What is Aggregation in Java?

Aggregation in Java is a specialized form of association that represents a "has-a" relationship between two classes, indicating that one class contains instances of another class. It's a key concept in object-oriented programming and describes how objects can be related without implying strong ownership.

Understanding Aggregation

• "Has-a" Relationship: This signifies that one class has an instance of another class as a member variable. For example, a Department has multiple Professor objects.

• Weak Relationship (Looser Coupling): Unlike composition, aggregation implies a weaker relationship. The contained object can exist independently of the container object. If the container object is destroyed, the contained object may continue to exist.

• Unidirectional: Aggregation is usually unidirectional, meaning that one class knows about the other, but not vice-versa. The Department knows about the Professor objects within it, but typically the Professor doesn't have a direct reference back to the Department.

Aggregation vs. Composition

Example in Java

// Professor class
class Professor {
    private String name;

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

    public String getName() {
        return name;
    }
}

// Department class
class Department {
    private String name;
    private Professor head;

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

    public String getName() {
        return name;
    }

     public Professor getHead() {
        return head;
    }
}

public class Main {
    public static void main(String[] args) {
        Professor professor1 = new Professor("Dr. Smith");
        Department scienceDepartment = new Department("Science", professor1);

        System.out.println(scienceDepartment.getName() + " Department Head: " + scienceDepartment.getHead().getName());
    }
}

In this example, a Department has a Professor object representing the department head. If the Department object is destroyed, the Professor object still exists. This showcases the independent lifecycle aspect of aggregation.

Key Benefits of Aggregation

• Code Reusability: Promotes reusability by allowing classes to utilize other classes as parts of their structure.
• Flexibility: Offers greater flexibility in designing relationships between classes.
• Reduced Coupling: Reduces dependencies between classes, making the system more maintainable and easier to change.

In conclusion, aggregation is a valuable tool in object-oriented design that allows you to model "has-a" relationships between classes in a flexible and maintainable way. It enhances code reusability and reduces coupling by enabling objects to exist independently of each other.