Difference between Association, Composition and Aggregation in Java, UML and Object Oriented Programming

In Object-oriented programming, one object is related to another to use functionality and service provided by that object. This relationship between two objects is known as the association in object-oriented general software design and is depicted by an arrow in Unified Modelling Language or UML. Both Composition and Aggregation are the forms of association between two objects, but there is a subtle difference between composition and aggregation, which is also reflected by their UML notation. We refer to the association between two objects as Composition when one class owns other classes and other classes can not meaningfully exist, when the owner is destroyed.

For example, Human class is a composition of several body parts including the Hand, Leg, and Heart. When a human object dies, all its body parts ceased to exist meaningfully, this is one example of Composition.

Programmers often confuse between Association, Composition, and Aggregation in Object-oriented design discussions, this confusion also makes the difference between Association, Composition, and Aggregation one of the popular questions in Java Interviews, only after the difference between abstract class and interface.

Another example of Composition is Car and its part e.g. engines, wheels, etc. Individual parts of the car can not function when a car is destroyed.  While in the case of Aggregation, including objects that can exist without being part of the main object like a Player which is part of a Team, can exist without a team and can become part of other teams as well.

Another example of Aggregation is Student in School class when School closed, Student still exists and then can join another School or so.  In UML notation, a composition is denoted by a filled diamond, while aggregation is denoted by an empty diamond, which shows their obvious difference in terms of the strength of the relationship.

The composition is stronger than Aggregation.  In Short, a relationship between two objects is referred to as an association, and an association is known as composition when one object owns another while an association is known as aggregation when one object uses another object.

In this OOPS tutorial, we will see a couple of more examples to understand the difference between Association, Composition, and Aggregation better.

Btw, if you are new to the world of object-oriented programming and design then I also suggest you go through a hands-on course like UML and Object-Oriented Design Foundations Course by Karlovy Neisztor on Udemy. It's a great course to understand the complete process of object-oriented analysis and design for creating quality software.

An Example of Association, Composition, and Aggregation in Java

Here is an example of composition and aggregation, in terms of Java Code. By looking at this code, you can gauge the differences between these two. By the way, Composition is also very much preferred in object-oriented design over inheritance, even Joshua Bloch has stated its importance in the classic book, Effective Java.

1. Composition: 

Since Engine is-part-of Car, the relationship between them is Composition. Here is how they are implemented between Java classes.

public class Car {

    //final will make sure engine is initialized
    private final Engine engine;  
       
    public Car(){
       engine  = new Engine();
    }
}

class Engine {
    private String type;
}

2. Aggregation: 

Since Organization has Person as employees, the relationship between them is Aggregation. Here is how they look like in terms of Java classes

public class Organization {
    private List employees;
}

public class Person {
    private String name;   
}

UML Diagram of Association, Composition, and Aggregation

UML has different notations to denote aggregation, composition, and association.  Association is denoted by the simple arrow while aggregation is denoted by an empty diamond-head arrow and composition is denoted by a filled diamond-head arrow.

When you draw a UML diagram for two related classes A and B, where A is associated with B then it's denoted by A -> B. A similar way is used to show aggregation and composition between two classes.

Here are UML notations for a different kind of dependency between the two classes.

As I said all three denotes relationship between objects and only differ in their strength, you can also view them as below, where composition represents the strongest form of relationship and association being the most general form.

Association vs Composition vs Aggregation

Here is the list of differences between Composition and Aggregation in point format, for quick review. As I said the key difference between them comes from the point that in the case of Composition, One object is OWNER of another object, while in the case of aggregation, one object is just a USER or another object.

1) If A and B two classes are related to each other such that, B ceased to exist when A is destroyed, then the association between two objects is known as Composition. An example is Car and Engine. While if A and B are associated with each other, such that B can exist without being associated with A, then this association is known as Aggregation.

See Head First Object-Oriented Analysis and Design for more examples of Composition and Association in OOP.

2) In the case of Composition A owns B e.g. Person is the owner of his Hand, Mind and Heart, while in the case of Aggregation, A uses B e.g. Organization uses People as an employee.

3) In UML diagram Association is denoted by a normal arrow head, while Composition is represented by filled diamond arrow head, and Aggregation is represented by an empty diamond arrow head, As shown in below and attached diagram in the third paragraph.

Association  A---->B
Composition  A-----<filled>B
Aggregation  A-----<>B

4) Aggregation is a lighter form of Composition, where a sub-part object can meaningfully exist without main objects.

5) In Java, you can use the final keyword to represent Composition. Since in Composition, Owner object expects a part object to be available and functions, by making it final, your provide a guarantee that, when Owner will be created, this part object will exist. This is actually a Java idiom to represent a strong form of association i.e. composition between two objects.

6) Another interesting word, which comes in handy to understand the difference between Composition and Aggregation in software design is "part-of" and "has". If one object is-part-of another object e.g. Engine is part of Car, then the association or relationship between them is Composition. On the other hand, if one object just has another object e.g. Car has the driver then it's Aggregation.

That's all on the difference between Association, Composition, and Aggregation in UML, Java, and Object-oriented design. Since object-oriented analysis is more about defining the relationship between objects, it's important to know what kind of relationship exists between them, composition and aggregation is a two way of representing the relationship between two objects.


Few more articles to improve your object-oriented programming and design skills

• Top 10 Object-Oriented Design Principles 
• Why Prefer Composition over Inheritance in Java
• Difference between overloading and overriding in Java
• Top 10 Interview Questions on Abstract class and Interface
• 20 questions on Java design pattern and Software Design
• 5 Books and Courses to learn Object-Oriented Programming
• 25 Software Design Interview Questions for Interviews
• 5 Best Books to learn Software Architecture
• My Favorite Courses to learn Software Architecture

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