Association, Composition and Aggregation

About basic object relationship

Association

1. one to one
2. one to many
3. many to one
4. many to many

Classification

• Aggregation
• Composition

Aggregation ( weak — both objects can live independent. Has a relationship)

1. Student + Department
2. Bank + Employee

Composition (Part of — object will have part of other object and dependent. Part of relationship)

1. Book + Library
2. Car + Engine

Aggregation vs Composition

1. Dependency: Aggregation implies a relationship where the child can exist independently of the parent. For example, Bank and Employee, delete the Bank and the Employee still exist. whereas Composition implies a relationship where the child cannot exist independent of the parent. Example: Human and heart, heart don’t exist separate to a Human
2. Type of Relationship: Aggregation relation is “has-a” and composition is “part-of” relation.
3. Type of association: Composition is a strong Association whereas Aggregation is a weak Association.