デザインパターン「Observer」 ref: http://qiita.com/shoheiyokoyama/items/d4b844ed29f84a80795b

DegitObserver.java

public class DegitObserver implements Observer {
    public void update(NumberGenerator generator) {
        System.out.println("DegitObserver:" + generator.getNumber());
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {

        }
    }
}

file6.java

DegitObserver:20
GraphObserver:********************
DegitObserver:41
GraphObserver:*****************************************
DegitObserver:41
GraphObserver:*****************************************
DegitObserver:35
GraphObserver:***********************************
DegitObserver:16
GraphObserver:****************
DegitObserver:49
GraphObserver:*************************************************
DegitObserver:44
GraphObserver:********************************************
DegitObserver:5
GraphObserver:*****
DegitObserver:0
GraphObserver:
DegitObserver:11
GraphObserver:***********
DegitObserver:16
GraphObserver:****************
DegitObserver:43
GraphObserver:*******************************************
DegitObserver:11
GraphObserver:***********
DegitObserver:47
GraphObserver:***********************************************
DegitObserver:24
GraphObserver:************************
DegitObserver:30
GraphObserver:******************************
DegitObserver:21
GraphObserver:*********************
DegitObserver:15
GraphObserver:***************
DegitObserver:20
GraphObserver:********************
DegitObserver:46
GraphObserver:**********************************************

GraphObserver.java

public class GraphObserver implements Observer {
    public void update(NumberGenerator generator) {
        System.out.print("GraphObserver:");
        int count = generator.getNumber();
        for (int i = 0; i < count; i++) {
            System.out.print("*");
        }
        System.out.println("");
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {

        }
    }
}

Main.java

public static void main(String[] args) {
        NumberGenerator generator = new RandomNumberGenerator();
        Observer degitObserver = new DegitObserver();
        Observer graphObserver = new GraphObserver();
        generator.addObserver(degitObserver);
        generator.addObserver(graphObserver);
        generator.execute();
    }

NumberGenerator.java

import java.util.ArrayList;
import java.util.Iterator;

public abstract class NumberGenerator {
    private ArrayList observers = new ArrayList();
    public void addObserver(Observer observer) {
        observers.add(observer);
    }
    public void deleteObserver(Observer observer) {
        observers.remove(observer);
    }
    public void notifyObservers() {
        Iterator iterator = observers.iterator();
        while (iterator.hasNext()) {
            Observer obserber = (Observer)iterator.next();
            obserber.update(this);
        }
    }

    public abstract int getNumber();
    public abstract void execute();
}

Observer.java

public interface Observer {
    public abstract void update(NumberGenerator generator);
}

Pull型の例

public void update() {
        // Observer自身が問い合わせる
        System.out.print("Number:" + this.subject.number);
}

RandomNumberGenerator.java

import java.util.Random;

public class RandomNumberGenerator extends NumberGenerator {
    private Random random = new Random();
    private int number;

    public int getNumber() {
        return number;
    }

    public void execute() {
        for (int i = 0; i < 20; i++) {
            number = random.nextInt(50);
            notifyObservers();
        }
    }
}