IS-A and HAS-A Relationship in Java (OOP Concepts)

In Object-Oriented Programming (OOP), IS-A and HAS-A relationships define how classes interact with each other.

• IS-A :

  represents inheritence between classes.

  eg: Audi is a car.

  Defined using extends or implements keyword.

  indicates that a subclass is a specialised version of its superclass.

• HAS-A :

  represents use of object of another class inside a class.

  eg: Audi has a Horn

    public class StaticDemo {
  
        class Car {
            void engine() {
                System.out.println("Car needs to have an engine.");
            }
        }
  
        class Audi extends Car {  // ✅ Audi IS-A Car (Inheritance)
            static Horn h1 = new Horn(); // ✅ Audi HAS-A Horn (Composition)
  
            Audi() {
                h1.featureHorn(); // Calling featureHorn() inside constructor
            }
  
            @Override
            void engine() {
                System.out.println("Audi has refined diesel engine.");
            }
        }
  
        public static void main(String[] args) {
            StaticDemo staticDemo = new StaticDemo();
            Audi a1 = staticDemo.new Audi(); // ✅ Audi IS-A Car (creating Audi object)
            a1.engine();
  
            // ✅ Audi HAS-A Horn (using static Horn instance)
            Audi.h1.featureHorn(); // Output: pow pow
        }
    }
  
    // Separate class for Horn
    class Horn {
        void featureHorn() {
            System.out.println("pow pow");
        }
    }
  

  🔹 Explanation of example:

  IS-A Relationship (Inheritance)

  • Audi extends Car → Audi IS-A Car

  • Audi inherits behavior from Car (engine() method).

HAS-A Relationship (Composition)

• Audi HAS-A Horn instance (static Horn h1).

• This means Audi contains a Horn but does not inherit from it.