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;  
3.   
4. public Address(String city, String state, String country) {  
5.     this.city = city;  
6.     this.state = state;  
7.     this.country = country;  
8. }  
9.   
10. }  

Employee.java

1. public class Emp {  
2. int id;  
3. String name;  
4. Address address;  
5.   
6. public Emp(int id, String name,Address address) {  
7.     this.id = id;  
8.     this.name = name;  
9.     this.address=address;  
10. }  
11.   
12. void display(){  
13. System.out.println(id+” “+name);  
14. System.out.println(address.city+” “+address.state+” “+address.country);  
15. }  
16.   
17. public static void main(String[] args) {  
18. Address address1=new Address(“gzb”,”UP”,”india”);  
19. Address address2=new Address(“gno”,”UP”,”india”);  
20.   
21. Emp e=new Emp(111,”varun”,address1);  
22. Emp e2=new Emp(112,”arun”,address2);  
23.       
24. e.display();  
25. e2.display();  
26.       
27. }  
28. }  

Output

111 varun
gzb UP india
112 arun
gno UP india