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Design Patterns • Composition vs Aggregation



When it comes to software engineering and design, understanding the concepts of composition and aggregation is crucial for creating robust and efficient domain models. These terms, often used in object-oriented programming, represent relationship patterns between objects that dictate how they interact and depend on each other. By dissecting these concepts and illustrating them with real-life examples, we can gain a deeper comprehension of their applications and significance in domain modeling.

Composition and aggregation are both associative relationships but differ in terms of ownership and life cycle of the involved objects. While both play pivotal roles in domain modeling, understanding their distinct characteristics is essential for their appropriate application in various scenarios. This article aims to demystify these concepts, providing clarity with practical examples.

Composition Domain Modeling

In domain modeling, composition and aggregation represent different types of relationships between objects. Composition implies a strong ownership, where the life cycle of the contained object is strictly bound to the container. When the container object is destroyed, so are its contents. This reflects a “part-of” relationship where the parts cannot exist independently of the whole.

Composition of an Engine and Wheel

In the figure below is an example class diagram to illustrate the concept of Composition in a UML class diagram. In this example, we’ll use a simple scenario of a Car and its integral parts, like Engine and Wheel. In Composition, these parts are entirely dependent on the Car for their existence.

Figure 1. Composition Class Diagram of an Engine and Wheel Relationship

The UML class diagram showing the composition relationship between a Car and its components, Engine and Wheels, underlining the concept that the existence of the components is dependent on the existence of the car.

Class Diagram: Composition of Engine and Wheel

Also available in: SVG | PlantText

In this diagram:

Composition of a Shopping Cart System

For a real-world service-oriented example of Composition, let’s consider an Online Shopping System. In this system, a ShoppingCart service is composed of Item services. The lifecycle of Item services is tightly coupled with the ShoppingCart - when the cart is cleared or deleted, the items within it cease to exist in that context.

Figure 2. Shopping Cart System Relationship Class Diagram

This UML diagram effectively illustrates the composition relationship in a service-oriented architecture, where the ShoppingCart service is composed of and manages the lifecycle of Item services. This setup highlights the dependency of Item services on their encompassing ShoppingCart service.

Shopping Cart System Relationship Class Diagram

Also available in: SVG | PlantText

In this diagram:

The example Java implementation below captures the essence of a composition relationship where the items are part of the shopping cart and are managed by it.

import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.ArrayList;
import java.util.List;

public class ShoppingCart {
    private List<Item> items = new ArrayList<>();

    public void addItem(Item item) {

    public void removeItem(Item item) {

    public void clearCart() {

public class Item {
    private String name;
    private double price;
    private int quantity;

    public Item(String name, double price, int quantity) {
        this.name = name;
        this.price = price;
        this.quantity = quantity;

In this code:

Aggregation in Domain Modeling

Aggregation, on the other hand, suggests a weaker relationship. It indicates a “has-a” relationship, where the container object holds references to other objects but does not strictly manage their life cycles. In aggregation, the contained objects can exist independently of the container.

Aggregation of Universities and Students

This time, we’ll consider a scenario involving a University and Student to depict the aggregation relationship. In Aggregation, the Student can exist independently of the University, unlike in Composition.

Figure 3. Aggregation Class Diagram for a University Student Relationship

This UML diagram illustrates the aggregation relationship between a University and Student, demonstrating that while the university contains students, these students have an existence independent of the university.

Aggregation Class Diagram for a University Student Relationship

Also available in: SVG | PlantText

In this diagram:

Aggregation of a Library Management System

For a real-world service-oriented example of Aggregation, let’s consider a Library Management System. In this system, a Library service aggregates Book services. While the Library manages and coordinates access to Book services, each Book service can exist independently, serving its information or getting loaned out to users. The Library does not own the Book services but merely coordinates them.

Figure 4. Aggregation Class Diagram for a Library Management System

This UML diagram efficiently illustrates the aggregation relationship in a service-oriented architecture, where the Library service aggregates multiple Book services without owning them.

Aggregation Class Diagram for a Library Management System

Also available in: SVG | PlantText

In this diagram:

Following the aggregation pattern for the Library Management System with Library and Book classes, here’s how you can implement it in Java with Lombok for reducing boilerplate code.

In this code, the aggregation relationship is characterized by the Library managing a collection of Book objects without having direct control over their lifecycle. The Book objects maintain their independent existence, which is a fundamental characteristic of aggregation in object-oriented design.

import lombok.Data;
import java.util.ArrayList;
import java.util.List;

public class Library {
    private List<Book> books;

    public Library() {
        this.books = new ArrayList<>();

    public void addBook(Book book) {

    public void removeBook(Book book) {

public class Book {
    private String title;
    private String author;

    public Book(String title, String author) {
        this.title = title;
        this.author = author;

In Conclusion

In this exploration of composition and aggregation in object-oriented programming, we’ve delved into the nuances of these two fundamental design patterns, crucial for structuring robust and efficient systems. Through real-world analogies and service-oriented examples, we’ve seen how these relationships dictate the interactions and dependencies between objects, influencing the design and functionality of software systems.

Composition, characterized by a strong, life-dependent relationship between objects, emphasizes the integral nature of components within a whole. Our example of the Online Shopping System, illustrated both in concept and Java code, demonstrates how items within a shopping cart are intrinsically linked to the cart itself, ceasing to exist independently once the cart is cleared or deleted.

On the other hand, aggregation, with its emphasis on a looser association where components can exist independently of the whole, was exemplified in the Library Management System. Here, books in a library exist as separate entities and retain their identity and existence beyond the scope of the library, a concept we observed through both conceptual explanation and Java implementation.

Both composition and aggregation offer unique advantages and are chosen based on the specific requirements and constraints of the system being designed. Understanding these patterns not only aids developers in making informed design decisions but also enhances the maintainability, scalability, and overall quality of the software.

As we continue to advance in the world of software development, the thoughtful application of such design patterns remains a cornerstone in the creation of efficient, resilient, and adaptable systems. Whether you’re a seasoned developer or a newcomer to the field, grasping the essence of composition and aggregation is a step towards mastering the art of software design and architecture.

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