Java: Problems with wait () and notify ()

Good evening, I'm doing a java project where I make connections between the client and a server. This client sends messages to another recipient. It is up to the Server to handle this message, forwarding it to the right Client. I am an error:

  

Exception in thread "Thread-4" java.lang.IllegalMonitorStateException       at java.lang.Object.wait (Native Method)       at java.lang.Object.wait (Object.java:502)       at network.WorkerRunnableOut.run (WorkerRunnableOut.java:34)       at java.lang.Thread.run (Thread.java:745)

What happens soon after connecting the Client to the server. I then leave some of my code to see where the error comes from.

public class WorkerRunnableOut implements Runnable{

protected Socket clientSocket = null;
protected String serverText = null;
protected ServerMulti server;
ArrayList <Message> msmDelevery;

public WorkerRunnableOut(ServerMulti server, Socket clientSocket, String serverText) {
    this.clientSocket = clientSocket;
    this.serverText = serverText;
    this.server = server;
    msmDelevery = new ArrayList<Message>();
}

public void run() {
    try {
        ObjectOutputStream output = new ObjectOutputStream(clientSocket.getOutputStream());
        long time = System.currentTimeMillis();
        output.write(("HTTP/1.1 200 OK\n\nWorkerRunnable: " +
                this.serverText + " - " +
                time +
                "").getBytes());
        while(msmDelevery.isEmpty()) {
            System.out.println("Servidor : nenhuma mensagem para entregar");
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }   
        for(int i = 0; i<server.msgToDelevery.size(); i++){
            if(msmDelevery.get(i)!=null) {
                Message m = msmDelevery.get(i);
                String id = msmDelevery.get(i).getDest();
                if(server.clientesOn.containsKey(id)) {
                    server.clientesOn.get(id).sendMessage(output, m);
                }

            }



        }

    } catch (IOException e) {
        e.printStackTrace();
    }
}

private void sendMessage(ObjectOutputStream o, Message m) {
    // TODO Auto-generated method stub
    try {
        o.writeObject(m);
        o.flush();
    } catch (IOException e) {
        System.out.println("Erro ao enviar para o cliente");
    }

}

public void addMessage(Message m) {
    msmDelevery.add(m);
    notify();
}

}

.

public class WorkerRunnableIn implements Runnable {
protected Socket clientSocket = null;
protected String serverText = null;
protected ServerMulti server;

public WorkerRunnableIn(ServerMulti server, Socket clientSocket, String serverText) {
    this.clientSocket = clientSocket;
    this.serverText = serverText;
    this.server = server;

}

public void run() {
    try {
        ObjectInputStream input = new ObjectInputStream(clientSocket.getInputStream());

        WorkerRunnableOut wout = new WorkerRunnableOut(server, clientSocket, serverText);
        new Thread(wout).start();

        while (true) {
            Message m = (Message) input.readObject();
            System.out.println("Teste de m >>> " + m);
            if(m.getDest()==null) {
                String usernames = m.getUser();
                server.clientesOn.put(usernames, wout);
                System.out.println("Adicionei " + usernames + " à lista de clientes On");
            }
            else {
                wout.addMessage(m);
                System.out.println("Entrei aqui");
            }
        }

    } catch (IOException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

Thank you