ObserverPattern-v1

Observer design pattern

1. Model the "independent" functionality with a "subject" abstraction
2. Model the "dependent" functionality with "observer" hierarchy
3. The Subject is coupled only to the Observer base class
4. Observers register themselves with the Subject
5. The Subject broadcasts events to all registered Observers
6. Observers "pull" the information they need from the Subject
7. Client configures the number and type of Observers


package designpattern.observer_v1;

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

abstract class Observer
{
    protected Subject subject;
    public abstract void update();
}

class Subject
{
    private List<Observer> observers = new ArrayList<>();
    private int state;

    public void add(Observer o)
    {
        observers.add(o);
    }

    public int getState()
    {
        return state;
    }

    public void setState(int value)
    {
        this.state = value;
        execute();
    }

    private void execute()
    {
        for(Observer observer : observers)
        {
            observer.update();
        }
    }
}

class HexObserver extends Observer
{
    public HexObserver(Subject subject)
    {
        this.subject = subject;
        this.subject.add(this);
    }

    @Override
    public void update()
    {
        System.out.println("Hexa   : " + Integer.toHexString(subject.getState()));
    }
}

class OctObserver extends Observer
{
    public OctObserver(Subject subject)
    {
        this.subject = subject;
        this.subject.add(this);
    }

    @Override
    public void update()
    {
        System.out.println("Octal  : " + Integer.toOctalString(subject.getState()));
    }
}
    class BinObserver extends Observer
    {
        public BinObserver(Subject subject)
        {
            this.subject = subject;
            this.subject.add(this);
        }

        @Override
        public void update()
        {
            System.out.println("Binary : " + Integer.toBinaryString(subject.getState()));
        }
    }

public class ObserverDemo
{
    public static void main(String[] args)
    {
        Subject sub = new Subject();
        // client configures the number and type ofObserver
        new HexObserver(sub);
        new OctObserver(sub);
        new BinObserver(sub);
        Scanner scan = new Scanner(System.in);

        for(int i = 0;; i++)
        {
            System.out.print("Enter a Number : ");

            sub.setState(scan.nextInt());
            System.out.println("");
        }
    }
}