Association, Aggregation, Composition and Dependency relationship

Understanding Dependency in Object-Oriented Relationships

A dependency relationship in object-oriented programming refers to a situation where one object relies on another object to function. This can happen in several ways, and the strength of the dependency varies depending on the type of relationship.

1. Association as a Type of Dependency

In association, objects can interact or use each other, but they don’t own each other. The dependency is weak because neither object depends on the other for its life cycle or existence.

Example of Dependency: A Student depends on a Course to enroll, but both objects can exist independently.
In terms of dependency: Student might depend on Course to perform a function (enroll), but the Course object does not depend on the Student object.

2. Aggregation as a Type of Dependency

In aggregation, one object (the whole) can contain other objects (the parts), but these parts can exist independently of the whole. The dependency is weaker than in composition but stronger than in simple association.

Example of Dependency: A Department aggregates Employee objects. The Department can exist without Employee objects (if no employees are assigned), and the Employee objects can exist without the Department.
In terms of dependency: Department and Employee are linked, but neither depends on the existence of the other for its life cycle. Both can be created or destroyed independently.

3. Composition as a Type of Dependency

In composition, one object owns another object, and the owned object cannot exist independently of the owner. This creates a stronger dependency relationship than association or aggregation.

Example of Dependency: A Car object has an Engine. If the Car is destroyed, its Engine is destroyed as well.
In terms of dependency: The Car and Engine have a strong dependency, where the Engine depends on the Car for existence. If the Car is removed, the Engine cannot exist.

Summarizing the Relationships and Their Dependency Strength

Relationship	Dependency	Ownership	Existence of Dependent Object
Association	Weak (objects interact but do not depend on each other)	None	Can exist independently
Aggregation	Medium (one object contains another, but they are still independent)	Containment but not ownership	Can exist independently
Composition	Strong (one object owns another and the owned object cannot exist without it)	Strong ownership (part-of)	Cannot exist independently

Clarifying the Terminology

Dependency: A relationship where one object relies on another. It’s the broadest concept, applicable to all three types of relationships.
Association: A weak dependency where objects can interact but do not have strong ties (objects can exist independently).
Aggregation: A medium-strength dependency where one object “owns” others, but they can exist independently.
Composition: A strong dependency where one object owns another, and the owned object cannot exist without the owner.

Final Thoughts

So, to answer your question:

Yes, association, aggregation, and composition can be considered types of dependency relationships, but with varying strengths of dependency.
Association represents the weakest form, aggregation is stronger (with some level of ownership), and composition represents the strongest form, where the dependency is tied to the life cycle of the objects involved.

By MOUSTAFA ALSAYEH