Composition vs. Aggregation vs. Association?

Composition

•  Composition is a strong type of Aggregation (strong association).
•  Part- whole relationship where part cannot exist without a whole
•  The life cycle of child is fully dependent on parent object, if parent object deletes all child object will also be deleted

Example:

Relationship between House and Rooms. House can contain multiple rooms, there is no independent life of room and if we delete the house, room will be automatically deleted.


final class Car {
       private final Engine engine;

       Car(EngineSpecs specs) {
              // lives within car object- Engine cannot exist without car
              engine = new Engine(specs);
       }

       void move() {
              engine.work();
       }
}
 

Here the Engine is completely encapsulated by the Car. There is no way for the outside world to get a reference to the Engine. The Engine lives and dies with the car. 


Aggregation

•  Aggregation is a weak association

•  Part- whole relationship where part can exist without a whole


Example 1:  
 
final class Car { 
       private Engine engine;

       void setEngine(Engine engine) {
              this.engine = engine;
       }

       void move() {
           if (engine != null)
              engine.work();
       }
}


 
Example 2: 
    


   public class Person {

       private Address address;

       public Person(Address address) {
              this.address = address;
       }
   }


Here the outside world can still have a reference to the Engine or Address and it can exist without Car or Person respectively.

Association


• Association is the relation between two objects. In other words its connectivity between two objects. Aggregation and compositions are form of association.
 

Newer Post Older Post

Leave a Reply

naeemgik. Powered by Blogger.