How to implement Composite Design Pattern in Java ?.

Program to demonstrate how to implement Composite Design Pattern in Java.

1. Geographical.java 

package com.hubberspot.designpattern.structural.composite;

// This acts as a interface for every State(Leaf) and
// Country(Composite)
public abstract class GeographicalArea {

    // Name of the region 
    private String areaName;    

    public GeographicalArea(String areaName) {

        this.areaName = areaName;
    }    

    public String getAreaName() {
        return areaName;
    }

    // Create methods which will throw UnsupportedOperationException

    public void addGeographicalArea(GeographicalArea geographicalArea) {

        throw new UnsupportedOperationException(
                "Invalid Operation. Not Supported");
    }
    public GeographicalArea getGeographicalArea(int areaIndex) {

        throw new UnsupportedOperationException(
                "Invalid Operation. Not Supported");
    }

    public long getPopulation() {

        throw new UnsupportedOperationException(
                "Invalid Operation. Not Supported");
    }

    // Create a abstract method which will be implemented
    // by State and Country respectively
    public abstract void displayAreaInfo();

}

2. Country.java

package com.hubberspot.designpattern.structural.composite;

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

// It extends GeographicalArea and has a list of 
// objects containing the GeographicalArea such as 
// State in this case.
public class Country extends GeographicalArea {

    List< GeographicalArea > listOfGeographicalArea = new ArrayList< GeographicalArea >();

    public Country(String areaName) {
        super(areaName);
    }

    // adding states to country
    @Override
    public void addGeographicalArea(GeographicalArea geographicalArea)
            throws UnsupportedOperationException {
        listOfGeographicalArea.add(geographicalArea);
    }

    // getting states one by one based on index
    @Override
    public GeographicalArea getGeographicalArea(int areaIndex)
            throws UnsupportedOperationException {

        return listOfGeographicalArea.get(areaIndex);

    }

    // displaying the information for the country
    @Override
    public void displayAreaInfo() {

        System.out.println("Country name : " + getAreaName());
        System.out.println("It has following info : ");

        long totalPopulation = 0;
        int index = 1;

        System.out.println("It has following states : ");
        for(GeographicalArea geographicalArea : listOfGeographicalArea) {

            System.out.println(index + " : "+ geographicalArea.getAreaName() );
            System.out.println("It has population around : " + geographicalArea.getPopulation());
            totalPopulation = totalPopulation + geographicalArea.getPopulation();
            index++;
        }

        System.out.println("Total Population of "+ getAreaName()+ " is : " + totalPopulation);

    }

}

3. State.java

package com.hubberspot.designpattern.structural.composite;

//It extends GeographicalArea
public class State extends GeographicalArea {

    // has a property called as population 
    private long population;


    public long getPopulation() {
        return population;
    }

    public State(String areaName, long population) {
        super(areaName);
        this.population = population;        
    }

    // displaying the information for the states
    @Override
    public void displayAreaInfo() {

        System.out.println("State name : " + getAreaName());
        System.out.println("State population : " + getPopulation());

    }

}

4. TestCompositePattern.java

package com.hubberspot.designpattern.structural.composite;

public class TestCompositePattern {

    public static void main(String[] args) {

        // Create two countries such as India and USA
        GeographicalArea countryIndia = new Country("India");
        GeographicalArea countryUSA = new Country("USA");

        // Create two Indian States with name and population
        GeographicalArea stateMadhyaPradesh = new State("Madhya Pradesh", 10000000);
        GeographicalArea stateMaharashtra = new State("Maharashtra", 20000000);

        // Create two USA States with name and population
        GeographicalArea stateNewYork = new State("New York", 19570261);
        GeographicalArea stateWashington = new State("Washington", 6897012);

        // Add the Indian States to India 
        countryIndia.addGeographicalArea(stateMadhyaPradesh);
        countryIndia.addGeographicalArea(stateMaharashtra);

        // Add the USA States to USA
        countryUSA.addGeographicalArea(stateNewYork);
        countryUSA.addGeographicalArea(stateWashington);

        // Display Info for country India 
        countryIndia.displayAreaInfo();
        System.out.println();

        // Display Info for country USA
        countryUSA.displayAreaInfo();

    }

}

Output of the program :