Design Pattern: Composite Pattern in Java

Composite pattern is used to handle the operation of a bunch of classes (having the same interface), to operate the same function of all items a the same time. Using Composite pattern the client can ignore the difference between the Composition and Leaf classes, and can use the composition considering the same interface.

Check complete details and explanations about the Composite pattern in the article: Composite Design Pattern

This article demonstrates Composite pattern implementations in Java. Check the following examples.

Example #1: Transport

In the example below we are implementing the Composite pattern for a transport system.

Take a look at the class diagram.

Transport Interface

package com.bigboxcode.composite;

public interface Transport {

    void start();

    void operate();

    void stop();

}

Bike Class

package com.bigboxcode.composite;

public class Bike implements Transport{
    @Override
    public void start() {
        System.out.println("Starting Bike...");
    }

    @Override
    public void operate() {
        System.out.println("Riding Bike");
    }

    @Override
    public void stop() {
        System.out.println("Stopping Bike...");
    }
}

Plane Class

package com.bigboxcode.composite;

public class Plane implements Transport {
    @Override
    public void start() {
        System.out.println("Starting Plane...");
    }

    @Override
    public void operate() {
        System.out.println("Flying Plane");
    }

    @Override
    public void stop() {
        System.out.println("Stopping Plane...");
    }
}

Car Class

package com.bigboxcode.composite;

public class Car implements Transport {
    @Override
    public void start() {
        System.out.println("Starting Car...");
    }

    @Override
    public void operate() {
        System.out.println("Driving Car");
    }

    @Override
    public void stop() {
        System.out.println("Stopping Car...");
    }
}

Composite Class

package com.bigboxcode.composite;

import java.util.ArrayList;
import java.util.List;

public class TransportGroup implements Transport {
    private List<Transport> transportList = new ArrayList<Transport>();

    @Override
    public void start() {
        for (Transport transport: transportList) {
            transport.start();
        }
    }

    @Override
    public void operate() {
        for (Transport transport: transportList) {
            transport.operate();
        }
    }

    @Override
    public void stop() {
        for (Transport transport: transportList) {
            transport.stop();
        }
    }

    public void addTransport(Transport transport) {
        transportList.add(transport);
    }

    public void removeTransport(Transport transport) {
        transportList.remove(transport);
    }
}

Demo

package com.bigboxcode.composite;

public class Main {
    public static void main(String[] args) {
        Bike bike = new Bike();
        Plane plane = new Plane();
        Car car = new Car();
        Car secondCar = new Car();

        TransportGroup transports = new TransportGroup();
        transports.addTransport(bike);
        transports.addTransport(plane);
        transports.addTransport(car);
        transports.addTransport(secondCar);

        System.out.println("-----------------Output with 4 transports------------------\n");

        transports.start();
        transports.operate();
        transports.stop();

        System.out.println("\n-----------------Output when plane is removed------------------\n");

        transports.removeTransport(plane);

        transports.start();
        transports.operate();
        transports.stop();
    }
}

Output

The output of the demo above will be like below.

-----------------Output with 4 transports------------------

Starting Bike...
Starting Plane...
Starting Car...
Starting Car...
Riding Bike
Flying Plane
Driving Car
Driving Car
Stopping Bike...
Stopping Plane...
Stopping Car...
Stopping Car...

-----------------Output when plane is removed------------------

Starting Bike...
Starting Car...
Starting Car...
Riding Bike
Driving Car
Driving Car
Stopping Bike...
Stopping Car...
Stopping Car...

Source Code

Use the following link to get the source code:

Example	Source Code Link
Example #1: Transport	GitHub