Association, aggregation, and composition in Object Oriented Programming (OOP)
Nikhil Joshi
Cloud & Software Architect | Azure Solutions | .NET Core | Angular | Microservices
In object-oriented programming (OOP), aggregation, association, and composition are all ways to define relationships between classes, but they differ in their strength and implications regarding ownership and lifecycle. Here’s a detailed overview:
1. Association
- Definition: A general relationship between two classes where one class uses or interacts with another. This relationship is often described as a "has-a" relationship.
- Characteristics:
- Can be one-to-one, one-to-many, or many-to-many.
- Does not imply ownership; both classes can exist independently.
- Example: A Teacher and Student relationship where a teacher can teach multiple students, and students can have multiple teachers.
2. Composition
- Definition: A strong form of aggregation that indicates a "whole-part" relationship with a strict ownership model. The lifecycle of the part is tied to the lifecycle of the whole.
- Characteristics:
- If the whole is destroyed, the parts are also destroyed.
- The parts typically cannot exist independently of the whole.
- Example: A House and Room relationship. If the house is destroyed, the rooms within it no longer exist.
3. Aggregation
- Definition: A specialized form of association that represents a "whole-part" relationship but with a weaker bond than composition. The part can exist independently of the whole.
- Characteristics:
- Implies a relationship where the contained objects can exist independently of the container.
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- Often visualized as a "has-a" relationship, but the parts are not exclusively owned by the whole.
- Example: A School and Student relationship. A school can have many students, but students can exist without being part of that specific school (e.g., transferring to another school).
Key Differences
- Ownership:
- Association: No ownership; classes are independent.
- Aggregation: Indicates a whole-part relationship, but parts can exist independently.
- Composition: Strong ownership; parts cannot exist without the whole.
- Lifecycle:
- Association: Independent lifecycles.
- Aggregation: Independent lifecycles for parts.
- Composition: Lifecycles tied together; if the whole is destroyed, so are the parts.
Summary
- Association: General relationship; no ownership or lifecycle dependency.
- Aggregation: Whole-part relationship; parts can exist independently.
- Composition: Strong whole-part relationship; parts depend on the whole for their existence.
Understanding these distinctions helps in designing systems that accurately represent real-world relationships and manage the interactions between objects effectively.