Association, Aggregation and Composition are terms that represent relationships among objects. They are very basic stuff of Object-Oriented Programming.

In this article, I’ll explain association, composition & aggregation. Let’s get started:

Table of Contents

1. Association
2. Composition
3. Aggregation

Association

Association is a relationship between two objects. Association can be one-to-one, one-to-many, many-to-one, many-to-many. Composition and aggregation are two types of association.

Example: A Car and a Engine are having an association.

Composition

Composition is a strong association. An association is said to composition if an object owns another object and another object cannot exist without the owner object.

Example: Car class contains Engine. Engine cannot exist without a Car. There exists composition between Car and Engine.

// Car must have Engine
public class Car {
   // engine is a mandatory part of the car
   private final Engine engine;

   Car() {
      engine = new Engine();
   }
}

// Engine Object
class Engine {}

Aggregation

Aggregation is a weak association. An association is said to be aggregation if both objects can exist independently.

Example: A Team has 0 or more players. A Player can be in a Team or not. There exists aggregation between Team and Player.

// Team
public class Team {
   // players can be 0 or more
   private List players;

   Team() {
      players = new Player();
   }
}

// Player Object
class Player {}

The article is over. Thanks for reading.