Polymorphism: overloading and overriding

Why this matters

It is what makes code flexible. You can write one loop that calls draw() and every shape draws itself correctly.

Example

class Calculator {
    int add(int a, int b) { return a + b; }
    int add(int a, int b, int c) { return a + b + c; }
    double add(double a, double b) { return a + b; }
}

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

class Dog extends Animal {
    @Override
    void sound() { System.out.println("Woof"); }
}

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

public class Main {
    public static void main(String[] args) {
        Calculator c = new Calculator();
        System.out.println(c.add(2, 3));
        System.out.println(c.add(2, 3, 4));
        System.out.println(c.add(2.5, 3.5));

        Animal[] animals = { new Dog(), new Cat(), new Animal() };
        for (Animal a : animals) {
            a.sound();      // decided at runtime
        }
    }
}

Step by step

1. 1The three add() methods share a name but differ in parameters — compile-time polymorphism (overloading).
2. 2Dog and Cat override sound() with their own version — runtime polymorphism.
3. 3`Animal a = new Dog();` is upcasting: the reference type is Animal, the object is a Dog.
4. 4At runtime the JVM calls the Dog version. This is dynamic method dispatch.

Key points

• Overloading: same name, different parameter list, same class, resolved at compile time
• Overriding: same signature, parent and child, resolved at runtime
• @Override helps the compiler catch mistakes
• Return type alone cannot overload a method

Common mistakes

• Thinking two methods differing only by return type are overloaded
• Changing the parameter list while intending to override (it becomes overloading)
• Trying to override a private or static method

Try it yourself

• Overload an area() method for square, rectangle and circle.
• Create Shape, Circle and Square and call draw() through a Shape reference.