Aggregation in Java

In this blog, we can see what is aggregation in java. How it differs from inheritance. And what is the advantage of it.

What is aggregation?

Any class is dependents the other class, that class has the dependent class as the property, And an aggregation represents the HAS-A relationship. So there are no parent class and child class.

Difference between Inheritance and aggregation

Represents IS-A relationshipRepresents HAS-A relationship
The child class inherits the parent class using extends keywordThere are no child and parent class association. So the class can have the dependent class as a property.
Tightly coupledLoosely coupled
Inheritance vs Aggregation

Advantage of aggregation

The main advantage of aggregation is code reusability and memory management. And it it loosely coupled. So If need the dependent object we can create objects of dependent class and initialize it at runtime. Otherwise, we can initialize as null.

Example for aggregation

We can take the Employee class. An employee may have two phone numbers or may have only one phone number. When the employee doesn’t have a secondary phone number we can have as null.

package com.samplecoder.aggregation;

public class Employee {
	private int id;
	private String name;
	private String qualification;
	private Long primaryPhoneNo;
	private Long secondayPhoneNo;
	public Employee(int id, String name, String qualification, Long primaryPhoneNo, Long secondayPhoneNo) {
		super(); = id; = name;
		this.qualification = qualification;
		this.primaryPhoneNo = primaryPhoneNo;
		this.secondayPhoneNo = secondayPhoneNo;

	public String toString() {
		return "Employee [id=" + id + ", name=" + name + ", qualification=" + qualification + ", primaryPhoneNo="
				+ primaryPhoneNo + ", secondayPhoneNo=" + secondayPhoneNo + "]";
package com.samplecoder.aggregation;

public class AggregationMain {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Employee obj1 = new Employee(1, "Praith", "MCA", Long.valueOf(1234567890), Long.valueOf(1122334455));
		Employee obj2 = new Employee(2, "Heshika", "MBA", 1122334455L, null);

Aggregation in Java