What is Is-A-Relationship in Java?

A relationship in Java means different relations between two or more classes. For example, if a class Bulb inherits another class Device, then we can say that Bulb is having is-a relationship with Device, which implies Bulb is a device.  

In Java, we have two types of relationship:

1. Is-A relationship: Whenever one class inherits another class, it is called an IS-A relationship.
2. Has-A relationship: Whenever an instance of one class is used in another class, it is called HAS-A relationship.

Is-A relationship 

IS-A Relationship is wholly related to Inheritance. For example - a kiwi is a fruit; a bulb is a device.

• IS-A relationship can simply be achieved by using extends Keyword.
• IS-A relationship is additionally used for code reusability in Java and to avoid code redundancy.
• IS-A relationship is unidirectional, which means we can say that a bulb is a device, but vice versa; a device is a bulb is not possible since all the devices are not bulbs.
• IS-A relationship is tightly coupled, which means changing one entity will affect another entity.

Advantage of IS-A relationship 

• Code Reusability.
• Reduce redundancy.

How to achieve IS-A relationship

IS-A relationship can simply be achieved by extending an interface or class by using extend keyword.

Let's understand the IS-A relationship with the help of a flowchart -

In the above flowchart, the class Bulb extends class Device, which implies that Device is the parent class of Bulb, and Class Bulb is said to have an Is-A relationship. Therefore we can say Bulb Is-A device.

Implementation of IS-A relationship

1. Class Device has a field named as deviceName.

2. Class Bulb extends Device that means Bulb is a type of Device.

3. Since Device is the parent class which implies that class Device can store the reference of an instance of class Bulb.

Example: Here is the implementation of the same, which is as follows: 

Device name is Bulb
Bulb is a Device

In the above java program Bulb class inherits the Device class. Therefore, we can say that Bulb is having an IS-A relationship with the class Device. Hence Bulb is a Device.