In object-oriented programming, composition and aggregation are two important concepts that describe the relationships between objects. These relationships determine how objects are connected and interact with each other.

Composition

Composition is a strong form of association where a class (known as the composite) contains one or more objects of another class (known as the component). The composite class fully owns the component objects, which means that the lifecycle of the component is tightly coupled with the composite class. In other words, when the composite object is destroyed, all its components are also destroyed.

Here’s an example in Java:

public class Car {
    private Engine engine;
    private Wheels wheels;

    // Other properties and methods

    // Composition: Car "has a" Engine and Wheels
    public Car() {
        engine = new Engine();
        wheels = new Wheels();
    }
}

In the above example, the Car class is composed of an Engine and Wheels objects. The engine and wheels objects are created inside the Car constructor, indicating that the Car owns and manages these objects.

Aggregation

Aggregation is a weaker form of association where a class (known as the aggregate) contains references to one or more objects of another class. Unlike composition, the lifecycle of the aggregated objects is independent of the aggregate class. The aggregate class does not fully own the aggregated objects, and they can exist independently.

Here’s an example in Java:

public class University {
    private List<Student> students;

    // Other properties and methods

    // Aggregation: University "has a" list of students
    public University() {
        students = new ArrayList<>();
    }

    public void addStudent(Student student) {
        students.add(student);
    }
}

In the above example, the University class aggregates a list of Student objects. The students list is not created inside the University constructor, indicating that the University does not fully own or create these objects. Students can exist independently of the University.

Conclusion

Composition and aggregation are important concepts in object-oriented programming. Composition represents a strong ownership relationship, where one class fully owns another class, while aggregation represents a weaker association where one class contains references to objects of another class.

Understanding these concepts is essential for designing complex object relationships and building modular and maintainable code.

#Java #ObjectOrientedProgramming