Design Pattern Observer JAVA

1. Observer Pattern adalah salah satu Software Design Pattern yang berguna untuk memantain state (keadaan) dari sesuatu dan mengabarkan state tersebut kepada bagian-bagian yang membutuhkan.
2. Kelebihan Observer Pattern = Dapat membuat sebuah method yang dipantau dan memantau oleh method lain
3. Kekurangan Observer Pattern = Subject (publisher) mungkin mengirim update yang tidak penting bagi Observer (subsriber)
4. Kapan digunakan Observer Pattern = Ketika anda membutuhkan object lain untuk menerima update saat object lain berubah
5. Apa yang dilakukan ketika tidak boleh menggunakan Observer Pattern =

Tambahan :
1. Register Observer: Observable dapat mendaftarkan observer yang akan melakukan subscribe
2. Notify Observer: Beri tahu observer ketika ada perubahan state
3. Remove Observer:  Unsubscibe dari observable (objek yang disubcribe)
4. Update Observer : Sesuatu yang dilakukan ketika mendapat notifikasi dari Observer

// Subject.java
public interface Subject {
    public void register(Observer o);
    public void removeRegister(Observer o);
    public void notifyObserver();
}

// Observer.java
public interface Observer {
    public void update(double sahamPadang, double sahamSunda, double sahamTegal);
}

// StockGrabber.java
import java.util.ArrayList;

public class StockGrabber implements Subject {
    private ArrayList<Observer> observers;
    private double sahamPadang;
    private double sahamSunda;
    private double sahamTegal;

    public StockGrabber() {
        observers = new ArrayList<Observer>();
    }

    public void register(Observer newObserver) {
        observers.add(newObserver);
    }

    public void removeRegister(Observer removeObserver) {
        int observerIndex = observers.indexOf(removeObserver);
        System.out.println("Observer " + (observerIndex + 1) + " removed.");
        observers.remove(observerIndex);
    }

    public void notifyObserver() {
        for(Observer observer : observers) {
            observer.update(sahamPadang, sahamSunda, sahamTegal);
        }
    }

    public void setSahamPadang(double newSahamPadang) {
        this.sahamPadang = newSahamPadang;
        notifyObserver();
    }

    public void setSahamSunda(double newSahamSunda) {
        this.sahamSunda = newSahamSunda;
        notifyObserver();
    }

    public void setSahamTegal(double newSahamTegal) {
        this.sahamTegal = newSahamTegal;
        notifyObserver();
    }
}

// StockObserver.java
public class StockObserver implements Observer {
    private double sahamPadang;
    private double sahamSunda;
    private double sahamTegal;

    private static int observerIDTracker = 0;

    private int observerID;

    private Subject stockGrabber;

    public StockObserver(Subject stockGrabber) {
        this.stockGrabber = stockGrabber;
        this.observerID = ++ observerIDTracker;

        System.out.println("New Observer " + observerID);

        stockGrabber.register(this);
    }

    public void update(double sahamPadang, double sahamSunda, double sahamTegal) {
        this.sahamPadang = sahamPadang;
        this.sahamSunda = sahamSunda;
        this.sahamTegal = sahamTegal;

        System.out.println("1. Saham Restaurant Makanan Padang  : " + sahamPadang
                + "\n2. Saham Restaurant Makanan Sunda  : " + sahamSunda
                + "\n3. Saham Restaurant Makanan Tegal  : " + sahamTegal
                + "\n");
    }
}

// GrabStocks.java
public class GrabStocks {
    public static void main(String[] args) {
        StockGrabber stockGrabber = new StockGrabber();
        StockObserver observer1 = new StockObserver(stockGrabber);

        stockGrabber.setSahamPadang(90000.30);
        stockGrabber.setSahamSunda(89000.90);
        stockGrabber.setSahamTegal(95000.10);

        StockObserver observer2 = new StockObserver(stockGrabber);

        stockGrabber.setSahamPadang(91000.50);
        stockGrabber.setSahamSunda(90000.30);
        stockGrabber.setSahamTegal(96000.80);

        Runnable getSahamPadang = new GetTheStocks(stockGrabber, 2, "Restaurant Padang", 91000.50);
        Runnable getSahamSunda = new GetTheStocks(stockGrabber, 2, "Restaurant Sunda", 90000.30);
        Runnable getSahamTegal = new GetTheStocks(stockGrabber, 2, "Restaurant Tegal", 96000.80);

        new Thread(getSahamPadang).start();
        new Thread(getSahamSunda).start();
        new Thread(getSahamTegal).start();
    }
}

// GetTheStocks.java
import java.text.*;

public class GetTheStocks implements Runnable {
    private int startTime;
    private String stock;
    private double saham;

    private Subject stockGrabber;

    public GetTheStocks(Subject stockGrabber, int newStartTime, String newStock, double newSaham) {
        this.stockGrabber = stockGrabber;
        startTime = newStartTime;
        stock = newStock;
        saham = newSaham;
    }

    public void run() {
        for(int i = 1; i <= 30; i++) {
            try {
                Thread.sleep(2000);
            }

            catch(InterruptedException e) {}

            double randNum = (Math.random()) + 10.50;

            DecimalFormat df = new DecimalFormat("##.##");

            saham = Double.valueOf(df.format((saham + randNum)));

            if(stock == "1. Restaurant Padang") ((StockGrabber)stockGrabber).setSahamPadang(saham);
            if(stock == "2. Restaurant Sunda") ((StockGrabber)stockGrabber).setSahamSunda(saham);
            if(stock == "3. Restaurant Tegal") ((StockGrabber)stockGrabber).setSahamTegal(saham);

            System.out.println(stock + " : " + df.format((saham + randNum)) + " " + df.format(randNum));
            System.out.println();
        }
    }
}