{"id":78940,"date":"2020-06-03T10:00:56","date_gmt":"2020-06-03T04:30:56","guid":{"rendered":"https:\/\/techvidvan.com\/tutorials\/?p=78940"},"modified":"2020-06-03T10:00:56","modified_gmt":"2020-06-03T04:30:56","slug":"java-association","status":"publish","type":"post","link":"https:\/\/techvidvan.com\/tutorials\/java-association\/","title":{"rendered":"Java Association – Aggregation and Composition in Java"},"content":{"rendered":"

What if there are two classes that have some kind of relationship between them?<\/p>\n

For example, the classes are Library and Books or Teacher and Student, don\u2019t you think these two classes may be associated or connected with each other.<\/p>\n

We can establish a relationship between them using Java Association<\/strong>. This relationship can be any type like one to one, one to many, many to one, or many to many.<\/p>\n

Actually, these separate classes<\/strong> are connected with the help of their objects<\/strong>. In this article, we will discuss Association in Java<\/strong>, along with its types and implementation examples in Java.<\/p>\n

We will learn each topic with a real-life example for your better understanding.<\/p>\n

What is Association in Java?<\/h3>\n

Association in Java is one of the building blocks and the most basic concept of object-oriented programming. Association is a connection or relationship between two separate classes.<\/p>\n

It shows how objects of two classes are associated with each other. The Association defines the multiplicity between objects. We can describe the Association as a has-a relationship between the classes.<\/p>\n

Association is a kind of relationship between classes whose objects have an independent lifecycle and there is no ownership between the objects. It can be one-to-one, one-to-many, many-to-one, many-to-many.<\/p>\n

Let\u2019s take a real-life example to understand these types:<\/p>\n

1. One-to-one<\/h4>\n

The best example of a one-to-one association is that one person or one individual can have only one passport. This is a one-to-one relationship between the person and the passport.<\/p>\n

2. One-to-many<\/h4>\n

Suppose, there is a Doctor and his patients. So, one doctor is associated with many patients. So this is an example of a one-to-many Association between a doctor and patients.<\/p>\n

3. Many-to-one<\/h4>\n

For example, there can be many books in one library, each book is associated with that library, and it can\u2019t be a part of another library. So, many books are related to one library.<\/p>\n

This is an example of a many-to-one Association between books and a library.<\/p>\n

4. Many-to-many<\/h4>\n

If we talk about a teacher and student, there can be many students associated with one teacher, and also, the teacher can be related to many students.<\/p>\n

So, the relationship between a teacher and student can be many-to-many.<\/p>\n

\"Association<\/a><\/h2>\n

Types of Java Association<\/h3>\n

There are two special forms of Association in Java. They are:
\n1.<\/strong> Composition
\n2.<\/strong> Aggregation<\/p>\n

UML Notations of Association, Composition, and Aggregation in Java<\/h4>\n

There are different UML notations for aggregation, associations, and composition. We can easily differentiate them from these notations.<\/p>\n

