What is polymorphism?

Polymorphism (Greek: "many forms") allows objects of different types to be treated through a common interface. The correct method is selected at runtime based on the actual object type. "One interface, multiple implementations." Types: (1) Runtime polymorphism (dynamic dispatch/method overriding): achieved through inheritance and virtual methods. The actual method called is determined at runtime: abstract class Animal { public abstract void speak(); } class Dog extends Animal { public void speak() { System.out.println("Woof!"); } } class Cat extends Animal { public void speak() { System.out.println("Meow!"); } } class Duck extends Animal { public void speak() { System.out.println("Quack!"); } } // Polymorphic code: List<Animal> animals = new ArrayList<>(); animals.add(new Dog()); animals.add(new Cat()); animals.add(new Duck()); for (Animal a : animals) { a.speak(); // Correct method called for each actual type! } // Woof! Meow! Quack!; (2) Compile-time polymorphism (method overloading / ad-hoc polymorphism): multiple methods with same name, different parameters: class Calculator { public int add(int a, int b) { return a + b; } public double add(double a, double b) { return a + b; } public int add(int a, int b, int c) { return a + b + c; } } // Compiler selects correct version based on argument types.. Parametric polymorphism (generics/templates): List<Integer> ints; List<String> strings;. Polymorphism enables the Open/Closed Principle — add new types without changing existing code that processes them.