HAS-A Relationship or Aggregation

Has-A Relationship in Java

In Java, a Has-A relationship is also known as composition. It is also used for code reusability in Java. In Java, a Has-A relationship simply means that an instance of one class has a reference to an instance of another class or an other instance of the same class. For example, a car has an engine, a dog has a tail and so on. In Java, there is no such keyword that implements a Has-A relationship. But we mostly use new keywords to implement a Has-A relationship in Java.

Consider we are storing the information of the student, then we may create a class like below –

Class Student {
   int roll;
   String sname;
   Address address;
}

In the above class we have entity reference “Address” which stores again its own information like street,city,state,zip. like below –

Class Address {
   String street;
   String state;
   String zip;
   String city;
}

so we can say that Student HAS-A Address Thus if the class has entity reference then entity will represent the HAS-A relationship.

Consider the following example –

Address.java

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

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

Student.java

public class Student {
 
   int roll;
   Address address;

   Student(int rollNo,Address addressDetail){
      roll = rollNo;
      address = addressDetail;
   }
 
    void printStudentDetails(Address address1) {
       System.out.println("Roll   : " + roll);
       System.out.println("Street : " + address1.street);
       System.out.println("City   : " + address1.city);
       System.out.println("State  : " + address1.state);
       System.out.println("Zip    : " + address1.zip);
    } 
 
    public static void main(String[] args) {
       Address address1;
       address1 = new Address("1-ST","PN","Mah","41");
       Student s1 = new Student(1,address1);
       s1.printStudentDetails(address1);
    }
}

Output :

Roll   : 1
Street : 1-ST
City   : PN
State  : Mah
Zip    : 41