Java - Has-A and Uses-A Relationship

Java - Has-A and Uses-A Relationship

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In Java, reuse code using an Is-A relationship or Has-A relationship.

• Is-A relationship is also known as inheritance
• Has-A relationship is also known as composition in Java.
• both used for code reusability in Java.

Is-A Relationship in Java

• In Java, Is-A relationship depends on inheritance.
  • Further inheritance is of two types, class inheritance and interface inheritance.
• inheritance is unidirectional in nature.
  • a Potato is a vegetable, a Bus is a vehicle.
  • a house is a building.
  • But not all buildings are houses.
• determine an Is-A relationship in Java.
  • 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, Has-A relationship is also known as composition.
• Has-A relationship: 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.

code 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;
    }
}

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


// ====================================================================


public class Pulsar extends Bike {
// Pulsar is a type of bike that extends the Bike class that shows that Pulsar is a Bike.
// All the methods like setColor( ), bikeInfo( ), setMaxSpeed( ) are used because of the Is-A relationship of the Pulsar class with the Bike class.

    public void PulsarStartDemo() {
        Engine PulsarEngine = new Engine();
        // Pulsar also uses an Engine's method, stop, using composition.
        PulsarEngine.stop();
    }
}


// ====================================================================

public class Demo  {
    public static void main(String[] args)  {
        Pulsar myPulsar = new Pulsar();
        myPulsar.setColor("BLACK");
        myPulsar.setMaxSpeed(136);
        myPulsar.bikeInfo();
        myPulsar.PulsarStartDemo();
    }
}