Types of Relationship between classes - Inheritance, Association, Aggregation,Composition

There are various relationship types that exist in java called Association, Aggregation, and composition.

Association represents the unidirectional or bidirectional relationship between two classes. If the Customer places an order, then this is a unidirectional association.

In Aggregation, the based object is standalone and can exist even if the object of the control class is dead. E.g. the association between Car and Wheel is the aggregation.

Aggregation vs composition Summary

Aggregation and Composition are a special type of association and differ only in the weight of the relationship.
 
Composition is a powerful form of “is part of” relationship collated to aggregation “Has-A”.
 
In Composition, the member object cannot exist outside the enclosing class while same is not true for Aggregation.

Inheritance:

Inheritance is basically Is-A relationship. We can achieve inheritance with the help of extends keyword

Pros: Code re-use, easier to understand
 
Cons: Tightly coupled, fragile, prone to be abused by developers

Composition:

Pros: Code re-use, great Flexibility, Loose coupling
 
Cons: A little harder to understand, complex design structure

Inheritance (IS-A relationship):

Class Vehicle{

}

Class Car extends Vehicle{

{

-->> Car IS-A Vehicle (Two classes are tightly coupled)

Association (HAS-A relationship):

Example 1:

Class Student{

String name;

int rollNo;

}

-->> Student HAS-A name

-->> Student HAS-A rollNo

Example 2:

Class Engine {}

class Car{

Engine e = new Engine();

}

-->> Car HAS-A Engine (Two classes are loosely coupled)

Car HAS-A musicPlayer -->>Weak bonding between classes -->>Aggregation

Car HAS-A engine -->> Strong bonding beween classes -->>composition