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What is aggregation in Java?

Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that Employee (class) has an object of Address class. Consider the following example.

Address.java

  1. public class Address {
  2. String city,state,country;
  4. public Address(String city, String state, String country) {
  5.     this.city = city;
  6.     this.state = state;
  7.     this.country = country;
  8. }
  10. }

Employee.java

  1. public class Emp {
  2. int id;
  3. String name;
  4. Address address;
  6. public Emp(int id, String name,Address address) {
  7.     this.id = id;
  8.     this.name = name;
  9.     this.address=address;
  10. }
  12. void display(){
  13. System.out.println(id+” “+name);
  14. System.out.println(address.city+” “+address.state+” “+address.country);
  15. }
  17. public static void main(String[] args) {
  18. Address address1=new Address(“gzb”,“UP”,“india”);
  19. Address address2=new Address(“gno”,“UP”,“india”);
  21. Emp e=new Emp(111,“varun”,address1);
  22. Emp e2=new Emp(112,“arun”,address2);
  24. e.display();
  25. e2.display();
  27. }
  28. }

Output

111 varun
gzb UP india
112 arun
gno UP india
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