Association, Aggregation and Composition
Association
- Association is a relation between two classes. To achieve association we used objects of classes.
- Association can be of between one to one, one to many, many to one and many to many objects of classes.
One to One:
If single class object has some relation with another single object of a class then it is called one to one association.
One to Many:
If single class object has some relation with more than one object of a class then it is called one to many association.
Many to One:
If multiple objects of class has relation with one object of class then it is called many to one relationship.
Many to Many:
If multiple objects of a class has relation with another multiple objects of class then it is called many to many relationship.
Aggregation and composition are forms of association.
Aggregation
Aggregation is a form of association where one class has an object(s) of another class in such a way that destruction of any one of class object wont destroy another, they are loosely associated.
Aggregation is called has-a relation.
Example:
Army has AirForce and AirForce has Pilot, in this example destruction of one does not necessary means another can’t exist.
import java.util.ArrayList;
import java.util.List;
class Army {
final AirForce airForce;
Army(AirForce airForce) {
this.airForce = airForce;
}
}
class AirForce {
final List pilots;
AirForce(List pilots) {
this.pilots = pilots;
}
}
class Pilot {
String name;
int age;
}
public class AssociationExample {
public static void main(String[] args) {
Pilot pilot = new Pilot();
List pilots = new ArrayList();
pilots.add(pilot);
AirForce airForce = new AirForce(pilots);
Army army = new Army(airForce);
}
} Composition
class Heart {
}
class HumanBody {
Heart heart = new Heart();
} UML of Association, Aggregation and Composition
UML diagram of Association:
 UML diagram of Aggregation:
UML diagram of Composition:
Generalization and Specialization
Generalization:
Generalization is a process where we define a parent or super class for smaller child classes and by doing that we are generalizing different smaller classes to one super or parent class.
Generalization is a bottom up approach.
Generalization can be achieved by creating sub classes and their one parent class.
Example:
Car class is generalized from ElectricCar and PetrolCar sub classes.
Specialization:
Specialization is a process where we define child or sub classes from one super class which are more specific and specialized in details.
Specialization is a top down approach.
Example:
Sub classes ElectricCar and PetrolCar from parent Car class is an example of specialization.
Diagram of Generalization and Specialization:
References
Reference article from oracle:Â UML