Aggregation vs Composition : Thinking at object level or class level

Have you ever wondered how to design your classes and objects to effectively represent complex relationships? Do you find yourself confusing between aggregation and composition? Well, wonder no more! In this article, we’ll delve into the world of object-oriented programming and uncover the secrets of aggregation and composition.

What’s the big deal about Aggregation and Composition?

In object-oriented programming, we create classes and objects to model real-world scenarios. These classes and objects interact with each other to achieve a specific goal. However, as the complexity of the system grows, so does the number of relationships between objects. This is where aggregation and composition come into play.

Aggregation and composition are two fundamental concepts in object-oriented programming that help us design and implement complex relationships between objects. They enable us to create objects that are composed of other objects or collections of objects, allowing us to model complex systems in a more efficient and effective manner.

Aggregation : The “has-a” Relationship

Aggregation is a relationship where one object (the container) contains a collection of other objects or references to other objects. The contained objects are not complete without the container, but they can exist independently.

• A university has departments.
• A department has students.
• A student has courses.

In the above example, the university is an aggregate of departments, a department is an aggregate of students, and a student is an aggregate of courses. Each of these objects can exist independently, but they are related to each other through aggregation.

Composition : The “part-of” Relationship

Composition is a relationship where one object (the container) contains other objects or references to other objects, and the contained objects cannot exist independently. The contained objects are an integral part of the container.

• A car has wheels.
• A wheel has a tire.
• A tire has a rim.

In the above example, a car is composed of wheels, a wheel is composed of a tire, and a tire is composed of a rim. Each of these objects cannot exist independently, and their existence is tied to the container object.

Key Differences between Aggregation and Composition

Characteristic	Aggregation	Composition
Relationship Type	Has-a	Part-of
Object Independence	Objects can exist independently	Objects cannot exist independently
Object Lifetime	Objects have separate lifetimes	Objects have the same lifetime as the container
Container-Contained Relationship	Container contains a reference to the contained object	Container contains the actual contained object

Designing with Aggregation and Composition

Now that we’ve covered the basics of aggregation and composition, let’s dive into some design principles to help you create effective objects and classes.

Identify the Relationship Type

When designing your classes and objects, identify the type of relationship between them. Ask yourself:

• Is it a “has-a” relationship? (Aggregation)
• Is it a “part-of” relationship? (Composition)

Keep it Simple and Focused

Keep your classes and objects simple and focused on a specific task. Avoid creating complex objects that perform multiple, unrelated tasks.

Use Interfaces and Abstract Classes

Use interfaces and abstract classes to define the contract and behavior of your objects. This allows for flexibility and polymorphism in your design.

Polymorphism and Inheritance

Use polymorphism and inheritance to create complex objects that can adapt to different scenarios. This enables you to write more flexible and reusable code.

Encapsulation and Abstraction

Use encapsulation and abstraction to hide the implementation details of your objects and expose only the necessary information to the outside world.

Best Practices for Aggregation and Composition

Aggregation Best Practices

When using aggregation, follow these best practices:

• Use a container class to manage the collection of objects.
• Use a weak reference to the contained objects to avoid strong coupling.
• Implement methods to add, remove, and iterate over the contained objects.

Composition Best Practices

When using composition, follow these best practices:

• Use a single class to manage the composition of objects.
• Use a strong reference to the contained objects to ensure they exist as long as the container exists.
• Implement methods to create, delete, and access the contained objects.

Common Pitfalls to Avoid

Over-Aggregation

Avoid over-aggregation by creating complex objects that contain too many other objects. This can lead to tight coupling and decreased flexibility.

Over-Composition

Avoid over-composition by creating objects that are too closely tied to their container. This can lead to inflexibility and decreased reusability.

Tight Coupling

Avoid tight coupling by using interfaces and abstract classes to define the contract and behavior of your objects.

God Objects

Avoid creating “god objects” that contain too much logic and functionality. Break down your objects into smaller, more focused classes.

Conclusion

In conclusion, aggregation and composition are powerful tools in object-oriented programming that enable us to model complex relationships between objects. By understanding the differences between aggregation and composition, and following best practices for each, you can create more effective and efficient designs. Remember to keep it simple, focused, and flexible, and avoid common pitfalls like over-aggregation, over-composition, tight coupling, and god objects.

Code Example

// Aggregation Example
public class University {
  private List<Department> departments;

  public University() {
    departments = new ArrayList<>();
  }

  public void addDepartment(Department department) {
    departments.add(department);
  }

  public void removeDepartment(Department department) {
    departments.remove(department);
  }
}

public class Department {
  private String name;

  public Department(String name) {
    this.name = name;
  }
}

// Composition Example
public class Car {
  private Wheel[] wheels;

  public Car() {
    wheels = new Wheel[4];
    for (int i = 0; i < 4; i++) {
      wheels[i] = new Wheel();
    }
  }

  public void replaceWheel(Wheel wheel, int index) {
    wheels[index] = wheel;
  }
}

public class Wheel {
  private Tire tire;

  public Wheel() {
    tire = new Tire();
  }
}

By following the principles and best practices outlined in this article, you’ll be well on your way to creating effective and efficient designs that utilize aggregation and composition to model complex relationships between objects.

Frequently Asked Question

Get ready to level up your object-oriented programming game with these frequently asked questions about aggregation vs composition!