IS-A and HAS-A Relationship in Java

1. IS-A Relationship (Inheritance)

Definition

An IS-A relationship represents inheritance in Java.
It means one class is a specialized version of another class.

Syntax

class Child extends Parent

Example

class Animal {
    void eat() {
        System.out.println("Eating...");
    }
}

class Dog extends Animal {
    void bark() {
        System.out.println("Barking...");
    }
}

Here:

Dog IS-A Animal.

• Dog inherits behavior from Animal.
• Enables method overriding and runtime polymorphism.

When to Use IS-A

• When there is a true hierarchical relationship.
• When polymorphism is required.
• Example:
  • Car IS-A Vehicle
  • Manager IS-A Employee

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2. HAS-A Relationship (Composition)

Definition

A HAS-A relationship represents composition.
It means one class contains another class as a member.

Example

class Engine {
    void start() {
        System.out.println("Engine started");
    }
}

class Car {
    private Engine engine = new Engine();  // HAS-A relationship

    void drive() {
        engine.start();
        System.out.println("Car is moving");
    }
}

Here:

Car HAS-A Engine.

Car is not an Engine.
Car contains an Engine.

When to Use HAS-A

• When building modular and flexible designs.
• When avoiding tight coupling.
• When following the principle:
  Favor Composition over Inheritance.

Examples: - Car HAS-A Engine - House HAS-A Room - Employee HAS-A Address

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Key Differences

IS-A HAS-A

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Inheritance Composition Uses extends Uses object reference Strong coupling More flexible Supports polymorphism Encourages modular design

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Design Insight

Bad Design:

class Car extends Engine  // Incorrect

Good Design:

class Car {
    Engine engine;
}

IS-A builds hierarchy.
HAS-A builds modular structure.