What is Association, Aggregation,Composition, Abstraction, Generalization

What is Association?

Association is a relationship between two objects. In other words, association defines the multiplicity between objects. You may be aware of one-to-one, one-to-many, many-to-one, many-to-many all these words define an association between objects. Aggregation is a special form of association. Composition is a special form of aggregation.

Example: A Student and a Faculty are having an association.

What is Aggregation?

Aggregation is a special case of association. A directional association between objects. When an object ‘has-a’ another object, then you have got an aggregation between them. Direction between them specified which object contains the other object. 

Aggregation is also called a “Has-a” relationship.

Aggregation is a relationship between two classes that is best described as a “has-a” and “whole/part” relationship. It is a more specialized version of the association relationship. 

The aggregate class contains a reference to another class and is said to have ownership of that class. Each class referenced is considered to be part-of the aggregate class.Ownership occurs because there can be no cyclic references in an aggregation relationship. 

If Class A contains a reference to Class B and Class B contains a reference to Class A then no clear ownership can be determined and the relationship is simply one of association.

For example, imagine a Student class that stores information about individual students at a school. Now let’s say there is a Subject class that holds the details about a particular subject (e.g., history, geography). If the Student class is defined to contain a Subject object then it can be said that the Student object has-a Subject object. The Subject object also makes up part-of the Student object, after all there is no student without a subject to study. The Student object is therefore the owner of the Subject object.


There is an aggregation relationship between Student class and the Subject class:

public class Subject {
private String name;
public void setName(String name)
this.name = name;
public String getName()
return name;
public class Student {
private Subject[] studyAreas = new Subject[10];
//the rest of the Student class

What is Composition?

Composition is a special case of aggregation. In a more specific manner, a restricted aggregation is called composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition.

Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.

What is Abstraction?

Abstraction is specifying the framework and hiding the implementation level information. Concreteness will be built on top of the abstraction. It gives you a blueprint to follow to while implementing the details. Abstraction reduces the complexity by hiding low level details.

Example: A wire frame model of a car.

What is Generalization?

Generalization uses a “is-a” relationship from a specialization to the generalization class. Common structure and behaviour are used from the specialization to the generalized class. At a very broader level you can understand this as inheritance. Why I take the term inheritance is, you can relate this term very well. Generalization is also called a “Is-a” relationship.

Example: Consider there exists a class named Person. A student is a person. A faculty is a person. Therefore here the relationship between student and person, similarly faculty and person is generalization.


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