How do I create a event that fires up every time a new element is added to ArrayList

Question

I want to create an method that fires up every time a new message is added to the groupchat arraylist. Pseudo code:

public void listenForChange(){
   while(true){
        if(new message is added to the groupchat){
             System.out.println(print the last added message);
        }
   }
 }

What I have tried, but doesn't work:

public class Groupe{
   ArrayList<String> groupechat;
   int liveChange;

 public void listenForChange() {
    while(true){
        if (groupchat.size() > liveChange){
            liveChange= gruppenchat.size();
            System.out.println(gruppenchat.get(liveChange-1));
        }
    }
}

Test class:

public class testGruppen extends Thread {
Gruppe gruppe;


public TestGroup(){
    gruppe= new Gruppe("Monday");
}

public void run() {
    System.out.println("listen");
    gruppe.listenForChange();
}
public static void main(String[] args) {
    testGruppen test = new testGruppen();
    test.start();
    test.gruppe.write("1"); // write just adds a new String to groupchat
    test.gruppe.write("2");
    test.gruppe.write("3");
    test.gruppe.write("4");

}

}

Output: 4 instead of 1\\n 2\\n 3\\n 4\\n

Answer 1

What about using decorator:

public static void main(String... args) {
    List<Integer> group = new FireEventListDecorator<>(new ArrayList<>());
    for (int i = 0; i < 3; i++)
        group.add(i);
}

public static class FireEventListDecorator<E> extends AbstractList<E> {

    private final List<E> delegate;

    public FireEventListDecorator(List<E> delegate) {
        this.delegate = delegate;
    }

    @Override
    public void add(int index, E element) {
        delegate.add(index, element);
        fireEvent(element);
    }

    @Override
    public E get(int index) {
        return delegate.get(index);
    }

    @Override
    public int size() {
        return delegate.size();
    }

    private void fireEvent(E element) {
        System.out.println("add: " + element);
    }
}

Answer 2

To avoid a CPU wasteful while (true) loop with polling, use a call-back method via an observer/listener pattern. One way to do this is to give your class that holds the ArrayList a PropertyChangeSupport instance, allow it to accept listeners, and then in the method that changes the ArrayList, notify listeners.

eg,

public class Group {
    // property listened to: ADD_TEXT
    public static final String ADD_TEXT = "add text";

    // the support object
    private PropertyChangeSupport support = new PropertyChangeSupport(this);
    private List<String> chatText = new ArrayList<>();

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        support.addPropertyChangeListener(ADD_TEXT, listener);
    }

    public void addText(String text) {
        String oldValue = "";
        String newValue = text;
        chatText.add(text + "\n");

        // notify listeners
        support.firePropertyChange(ADD_TEXT, oldValue, newValue);
    }
}

And then it can be used like so:

public class TestGroupChat {
    public static void main(String[] args) {
        final Group group = new Group();
        group.addPropertyChangeListener(new GroupListener());
        final String[] texts = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday"};

        new Thread(() -> {
            for (String text : texts) {
                group.addText(text);
                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e) {}
            }
        }) .start();
    }

    private static class GroupListener implements PropertyChangeListener {
        @Override
        public void propertyChange(PropertyChangeEvent evt) {
            // call back method that is called any time the listened-to
            // property has been changed
            System.out.println("Notification: "+ evt.getNewValue());
        }
    }
}

Answer 3

You should take a look at LinkedBlockingQueue class . This class is useful when you want to wake up a thread when a new element is added to a queue. In the example below, everytime you add a new message to the queue, the thread will print the message and wait for the next message.

public class Foo extends Thread {

    LinkedBlockingQueue<String> messagesQueue;

    public Foo(LinkedBlockingQueue<String> messagesQueue) {
        this.messagesQueue = messagesQueue;
    }

    @Override
    public voir run() {
        while(true) {
            String message = messagesQueue.take();
           //The thread will sleep until there is a new message in the queue.
           System.out.println(message);
        }
    }

}