Aggregation in Java

An aggregation is a special form of association. Aggregation is used when we need to use property and behavior of a class without modifying it in the class. Aggregation is also used to achieve code reusability.

• We have two types of relationships one is HAS A and other is IS A relationship and Aggregation represents IS A relationship.
• It is a special form of association i.e. unidirectional association. To understand it, let's take an example: There is an officer and a no. of employees are working for him. Then we can say Officer ‘HAS A ‘employee working for him but vice versa is not true.
• In aggregation both the entity survive individually. If we end one entity that doesn't affect the other entity.

import java.util.*;
// employee class
class Employee
{
String name;
int id ;
String oname;// officer name under whom he work
Employee(String name, int id, String oname)
{
this.name = name;
this.id = id;
this.oname = oname;
}
}
/* Officer Class contains list of employee
Objects. It is associated with Employee
Class through its Object(s). */
class Officer
{
String name;
private List< Employee> employees;
Officer (String name, List<Employee> employees)
{
this.name = name;
this.employees = employees;
}
public List<Employee> getemployees()
{
return employees;
}
}

/* Company class contains list of Employee
Objects. It is associated with Officer
Class through its Object(s).*/

class Company
{
String cName;
private List<Officer> officers;
Company (String cName, List<Officer> officer)
{
this.cName = cName;
this.officers = officer;
}
// count total employees of all officers
// in a given company
public int getTotalEmployeesInCompany()
{
int noOfEmployees = 0;
List<Employee> employees;
for(Officer oname : officers)
{
employees = oname.getemployees();
for(Employee e : employees)
{
noOfEmployees++;
}
}
return noOfEmployees;
}
}
// main method
class Main
{
public static void main (String[] args)
{
Employee e1 = new Employee("Mia", 1, "Sachin sir ");
Employee e2 = new Employee(" Mira", 2, "Sachin sir ");
Employee e3 = new Employee("Mahesh ", 3, "Ashish sir ");
Employee e4 = new Employee("Manish ", 4, " Ashish sir ");
// making a List of
// Sachin sir Employees.
List <Employee> Sachinsir_employee = new ArrayList<Employee>();
Sachinsir_employee.add (e1);
Sachinsir_employee.add (e2);
// making a List of
// AshishsirEmployees
List <Employee> Ashishsir_employee = new ArrayList<Employee>();
Ashishsir_employee.add (e3);
Ashishsir_employee.add (e4);
Officer o1= new Officer ("Sachinsir", Sachinsir_employee);
Officer o2 = new Officer ("Ashishsir", Ashishsir_employee);
List <Officer > officers = new ArrayList<Officer>();
officers.add(o2);
officers.add(o1);
// creating an instance of Company.
Company company= new Company("ABC", officers);
System.out.println("Total employees in company: ");
System.out.println(company.getTotalEmployeesInCompany());
}
}

Total employees in company: 
4

Example explanation:

Let's see diagrammatically what happened in this example: