ObserverPattern.java

import java.util.ArrayList;

interface Subject {
    public void registerObserver(Observer observer);
    public void removeObserver(Observer observer);
    public void notifyAllObservers();
}

class DataStore implements Subject{
    private ArrayList<Observer> observers;
    public DataStore() {
        this.observers = new ArrayList<>();
    }
    @Override
    public void registerObserver(Observer observer) {
        this.observers.add(observer);
    }
    @Override
    public void removeObserver(Observer observer) {
        int index = this.observers.indexOf(observer);
        if(index >= 0) {
            this.observers.remove(index);
        }
    }
    @Override
    public void notifyAllObservers() {
        for(Observer observer: this.observers) {
            observer.update();
        }
    }
    public void dataChanged() {
        notifyAllObservers();
    }
    public void setData() {
        // We can set data in this object as needed.
        dataChanged();
    }
}

interface Observer {
    public void update();
}

class GraphView implements  Observer{
    private Subject dataStore;
    public GraphView(Subject dataStore) {
        this.dataStore = dataStore;
        dataStore.registerObserver(this);
    }
    @Override
    public void update() {
        //Change the state of this object as needed here
        System.out.println("Update called in GraphView");
    }

}

class GridView implements  Observer{
    private Subject dataStore;
    public GridView(Subject dataStore) {
        this.dataStore = dataStore;
        dataStore.registerObserver(this);
    }
    @Override
    public void update() {
        //Change the state of this object as needed here
        System.out.println("Update called in GridView");
    }

}

public class ObserverPattern {
    public static void main(String[] args) {
        DataStore dataStore = new DataStore();
        Observer graphView = new GraphView(dataStore);
        Observer gridView = new GridView(dataStore);
        //More and more views can be added here if neededd
        System.out.println("Before change of state in observer");
        dataStore.setData(); //Some data to be passed.
        System.out.println("After change of state in observer");
    }
}