Class Association, Aggregation and Composition

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In this article, we will try to understand three important concepts: association, aggregation, and composition.
We will also try to understand in what kind of scenarios we need them. These three concepts have really confused a lot of developers and in this article; my attempt would be to present the concepts in a simplified manner with some real world examples.



In the world of Object Oriented Programming language, Class is known as the basic building block of an application. The class is kind of a container or capsule or a cell, which encapsulate the set of methods, attribute and properties to provide its indented functionalities to the outer world. Encapsulation is very basic property of a Class which is responsible for hiding class data from outside world.  Encapsulation also controls the exposure of function/properties of class so that it can be used/ consumed by other class. 
In order to modularize/ define the functionality of a one class, that class can uses functions/ properties exposed by another class in many different ways. According to Object Oriented Programming there are several techniques, classes can use to link with each other and they are named Association, Aggregation, and Composition.


In this article, we will try to understand three important concepts: associationaggregation, and composition.
We will also try to understand in what kind of scenarios we need them. These three concepts have really confused a lot of developers and in this article; my attempt would be to present the concepts in a simplified manner with some real world examples.


Association is the simplest form of relationship between two classes. In Association type of relation, all objects have their own lifecycle and there is no owner or dependency between two classes.
For example relation between a student and teacher is an Association. Multiple Students can associate with a single Teacher and single Student can associate with multiple Teachers, but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently. Creation or deletion of one object (Student) does not affect other object (Teacher).  The Course (or Subject) object can combine Student and Teacher in many to many relationships. 



Aggregation is a specialize form of Association where all object have their own lifecycle but there is parent - child relationship between these objects (But this doesn’t make true that child can’t be used by other Parent). Let’s take an example of Department and Teacher. A single Teacher cannot belong to multiple Departments, but if we delete the Department, Teacher object will not destroy. We can think about “has-a” relationship.


Composition is a strong type of Aggregation. Here the Parent and Child objects have coincident lifetimes. Child object does not have its own lifecycle and if parent object gets deleted, then all of its child objects will also be deleted. In short child object depends on parent object. Let’s take an example of relationship between House and Rooms. House can contain multiple Rooms there is no independent life of Room and any Room cannot belongs to two different houses.  If we delete the House, Room will automatically delete. 






No owner

Single owner

 Single owner

Life time

Have their own lifetime

Have their own lifetime

 Owner's life time

Child object

Child objects all are independent

Child objects belong to a parent

 Child objects belong to a  single parent


Practical implementation of Relationship type

When we look at the real world through OOPs point of view, you will feel that these real world objects have relationships and these relations are implemented through 3 different ways i.e. Association, Aggregation and Composition. 
Let’s consider the simple requirement listed below:
1. Manager is an employee of XYZ Company.
2. Manager uses a swipe card to enter XYZ premises.
3. Manager has workers who work under him.
4. Project success depends on Manager.
5. Manager's incentive depends on project success. 
If you carefully go through these five requirements, you can easily visualize four relationships:-
·   Inheritance [Is a]
·   Association [uses]
·   Aggregation [has]
·   Cmposition [depends
Let’s understand them one by one. 

Requirement 1: The “IS A” relationship: Inheritance 

If you look at the first requirement (Manager is an employee of XYZ limited corporation), it’s a parent child relationship or inheritance relationship. The sentence above specifies that Manager is a type of employee, in other words we will have two classes: parent class Employee, and a child class Manager which will inherit from the Employee class. 
Note: As the scope of this article is only limited to aggregationassociation, and composition, we will not discuss inheritance in details. 

Requirement 2: The “Using” relationship: Association 

Requirement 2 is an interesting requirement (Manager uses a swipe card to enter XYZ premises). In this requirement, the Manager object and the Swipecard object use each other but they have their own object life time. In other words, they can exist without each other. The most important point in this relationship is that there is no single owner.

The above diagram shows how the SwipeCard class uses the Manager class and the Manager class uses the SwipeCard class. You can also see how we can create objects of the Manager Class and SwipeCard class independently and they can have their own object life time. 
This relationship is called the “Association” relationship 
Requirement 3: The “has” relationship with Parent: Aggregation
The third requirement from our list (Manager has workers who work under him) denotes the same type of relationship like association but with a difference that one of them is an owner. So as per the requirement,   
The Manager object will own Worker objects. 
But… the Worker object can have its own life time which is completely disconnected from the Manager object. Looking from a different perspective, it means that if the Manager object is deleted, the Worker object does not die.

This relationship is termed as an “Aggregation” relationship. 


Requirements 4 and 5: The “depends” relationship: Composition 

The last two requirements are actually logically one. If you read closely, the requirements are as follows: Project success depends on Manager and Manager's incentive depends on project success. 

So the conclusion from analyzing the above requirements is that Manager and the Project objects are dependent on each other. 

The lifetimes of both the objects are the same. In other words, the project will not be successful if the manager is not good, and the manager will not get good incentive if the project fails. 

Below is how the class formation will look like. You can also see that when I go to create the project object, it needs the manager object.

This relationship is termed as the composition relationship. In this relationship, both objects are heavily dependent on each other. In other words, if one goes for garbage collection the other also has to be garbage collected, or putting from a different perspective, the lifetime of the objects are the same. That’s why I have put in the heading “Death” relationship. 

Putting things together 

Below is a visual representation of how the relationships have emerged from the requirements.



Association or Aggregation or Compositions are all data access techniques which come under Encapsulation.  Use of Association, Aggregation and Compositions techniques usually results in design practices like SOLID or various design patterns.  
The point of using patterns, practices and such is to describe a solution to a specific problem that is also maintainable and extensible. You simply based on your experience and requirement to tell where to use which pattern or technique to achieve your programming goals. 

Reference :

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About the Author

Full Name: Nakul Lande
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Member Since: 7/24/2012 5:13:29 PM
Country: United States
Nakul Lande

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