Aggregation in Java

Last Updated: 28 May, 2022

Aggregation represents HAS-A relationship between classes in Java, It is a special kind of association. which means a class contains reference of another class.

• It is a unidirectional association i.e. a one-way relationship. For example, a Employee can have Address but vice versa is not possible.
• In Aggregation, both the entries can survive individually which means ending one entity will not affect the other entity.

For example:

class Employee { int emp_id; String emp_name; Address address; // codes }

Employee has an entity reference Address, so relationship is Employee HAS-A Address.

Example: Aggregation implementation in a Java

Class: Address.java

public class Address { private Integer address_id; private String address_line1; private String city; private String state; public Address(Integer address_id, String address_line1, String city, String state) { this.address_id = address_id; this.address_line1 = address_line1; this.city = city; this.state = state; } // Setter-Getter Methods }

Class: Employee.java

public class Employee { private Integer emp_id; private String emp_name; private Address address; public Employee(Integer emp_id, String emp_name, Address address) { this.emp_id = emp_id; this.emp_name = emp_name; this.address = address; } // Setter-Getter Methods void displayEmpInfo() { System.out.println("Emp ID: " + emp_id + "\nEmp Name: " + emp_name); System.out.println("Employee Address Details:"); System.out.println("ID: " + address.getAddress_id() + "\nAddress: " + address.getAddress_line1() + "\nCity: " + address.getCity() + "\nState: " + address.getState()); } }

Class: AggregationExample.java

public class AggregationExample { public static void main(String[] args) { Address address1 = new Address(101, "MG Road", "Bangalore", "Karnataka"); Employee empObj = new Employee(75, "John", address1); empObj.displayEmpInfo(); } }

Output

Emp ID: 75
Emp Name: John
Employee Address Details:
ID: 101
Address: MG Road
City: Bangalore
State: Karnataka

Why do we need Aggregation in Java?

For maintaining code reusability.