IS-A, HAS-A, USES-A RELATIONS IN JAVA

A business has many objects. All these objects must be related to each other. Java supports three types of relations between objects –

1. IS-A (Inheritance) – We should establish IS-A relation between classes for grouping classes as one category or family to establish child-parent relation so, that child classes objects can be used wherever parent class referenced variable is used.

In the above example : Student and Employee objects are grouped as one family of classes and they got the type Person. Then these two objects inherit the properties and behaviors of Person. Now these objects are also of type Person, this is the reason we call them as

• ·        Student IS-A Person
• ·        Employee IS-A Person

We can implement this IS-A relation using the following two keywords –

• ·        extends – It is used between two classes or two interfaces.
• ·        implements – It is used between a class and interface.

2. HAS-A (Composition) – We should establish HAS-A relation between classes, if one object cannot exist without another object. For example, in the above example a student or employee object cannot exist without address. Hence, we must establish HAS-A relation between these objects. This is the reason we call them as –

• ·        Student HAS-A Address
• ·        Employee HAS-A Address

This relation is implemented by storing other object’s instance, using its non-static variable in our object’s classes. Hence, we must create Address class type non-static variable to store its object in Student and Employee classes.

Example –

class Address{

}

class Student

{

          Address add = new Address();

}

Class Employee

{

          Address add = new Address();

}

3. USES-A (Aggregation) – We should establish USES-A relation between classes if one object uses another object for performing one of its operations. Example – An application to create Student and Bike objects for developing USES-A relation between these two objects.

class Bike{

          String bikenumber;

          String bikename;

          int modelnumber;

          String enginenumber;

}

class Student{

          int sno;

          String sname;

          String course;

          void goingToCollegeBy(Bike b)

          {

                   System.out.println(this.sname+" : is going to college by : "+b.bikename+" bike");

          }

}

public class Test {

          public static void main(String[] args) {

                   Bike ktm = new Bike();

                   ktm.bikename="KTM-220";

                   ktm.bikenumber="3122";

                   ktm.enginenumber="44321as";

                   ktm.modelnumber=2016;

                   Student s = new Student();

                   s.sno=26;

                   s.sname="Rakesh";

                   s.course="MCA";

                   s.goingToCollegeBy(ktm);

          }

}

Output –

Rakesh : is going to college by : KTM-220 bike