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Piecing Together Relationships in Java: Aggregation vs Composition


Understanding object-oriented principles is key to crafting robust Java applications. Two fundamental relationships that often cause confusion are aggregation and composition. This post will clarify these concepts with real-world examples.

The Basics of Object Relationships

Start with a brief introduction to object-oriented design and the importance of relationships like association, which is the base concept for both aggregation and composition.

Aggregation Explained

Define aggregation as a “has-a” relationship with a softer dependency, where the child can exist independently of the parent.

Real-World Example:

Discuss a scenario with a Car class and a Wheel class. A car “has-a” wheel, but wheels can exist independently of the car.

Java Code Snippet:

Provide a simple Java class example illustrating aggregation.

class Wheel {
    // Wheel properties and methods

class Car {
    private Wheel wheel;

    Car(Wheel wheel) {
        this.wheel = wheel;
    // Car properties and methods

Composition Explained

Describe composition as a stronger “has-a” relationship where the child’s existence is dependent on the parent’s lifecycle.

Real-World Example:

Consider a House and a Room class. A house “has-a” room, and a room doesn’t typically exist without a house.

Java Code Snippet:

Show a Java class example that demonstrates composition.

class Room {
    // Room properties and methods

class House {
    private List<Room> rooms = new ArrayList<>();

    void addRoom(Room room) {
    // House properties and methods

Key Differences

Highlight the main differences:

  • Lifespan: In aggregation, the child can outlive the parent, while in composition, the child cannot.
  • Ownership: Aggregation implies shared ownership, whereas composition implies sole ownership.

When to Use Which

Offer guidance on when to use aggregation vs. composition:

  • Use aggregation when you want to share an object among multiple owners.
  • Use composition when you want to strictly control the lifespan of the child objects.

Best Practices

Provide tips for deciding between aggregation and composition based on the nature of the relationship you want to establish between objects.


Emphasize that understanding these relationships is crucial for designing an efficient, organized system in Java. They help to build hierarchies that reflect real-world situations, contributing to code that is both reusable and easy to maintain.

Call to Action

Encourage readers to look at their own codebase to identify where they might refactor to use aggregation or composition for cleaner and more maintainable code.

Remember to make your explanations and examples as clear and concise as possible to ensure they are understandable to both beginner and experienced Java developers.



About Author

As Neelabh Singh, I am a Senior Software Engineer with 6.6 years of experience, specializing in Java technologies, Microservices, AWS, Algorithms, and Data Structures. I am also a technology blogger and an active participant in several online coding communities.

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