Introduction
In Java, we can reuse our code using an Is-A relationship or using a Has-A relationship. An Is-A relationship is also known as inheritance and a Has-A relationship is also known as composition in Java.
Is-A Relationship in Java
In Java, an Is-A relationship depends on inheritance. Further inheritance is of two types, class inheritance and interface inheritance. It is used for code reusability in Java. For example, a Potato is a vegetable, a Bus is a vehicle, a Bulb is an electronic device and so on. One of the properties of inheritance is that inheritance is unidirectional in nature. Like we can say that a house is a building. But not all buildings are houses. We can easily determine an Is-A relationship in Java. When there is an extends or implement keyword in the class declaration in Java, then the specific class is said to be following the Is-A relationship.
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.
![IS-A and HAS-A relationship]()
Example
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;
}
}
In the code above the Bike class has a few instance variables and methods.
package relationsdemo;
public class Pulsar extends Bike
{
public void PulsarStartDemo()
{
Engine PulsarEngine = new Engine();
PulsarEngine.stop();
}
}
Pulsar is a type of bike that extends the Bike class that shows that Pulsar is a Bike. Pulsar also uses an Engine's method, stop, using composition. So it shows that a Pulsar has an Engine.
package relationsdemo;
public class Engine
{
public void start()
{
System.out.println("Started:");
}
public void stop()
{
System.out.println("Stopped:");
}
}
The Engine class has the two methods start( ) and stop( ) that are used by the Pulsar class.
package relationsdemo;
public class Demo
{
public static void main(String[] args)
{
Pulsar myPulsar = new Pulsar();
myPulsar.setColor("BLACK");
myPulsar.setMaxSpeed(136);
myPulsar.bikeInfo();
myPulsar.PulsarStartDemo();
}
}
In the code above we make an object of the Pulsar class and then initialize it. All the methods like setColor( ), bikeInfo( ), setMaxSpeed( ) are used here because of the Is-A relationship of the Pulsar class with the Bike class.
Output
![example of IS-A and HAS-A relationship]()
Summary
In this article you learn about the Is-A and Has-A relationships in Java.