Association, Composition and Aggregation in Java

In object-oriented programming, relationships between classes play a crucial role in defining how objects interact with each other. Java, being an object-oriented language, provides mechanisms to model these relationships through association, aggregation, and composition.

• These relationships help model interactions between objects in real-world applications.
• They define the level of dependency and ownership between classes.
• Understanding them improves software design, maintainability, and code reusability.

👁 Association, Aggregation, and Composition

Association

Association is a relationship between two independent classes where one object uses or interacts with another object. Both objects can exist independently of each other.

• Represents a Uses-A relationship.
• Objects are loosely coupled.
• Both classes can exist independently.

Types of Association

• Unidirectional Association: One class knows about another, but not vice versa. Example: A Student has a LibraryCard, but the LibraryCard doesn't know about the Student.
• Bidirectional Association: Both classes know about and interact with each other. Example: A Teacher is assigned to a Classroom, and the Classroom knows its Teacher.

Example:

Ridhi belongs to bank ICICI
Vijay belongs to bank ICICI

Explanation: In the above example, two separate classes Bank and Employee are associated through their Objects. Bank can have many employees, So, it is a one-to-many relationship. 

👁 Association in Java

Aggregation

Aggregation is a special form of Association that represents a Has-A relationship with weak ownership. The contained object can exist independently of the container object.

• Represents a Has-A relationship.
• It is a weak association.
• Child objects can exist independently of the parent object.

Example:

Total students in institute: 4

Explanation:

• There is an Institute which has no. of departments like CSE, EE. Every department has no. of students.
• So, we make an Institute class that has a reference to Object or no. of Objects (i.e. List of Objects) of the Department class.
• That means Institute class is associated with Department class through its Object(s).
• And Department class has also a reference to Object or Objects (i.e. List of Objects) of the Student class means it is associated with the Student class through its Object(s). 

It represents a Has-A relationship. In the above example: Student Has-A name. Student Has-A ID. Department Has-A Students as depicted from the below media. 

👁 Aggregation Example

Note: Code reuse is best achieved by aggregation.  

Composition

Composition is a strong form of Aggregation that represents a Part-Of relationship. In Composition, the lifecycle of the contained object depends completely on the container object.

• Represents a Part-Of relationship.
• Strong ownership exists between objects.
• Child objects cannot exist without the parent object.

Example:

Total Rooms: 4
Room names: 
- Living Room
- Bedroom
- Kitchen
- Bathroom

Explanation: In the above example, if the house is destroyed, its room also get destroyed this represents a complete composition relationship, where the room is the part of the house and the life cycle of room completely depends on the house.

Difference Between Association, Aggregation and Composition