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
Studentdepends on aCourseto enroll, but both objects can exist independently. - In terms of dependency:
Studentmight depend onCourseto perform a function (enroll), but theCourseobject does not depend on theStudentobject.
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
DepartmentaggregatesEmployeeobjects. TheDepartmentcan exist withoutEmployeeobjects (if no employees are assigned), and theEmployeeobjects can exist without theDepartment. - In terms of dependency:
DepartmentandEmployeeare 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
Carobject has anEngine. If theCaris destroyed, itsEngineis destroyed as well. - In terms of dependency: The
CarandEnginehave a strong dependency, where theEnginedepends on theCarfor existence. If theCaris removed, theEnginecannot 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.
