Association in Java

Association in java is the one of a most important topics. The inheritance in java, aggregation in java and composition in java are part of Java association. In this post, we will read aggregation and composition in java.

Here is the table content of the article will we will cover this topic.

1. What is the Association in Java?
2. Inheritance(Is – A relationship)
3. Aggregation((has-A relationship)
4. Composition (Part of relationship)

What is the Association in Java?

In this article, we will read what is Association in java and how we can achieve it. Association is a connection between two separate classes that build up through their Objects. In Java, the association can have one-to-one, one-to-many, and many-to-many relationships.

The association builds a relationship between the classes and tells how much a class knows about another class. This relationship can be unidirectional or bi-directional. If you are a beginner, you should read the concept of Inheritance in Java. As you know Java is an Object-Oriented language, to provide the flexibility an Object can communicate with other Objects and provide functionality.

One to One: A person can have only one voter id card and a voter id card can be linked to only one person. That is a “one-to-one” relationship.  
One to many: A Bank can have many Employees, so it is a “one-to-many” relationship.
Many to one: A Country can have many states, which is a “many-to-one” relationship.
Many to Many: A Bank can have multiple customers and a customer can have multiple bank accounts.

Association in java

Types of Association in Java

  1. Inheritance (Is – A relationship)
  2. Aggregation (has-A relationship)
  3. Composition (Part of relationship)
Association in java

1. Inheritance in Java(Is – A)

It’s an important concept/feature of Object-Oriented Programming. Inheritance is a  mechanism by which one class can acquire/inherit the features(fields and methods) of another class. In this article, we will read the concept of Inheritance in javaIt means in java one class can inherit the variables and methods of another class. It is also known as the Is-A relationship. You can read inheritance in Java in detail.

2. Aggregation in java(Has-A)

  • Aggregation represents the Has-A relationship.
  • It is a one-way relationship and called unidirectional association. For example, Bank can have employees but vice versa is not possible and that’s why it unidirectional in nature.
  • In JAVA Aggregation,  both the class’s object will not affect each other.

Example of aggregation( Has-A): Let’s take the example of a Bank and a customer. A customer belongs to Bank. If a customer is not coming to the bank , the bank will not be closed because it may still be functional. We are taking an example of a Bank and a customer. It will show the HAS-A relationship between Bank and customer. A bank HAS-A customer.

class Bank
	String nameOfBank;;
	int IFSC;
	Bank(String nameOfBank, int IFSC)
		this.nameOfBank = nameOfBank;
		this.IFSC = IFSC;
	public void displayAllDetails(Customer customer)
		System.out.println("Name of Bank = "+ nameOfBank);
		System.out.println("IFSC of Bank = "+ IFSC);
		System.out.println("Name of Customer = "+ customer.nameOfCustomer);
		System.out.println("Account number of Customer = "+ customer.accountNumber);
		System.out.println("Amount of Customer = "+ customer.amount);

class Customer
	String nameOfCustomer;
	int accountNumber;
	int amount;
	Customer(String nameOfCustomer,	int accountNumber, int amount)
		this.nameOfCustomer = nameOfCustomer;
		this.accountNumber = accountNumber;
		this.amount = amount;

class Branch 
	public static void main(String arg[])
		Bank bank = new Bank("AXIS", 12345);
		Customer customer = new Customer("Ram", 123456789, 100000);

Name of Bank = AXIS
IFSC of Bank = 12345
Name of Customer = Ram
Account number of Customer = 123456789
Amount of Customer = 100000

In this example, we have two classes Bank and Customer. We need only some information from the Customer. So, we have created an object of Customer and send it as an argument. After that, we used the object of the Customer.

3. Composition in java(Part of)

It is a restricted form of Aggregation. In composition two entities are highly dependent on each other. One entity cannot exist without the other.

  • It represents a part-of relationship.
  • In composition, both the entities are dependent on each other.
  • It is a highly restricted form.

Example of Composition( Part – Of): For example, a car has an engine. Composition makes strong relationship between the objects. It means that if we destroy the owner object, its members also will be destroyed with it. For example, if the Car is destroyed the engine is destroyed as well.

class Car
    //final will make sure engine is initialized
    private final Engine engine;  
    String nameOfCar;
    String modell;
    public Car(String nameOfCar, String model)
       engine  = new Engine("POWERHIGH", "ABC");
       this.nameOfCar = nameOfCar;
       this.model = model;
    public void showAlldetails()
    	System.out.println("Name of Car ="+nameOfCar);
    	System.out.println("Name of Model ="+model);
    	System.out.println("Engine used in Car ="+engine.typeOfEngine);
    	System.out.println("Engine name of Car ="+engine.nameOfEngine);

class Engine 
    String typeOfEngine;
    String nameOfEngine;
    Engine(String typeOfEngine, String nameOfEngine)
    	this.typeOfEngine = typeOfEngine;
    	this.nameOfEngine = nameOfEngine;

class ShowRoom
	public static void main(String arg[])
		Car car = new Car("BMW", "5X");

Name of Car =BMW
Name of Model =5X
Engine used in Car =POWERHIGH
Engine name of Car =ABC

In this example, we have two classes Car and Engine. The Engine class is totally depending on Car. Because Engine is a Part-Of car. Without an engine, the car is nothing. So, in JAVA Car class we created an object of Engine class which initialized in the Car class. It is a highly dependent class.

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