Java - Inheritance & Polymorphism

Inheritance & Polymorphism

extends and super

class Animal { void speak() { System.out.println("..."); } }
class Dog extends Animal {
  @Override void speak() { System.out.println("Woof"); }
}
Animal a = new Dog();
a.speak(); // Woof (dynamic dispatch)

Method Overriding and final

Use @Override to catch mistakes. final prevents further overriding, final class prevents inheritance.

Constructors and super()

class Base { Base(int n) {} }
class Sub extends Base { Sub() { super(42); } }

Composition over Inheritance

Prefer composition when reuse does not model an "is-a" relationship.

Liskov Substitution Principle (LSP)

Subtypes must honor supertype contracts. Avoid strengthening preconditions or weakening postconditions.

Try it

1. Create a small hierarchy (Animal → Dog/Cat) and override a method; observe dynamic dispatch.
2. Mark a class final and attempt to extend it (should fail).