HAS-A vs IS-A Relationship

1. Introduction

Object-Oriented Programming (OOP) models real-world entities using relationships between classes.
Two fundamental relationships in Java are:

• IS-A Relationship (Inheritance)
• HAS-A Relationship (Composition / Aggregation)

Understanding these relationships helps you design clean, maintainable, and scalable systems.

2. IS-A Relationship (Inheritance)

Definition

IS-A represents inheritance.
It means one class is a type of another class.

Example:

Dog IS-A Animal
Car IS-A Vehicle
Manager IS-A Employee

In Java, IS-A is implemented using the extends keyword.

Example

class Animal {
    void eat() { }
}

class Dog extends Animal {
    void bark() { }
}

Dog inherits from Animal. Therefore, Dog is an Animal.

When to Use IS-A?

Use IS-A when:

• The subclass is a specialized version of the parent class
• The relationship is logically correct
• You want to enable polymorphism and method overriding

Example:

Circle IS-A Shape
SavingsAccount IS-A BankAccount

Pitfall: Misusing IS-A

Not every relationship should be inheritance.

Example:

Bird IS-A Animal  → correct  
Bird HAS-A Wing   → correct  
Bird IS-A Wing    → incorrect

3. HAS-A Relationship (Composition / Aggregation)

Definition

HAS-A means one class contains another class as a field.

It represents:

• Composition (strong ownership)
• Aggregation (weak ownership)

Examples:

Car HAS-A Engine  
Library HAS-A Book  
Student HAS-A Address

Example: Composition

class Engine {
    void start() { }
}

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

Car has an Engine.

4. Composition vs Aggregation (Under HAS-A)

4.1 Composition (Strong HAS-A)

• Child cannot exist without parent
• Lifecycle is tightly dependent
• Example: House HAS-A Room

Code:

class Room { }

class House {
    private Room room = new Room();
}

If the House object is destroyed, the Room object is also destroyed.

4.2 Aggregation (Weak HAS-A)

• Child can exist independently
• Loose relationship
• Example: Department HAS-A Student

class Student { }

class Department {
    private List<Student> students;
}

Students can exist even without a Department.

5. Differences Between IS-A and HAS-A

6. Real-World Examples

Example 1 — University System

Professor IS-A Employee  
Department HAS-A Professor  
University HAS-A Department

Example 2 — Banking System

SavingsAccount IS-A BankAccount  
Customer HAS-A Address  
ATM HAS-A CashDispenser

Example 3 — Applications

Button IS-A UIComponent  
Window HAS-A Button  

7. When to Use Which?

Use IS-A (Inheritance) when:

• The subclass is a special version of the parent class
• You need polymorphism
• You want to override behavior

Use HAS-A (Composition) when:

• You want to build functionality by combining objects
• Relationship is part-whole
• You want flexibility (composition is more flexible than inheritance)

8. Best Practices

• Prefer HAS-A (composition) over IS-A when possible (Composition is more flexible and avoids tight coupling)

• Use inheritance cautiously to avoid deep hierarchies

• Check logical correctness: If you can say X is a Y, use inheritance. If you say X has a Y, use composition.

9. Summary

• IS-A represents inheritance; implemented using extends.
• HAS-A represents composition/aggregation; implemented using fields.
• IS-A is used for specialization; HAS-A for combining objects.
• Composition is generally preferred over inheritance due to flexibility.
• Understanding both relationships is essential for good object-oriented design.

This completes HAS-A vs IS-A Relationship in Java.

Written By: Shiva Srivastava