If a class have an entity reference, it is known as Aggregation. Aggregation represents HAS-A relationship.

Aggregation in Java is a mechanism of using an object of one class as a member variable of another class. It is a way to achieve a "has-a" relationship between objects, where one object is composed of or contains another object. The aggregated object can be accessed and used by the containing object to provide additional functionality or behavior.

To implement aggregation in Java, we need to declare a member variable of the type of the aggregated object in the class that will contain it. We can then instantiate the aggregated object in the constructor or using a setter method, and use it as needed in the containing class.

public class Student { private String name; private int age; private Address address; // Aggregated object

public Student(String name, int age, Address address) { this.name = name; this.age = age; this.address = address; }

// Getters and setters for name, age, and address }

public class Address { private String street; private String city; private String state;

public Address(String street, String city, String state) { this.street = street; this.city = city; this.state = state; }

// Getters and setters for street, city, and state }

// Example usage Address address = new Address("123 Main St", "Anytown", "CA"); Student student = new Student("John Doe", 20, address);

// Accessing the student's address System.out.println(student.getAddress().getStreet());

In this example, the Student class contains an Address object as an aggregated object. The Address object is instantiated in the Student constructor and can be accessed using the getAddress() method.

Why we need Aggregation? To maintain code re-usability. To understand this lets take the same example again.

How does Aggregation work in Java? Aggregation is basically an association between two classes, as explained above. This is achieved by defining two classes in the JAVA program.

Conclusion Hence aggregation in the JAVA language is one of the key features used abundantly by developers. As it supports the feature of code reusability, thereby reduces the bulkiness of the code. Code looks clearer, and implementation along with maintenance becomes much easier. It is well-advised to use this concept while working on JAVA projects.