The following diagram shows the UML notations for each of them.<\/p>\n

\"Java<\/a><\/p>\n

Example of Association in Java<\/strong><\/p>\n

package com.techvidvan.association;\nclass Person {\n  String name;\n  long id;\n  Person(String name, long id) {\n    this.name = name;\n    this.id = id;\n  }\n}\nclass Passport extends Person {\n  String personName;\n  Passport(String name, long id) {\n    super(name, id);\n    this.personName = name;\n  }\n}\npublic class GovernmentAgency {\n  public static void main(String args[]) {\n    Passport obj = new Passport(\"Divya\", 99884444);\n    System.out.println(obj.personName + \" is a person with a passport number: \" + obj.id);\n  }\n}<\/pre>\n
Output:<\/strong>
\nDivya is a person with a passport number: 99884444<\/p>\n
In the above example, there is a one to one association between two classes: Person<\/strong> and Passport<\/strong>. Both the classes represent two separate entities.<\/p>\n
Aggregation in Java<\/h4>\n
Aggregation in Java is a special kind of association. It represents the Has-A relationship between classes. Java Aggregation allows only one-to-one relationships.<\/p>\n
If an object is destroyed, it will not affect the other object, i.e., both objects can work independently.<\/p>\n
Let\u2019s take an example. There is an Employee in a company who belongs to a particular Department. If the Employee<\/strong> object gets destroyed still the Department<\/strong> can work independently.<\/p>\n
The end of the Employee object will not affect or destroy the Department object. The Aggregation is represented as a line with a diamond.<\/p>\n
Here the Department has-a Employee and therefore both are in the Aggregation with each other.<\/p>\n
\"Aggregation<\/a><\/p>\n
package com.techvidvan.association;\nclass Employee {\n  int id;\n  String name;\n  String dept;\n  Employee(int id, String name, String dept) {\n    this.id = id;\n    this.name = name;\n    this.dept = dept;\n    System.out.println(\"\\nEmployee name is \" + name);\n    System.out.println(\"Employee Id is \" + id);\n    System.out.println(\"Employee belongs to the \" + dept + \" Department\");\n  }\n}\nclass Department {\n  String deptName;\n  int noOfemployees;\n  Department(String name, int numberOfemployees) {\n    this.deptName = name;\n    this.noOfemployees = numberOfemployees;\n  }\n}\nclass University {\n  String universityName;\n  int noOfdepartments;\n  University(String name, int departments) {\n    this.universityName = name;\n    this.noOfdepartments = departments;\n  }\n}\npublic class AggregationDemo {\n  public static void main(String[] args) {\n    Employee e1 = new Employee(101, \"Rishi\", \"Engineering\");\n    Employee e2 = new Employee(167, \"Rohan\", \"Management\");\n    Employee e3 = new Employee(125, \"Sneha\", \"Accounts\");\n  }\n}<\/pre>\n
Output:<\/strong>
\nEmployee name is Rishi
\nEmployee Id is 101
\nEmployee belongs to the Engineering Department<\/p>\n
Employee name is Rohan
\nEmployee Id is 167
\nEmployee belongs to the Management Department<\/p>\n
Employee name is Sneha
\nEmployee Id is 125
\nEmployee belongs to the Accounts Department<\/p>\n
Composition in Java<\/h4>\n
The composition is another form of aggregation which is considered as the restricted form of Association.<\/p>\n
In this type of association, the entities are completely dependent on each other, unlike the aggregation. Composition allows for one-to-many relationships between objects.<\/p>\n
It represents a part-of relationship between two objects. One entity cannot exist without the other. Composition in Java represents a one-to-many relationship.<\/p>\n
Suppose, there is a House<\/strong> and inside the house, there are many rooms<\/strong>. We consider the relationship between the house and the rooms.<\/p>\n
A single house can have multiple rooms but a single room can not have multiple houses. And, if we delete the house, the rooms will automatically be deleted.<\/p>\n
So the two entities: house and rooms, are dependent on each other. Room is a part of the House and they two hold a relation of composition.<\/p>\n
\"Composition<\/a><\/p>\n
package com.techvidvan.association;\nimport java.util. * ;\nclass Room {\n  public String roomName;\n  public int roomNo;\n  Room(String name, int number) {\n    this.roomName = name;\n    this.roomNo = number;\n  }\n}\nclass House {\n  private final List < Room > rooms;\n  House(List < Room > rooms) {\n    this.rooms = rooms;\n  }\n  public List < Room > getTotalRoomsInHouse() {\n    return rooms;\n  }\n}\npublic class CompositionDemo {\n  public static void main(String[] args) {\n    Room room1 = new Room(\"Dining Room\", 2);\n    Room room2 = new Room(\"Bed Room\", 5);\n    Room room3 = new Room(\"Living Room\", 3);\n    List < Room > books = new ArrayList < Room > ();\n    books.add(room1);\n    books.add(room2);\n    books.add(room3);\n    House house = new House(books);\n    List < Room > rooms = house.getTotalRoomsInHouse();\n    for (Room room: rooms) {\n      System.out.println(\"The Room Number of \" + room.roomName + \" is: \" + room.roomNo);\n    }\n  }\n}<\/pre>\n
Output:<\/strong>
\nThe Room Number of Dining Room is: 2
\nThe Room Number of Bed Room is: 5
\nThe Room Number of Living Room is: 3<\/p>\n
Conclusion<\/h3>\n
In this article, we discussed Association in Java. If we want to establish connections between two classes then we can use any type of association according to the need.<\/p>\n
If the objects are independent of each other, we use Aggregation and if they are dependent then we use Composition.<\/p>\n
In Aggregation, if one object dies, the other still remains alive but in Composition, if one dies then the other automatically dies.<\/p>\n
We covered everything with real-life so that you can connect it in a better way. We hope you enjoyed reading this article.<\/p>\n","protected":false},"excerpt":{"rendered":"
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