Observer

1. what is Observer patten
    Observer adalah design pattern yang dimana suatu objek mempunyai daftar objek yang memiliki ketergantungan / dependencies. desing pattern observer dapat digunakan  setiap kali subjek harus diamati oleh satu atau lebih observer.
2. what are their benefit
    Kita bisa melihat stock barang dan menupdate barang yang telah habis sehingga kita tidak bingung ketika ada perubahan pada stock tersebut.
3. what are their drawback
    Kelemahan nya adalah observer ini hanya cocok untuk system stock seperti toko online.
4. when to use it at best
    > Ketika kita membutuhkan banyak objek lain untuk menerima pembaruan ketika objek lain berubah.
    > Ketika sebuah bisnis yang memiliki pasar saham dengan ribuan stock perlu mengirim pembaruan ke objek yang mewakili saham individual.
    > subjek(penerbit) bisa mengirim banyak stok kepada pelanggan.
5. when to NOT use it
    ketika kita tidak memerlukan penambahan objek yang akan dirubah dan apabila kita tidak memerlukan pelanggan karena pattern inin hanya cocok untuk perusahaan yang seperti swalayan atau mall.



package com.rahmadnet;

public class GrabStocks
{
    public static void main(String[] args)
    {
        StockGrabber stockGrabber = new StockGrabber();

        StockObserver observer1 = new StockObserver(stockGrabber);

        stockGrabber.setIBMPrice(197.00);
        stockGrabber.setAAPLPrice(677.60);
        stockGrabber.setGOOGPrirece(676.40);

        StockObserver observer2 = new StockObserver(stockGrabber);

        stockGrabber.setIBMPrice(20.00);
        stockGrabber.setAAPLPrice(20.00);
        stockGrabber.setGOOGPrirece(20.00);

        stockGrabber.unregister(observer1);

        stockGrabber.setIBMPrice(197.00);
        stockGrabber.setAAPLPrice(677.60);
        stockGrabber.setGOOGPrirece(676.40);


        Runnable getIBM = new GetTheStock(stockGrabber, 2, "IBM", 2019.00);
        Runnable getAAPL = new GetTheStock(stockGrabber, 2, "AAPL", 2019.00);
        Runnable getGOOG = new GetTheStock(stockGrabber, 2, "GOOG", 2019.00);

        new Thread(getIBM).start();
        new Thread(getAAPL).start();
        new Thread(getGOOG).start();
    }
}

package com.rahmadnet;

public interface Subject
{
    public void regieter(Observer o);
    public void unregister(Observer o);
    public void notifyObserver();
}


package com.rahmadnet;

import java.util.ArrayList;

public class StockGrabber implements Subject
{

    private ArrayList<Observer> observer;
    private double ibmPrice;
    private double aaplPrice;
    private double googPrice;

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

    @Override
    public void regieter(Observer newObserver)
    {
        observer.add(newObserver);
    }

    @Override
    public void unregister(Observer deleteObserver)
    {
        int observerIndex = observer.indexOf(deleteObserver);

        System.out.println("Observer " + (observerIndex + 1) + " delete");

        observer.remove(observerIndex);
    }

    @Override
    public void notifyObserver()
    {
        for(Observer observer : observer)
        {
            observer.update(ibmPrice, aaplPrice, googPrice);
        }
    }

    public void setIBMPrice(double newIBMPrice)
    {
        this.ibmPrice = newIBMPrice;
        notifyObserver();
    }

    public void setAAPLPrice(double newAAPLPrice)
    {
        this.aaplPrice = newAAPLPrice;
        notifyObserver();
    }

    public void setGOOGPrirece(double newGOOGPrice)
    {
        this.googPrice = newGOOGPrice;
        notifyObserver();
    }

}

package com.rahmadnet;

public interface Observer
{
    public void update(double ibmPrice, double aaplPrice, double googPrice);
}


package com.rahmadnet;

public class StockObserver implements Observer
{

    private double ibmPrice;
    private double aaplPrice;
    private double googPrice;

    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 " + this.observerID);

        stockGrabber.regieter(this);
    }


    @Override
    public void update(double ibmPrice, double aaplPrice, double googPrice) {

        this.ibmPrice = ibmPrice;
        this.aaplPrice = aaplPrice;
        this.googPrice = googPrice;

        printThePrices();

    }

    public void printThePrices()
    {
        System.out.println(observerID + "\nIBM : " + ibmPrice +
                        "\nAAPL : " + aaplPrice + "\nGOOG : "+ googPrice + "\n");
    }

}

package com.rahmadnet;

import java.text.DecimalFormat;

public class GetTheStock implements Runnable
{
    private int startTime;
    private String stock;
    private double price;

    private Subject stockGrabber;

    public GetTheStock(Subject stockGrabber, int newStartTime, String newStock,
    double newPrice)
    {
        this.stockGrabber = stockGrabber;

        startTime = newStartTime;

        stock = newStock;
        price = newPrice;
    }

    @Override
    public void run() {

        for(int i = 1; i <= 20; i++)
        {
            try
            {
                Thread.sleep(1000);
            }
            catch (InterruptedException e) {}

            double randNum = (Math.random() * (.06) - .03);

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

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

            if(stock == "IBM") ((StockGrabber)stockGrabber).setIBMPrice(price);

            if(stock == "AAPL") ((StockGrabber)stockGrabber).setAAPLPrice(price);

            if(stock == "GOOG") ((StockGrabber)stockGrabber).setGOOGPrirece(price);

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

            System.out.println();
        }
    }
}