OOP

Why composition beats inheritance for change. A Decorator-style stack can be assembled at runtime.

// PROBLEM: capabilities × kinds explode an inheritance tree.
// {Duck, Penguin, Robot} × {can fly?, can swim?} → FlyingSwimmingDuck,
// NonFlyingSwimmingPenguin, ... one subclass per combination (up to 3×4 = 12),
// and "fly" logic is copy-pasted across unrelated branches (Bird, Plane).
class Bird extends Animal { void fly() {} }
class Plane extends Vehicle { void fly() {} }   // same behaviour, duplicated

// COMPOSITION: behaviours are objects injected at runtime.
interface FlyBehaviour { void fly(); }
class JetFly implements FlyBehaviour {
  public void fly() { System.out.println("fast"); }
}
class NoFly implements FlyBehaviour {
  public void fly() { /* nothing */ }
}

class Character {
  private final FlyBehaviour fly;          // injected → composition
  Character(FlyBehaviour fly) { this.fly = fly; }
  void move() { fly.fly(); }
}

// Now a bird that cannot fly is just: new Character(new NoFly());
// Adding "glide" never touches Character.