaces useful. A caller can depend on a PaymentMethod and accept card, bank or wallet implementations without a type switch. The cost is indirection, but the JVM optimises monomorphic and bimorphic call sites effectively.