Is-A and Has-A Relationship in Java

1. Overview

• IS-A Relationship:

The concept of IS-A is totally based on Inheritance, which can be of two types Class Inheritance or Interface Inheritance. It is just like saying "A is a B type of thing". For example, Apple is a Fruit, Car is a Vehicle etc. Inheritance is uni-directional. For example, House is a Building. But Building is not a House.

It is a key point to note that you can easily identify the IS-A relationship. Wherever you see an extends keyword or implements keyword in a class declaration, then this class is said to have IS-A relationship.

• HAS-A Relationship: 

Composition(HAS-A) simply means the use of instance variables that are references to other objects. For example, Maruti has Engine, or House has Bathroom.

2. Description

2.1 Is-A

package relationsdemo;  
public class Bike  
{  
    private String color;  
    private int maxSpeed;  
    public void bikeInfo()  
    {  
        System.out.println("Bike Color= "+color + " Max Speed= " + maxSpeed);  
    }  
    public void setColor(String color)  
    {  
        this.color = color;  
    }  
    public void setMaxSpeed(int maxSpeed)  
    {  
        this.maxSpeed = maxSpeed;  
    }  
}  

package relationsdemo;  
public class Pulsar extends Bike  
{  
    public void PulsarStartDemo()  
    {  
        Engine PulsarEngine = new Engine();  
        PulsarEngine.stop();  
    }  
}  

2.2 Has-A

package relationsdemo;  
public class Pulsar extends Bike  
{  
    public void PulsarStartDemo()  
    {  
        Engine PulsarEngine = new Engine();  
        PulsarEngine.stop();  
    }  
}  

package relationships;
public class Engine {
	public void start(){
		System.out.println("Engine Started:");
	}
	public void stop(){
		System.out.println("Engine Stopped:");
	}
}

3. Reference

https://www.c-sharpcorner.com/UploadFile/3614a6/is-a-and-has-a-relationship-in-java/

https://www.w3resource.com/java-tutorial/inheritance-composition-relationship.php