Association, Aggregation and Composition Relationships

Association, Aggregation and Composition Relationships

1. Association

       It is a simple structural connection or channel between classes and is a relationship where all objects have their own lifecycle and there is no owner. Lets take an example of Department and Student. Multiple students can associate with a single Department and single student can associate with multiple Departments, but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently. Here is respective Model and Code for the above example.

2. Aggregation

       It is a specialize form of Association where all object have their own lifecycle but there is a ownership like parent and child. Child object can not belong to another parent object at the same time. We can think of it as "has-a" relationship. Implementation details: 1. Typically we use pointer variables that point to an object that lives outside the scope of the aggregate class 2. Can use reference values that point to an object that lives outside the scope of the aggregate class 3. Not responsible for creating/destroying subclasses Lets take an example of Employee and Company. A single Employee can not belong to multiple Companies (legally!! ), but if we delete the Company, Employee object will not destroy. Here is respective Model and Code for the above example.

3. Composition

       It is again specialize form of Aggregation. It is a strong type of Aggregation. Here the Parent and Child objects have coincident lifetimes. Child object dose not have it's own lifecycle and if parent object gets deleted, then all of it's child objects will also be deleted

       Implementation details:

1.       Typically we use normal member variables

2.       Can use pointer values if the composition class automatically handles allocation/deallocation

3.       Responsible for creation/destruction of subclasses

       Lets take an example of a relationship between House and it's Rooms. House can contain multiple rooms there is no independent life for room and any room can not belong to two different house. If we delete the house room will also be automatically deleted. Here is respective Model and Code for the above example.