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Observer.java

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import java.util.ArrayList;

interface Subject {

public void registerObserver(Observe o);

public void removeObserver(Observe o);

public void notifyAllObservers();

}

interface Observe {

public void update(Subject s);

}

class VideoSite implements Subject {

private ArrayList<Observe> userList;

private ArrayList<String> videos;

public VideoSite() {

userList = new ArrayList<>();

videos = new ArrayList<>();

}

@Override

public void registerObserver(Observe o) {

userList.add(o);

}

@Override

public void removeObserver(Observe o) {

userList.remove(o);

}

@Override

public void notifyAllObservers() {

for (Observe o : userList) {

o.update(this);

}

}

public void addVideos(String video) {

this.videos.add(video);

notifyAllObservers();

}

public ArrayList<String> getVideos() {

return videos;

}

public String toString() {

return videos.toString();

}

}

class VideoFan implements Observe {

private String name;

public VideoFan(String name) {

this.name = name;

}

@Override

public void update(Subject s) {

System.out.println(this.name + ", new videos are available! ");

System.out.println(s);

}

}

public class Observer {

public static void main(String[] args) {

VideoSite vs = new VideoSite();

vs.registerObserver(new VideoFan("Siri"));

vs.registerObserver(new VideoFan("Alexa"));

vs.registerObserver(new VideoFan("Cornor"));

vs.addVideos("Big Bang Theories");

}

}