Observer Pattern | Set 2 (Implementation)

We strongly recommend to refer below Set 1 before moving on to this post.

Observer Pattern -Introduction

In Set 1, we discussed below problem, a solution for the problem without Observer pattern and problems with the solution.

Suppose we are building a cricket app that notifies viewers about the information such as current score, run rate etc. Suppose we have made two display elements CurrentScoreDisplay and AverageScoreDisplay. CricketData has all the data (runs, bowls etc.) and whenever data changes the display elements are notified with new data and they display the latest data accordingly

Applying Observer pattern to above problem:
Let us see how we can improve the design of our application using observer pattern. If we observe the flow of data we can easily see that the CricketData and display elements follow subject-observers relationship.

New Class Diagram:

Java Implementation:

// Java program to demonstrate working of
// onserver pattern
import java.util.ArrayList;
import java.util.Iterator;

// Implemented by Cricket data to communicate
// with observers
interface Subject
{
    public void registerObserver(Observer o);
    public void unregisterObserver(Observer o);
    public void notifyObservers();
}

class CricketData implements Subject
{
    int runs;
    int wickets;
    float overs;
    ArrayList<Observer> observerList;

    public CricketData() {
        observerList = new ArrayList<Observer>();
    }

    @Override
    public void registerObserver(Observer o) {
        observerList.add(o);
    }

    @Override
    public void unregisterObserver(Observer o) {
        observerList.remove(observerList.indexOf(o));
    }

    @Override
    public void notifyObservers()
    {
        for (Iterator<Observer> it =
              observerList.iterator(); it.hasNext();)
        {
            Observer o = it.next();
            o.update(runs,wickets,overs);
        }
    }

    // get latest runs from stadium
    private int getLatestRuns()
    {
        // return 90 for simplicity
        return 90;
    }

    // get latest wickets from stadium
    private int getLatestWickets()
    {
        // return 2 for simplicity
        return 2;
    }

    // get latest overs from stadium
    private float getLatestOvers()
    {
        // return 90 for simplicity
        return (float)10.2;
    }

    // This method is used update displays
    // when data changes
    public void dataChanged()
    {
        //get latest data
        runs = getLatestRuns();
        wickets = getLatestWickets();
        overs = getLatestOvers();

        notifyObservers();
    }
}

// This interface is implemented by all those
// classes that are to be updated whenever there
// is an update from CricketData
interface Observer
{
    public void update(int runs, int wickets,
                      float overs);
}

class AverageScoreDisplay implements Observer
{
    private float runRate;
    private int predictedScore;

    public void update(int runs, int wickets,
                       float overs)
    {
        this.runRate =(float)runs/overs;
        this.predictedScore = (int)(this.runRate * 50);
        display();
    }

    public void display()
    {
        System.out.println("\nAverage Score Display: \n"
                           + "Run Rate: " + runRate +
                           "\nPredictedScore: " +
                           predictedScore);
    }
}

class CurrentScoreDisplay implements Observer
{
    private int runs, wickets;
    private float overs;

    public void update(int runs, int wickets,
                       float overs)
    {
        this.runs = runs;
        this.wickets = wickets;
        this.overs = overs;
        display();
    }

    public void display()
    {
        System.out.println("\nCurrent Score Display:\n"
                           + "Runs: " + runs +
                           "\nWickets:" + wickets +
                           "\nOvers: " + overs );
    }
}

// Driver Class
class Main
{
    public static void main(String args[])
    {
        // create objects for testing
        AverageScoreDisplay averageScoreDisplay =
                          new AverageScoreDisplay();
        CurrentScoreDisplay currentScoreDisplay =
                          new CurrentScoreDisplay();

        // pass the displays to Cricket data
        CricketData cricketData = new CricketData();

        // register display elements
        cricketData.registerObserver(averageScoreDisplay);
        cricketData.registerObserver(currentScoreDisplay);

        // in real app you would have some logic to
        // call this function when data changes
        cricketData.dataChanged();

        //remove an observer
        cricketData.unregisterObserver(averageScoreDisplay);

        // now only currentScoreDisplay gets the
        // notification
        cricketData.dataChanged();
    }
}

Output:

Average Score Display: 
Run Rate: 8.823529
PredictedScore: 441

Current Score Display:
Runs: 90
Wickets:2
Overs: 10.2

Current Score Display:
Runs: 90
Wickets:2
Overs: 10.2

Note: Now we can add/delete as many observers without changing the subject.

References:

1. https://en.wikipedia.org/wiki/Observer_pattern
2. Head First Design Patterns book (highly recommended)

This article is contributed by Sulabh Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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