Aggregation is a special form of association. It is a relationship between two classes like association, however its a directional association, which means it is strictly a one way association. It represents a HAS-A relationship.

Aggregation Example in Java

For example consider two classes Student class and Address class. Every student has an address so the relationship between student and address is a Has-A relationship. But if you consider its vice versa then it would not make any sense as an Address doesn’t need to have a Student necessarily. Lets write this example in a java program.
Student Has-A Address

class Address
{
   int streetNum;
   String city;
   String state;
   String country;
   Address(int street, String c, String st, String coun)
   {
       this.streetNum=street;
       this.city =c;
       this.state = st;
       this.country = coun;
   }
}
class StudentClass
{
   int rollNum;
   String studentName;
   //Creating HAS-A relationship with Address class
   Address studentAddr; 
   StudentClass(int roll, String name, Address addr){
       this.rollNum=roll;
       this.studentName=name;
       this.studentAddr = addr;
   }
   public static void main(String args[]){
       Address ad = new Address(55, "Agra", "UP", "India");
       StudentClass obj = new StudentClass(123, "Chaitanya", ad);
       System.out.println(obj.rollNum);
       System.out.println(obj.studentName);
       System.out.println(obj.studentAddr.streetNum);
       System.out.println(obj.studentAddr.city);
       System.out.println(obj.studentAddr.state);
       System.out.println(obj.studentAddr.country);
   }
}

Output:

123
Chaitanya
55
Agra
UP
India

The above example shows the Aggregation between Student and Address classes. You can see that in Student class I have declared a property of type Address to obtain student address. Its a typical example of Aggregation in Java.

Why we need Aggregation?

To maintain code re-usability. To understand this lets take the same example again. Suppose there are two other classes College and Staff along with above two classes Student and Address. In order to maintain Student’s address, College Address and Staff’s address we don’t need to use the same code again and again. We just have to use the reference of Address class while defining each of these classes like:

Student Has-A Address (Has-a relationship between student and address)
College Has-A Address (Has-a relationship between college and address)
Staff Has-A Address (Has-a relationship between staff and address)

Hence we can improve code re-usability by using Aggregation relationship.

So if I have to write this in a program, I would do it like this:

class Address
{
   int streetNum;
   String city;
   String state;
   String country;
   Address(int street, String c, String st, String coun)
   {
       this.streetNum=street;
       this.city =c;
       this.state = st;
       this.country = coun;
   }
}
class StudentClass
{
   int rollNum;
   String studentName;
   //Creating HAS-A relationship with Address class
   Address studentAddr; 
   StudentClass(int roll, String name, Address addr){
       this.rollNum=roll;
       this.studentName=name;
       this.studentAddr = addr;
   }
   ...
}
class College
{
   String collegeName;
   //Creating HAS-A relationship with Address class
   Address collegeAddr; 
   College(String name, Address addr){
       this.collegeName = name;
       this.collegeAddr = addr;
   }
   ...
}
class Staff
{
   String employeeName;
   //Creating HAS-A relationship with Address class
   Address employeeAddr; 
   Staff(String name, Address addr){
       this.employeeName = name;
       this.employeeAddr = addr;
   }
   ...
}

As you can see that we didn’t write the Address code in any of the three classes, we simply created the HAS-A relationship with the Address class to use the Address code. The dot dot(…) part in the above code can be replaced with the public static void main method, the code in it would be similar to what we have seen in the first example.