CODEWITHSUNDEEP
javalisteneractionlistener
Kevik
Kevik

Reputation: 9351

custom listener is null

I have used listeners with Android and never had any problems, however today when it tried to create a simple listener in Java I am getting NullPointerException, what is the cause of this and how can I fix it?

this includes 3 classes, ListenerMainStart.java, SendMessageClass.java, and ReceiveMessageClass.java

ListenerMainStart.java

public class ListenerMainStart {

public static void main(String[] args) {

    new SendMessageClass();

}
}

SendMessageClass.java

public class SendMessageClass {

public OnStringRequestedListener onStringListener;

public Timer timer;

public SendMessageClass() {

    timer = new Timer();
    timer.schedule(new TimerAction(), 3000);

}

public void SetOnStringRequestedListener(OnStringRequestedListener listener) {
       System.out.println("value of onStringListener " + onStringListener.toString());
       onStringListener = listener;

    }

public interface OnStringRequestedListener {

public void passString(String sendString);

}

public class TimerAction extends TimerTask {

@Override
public void run() {

    if (onStringListener!=null){
   // pass string to other class, ONSTRINGLISTENER IS ALWASY NULL
   onStringListener.passString("string sent from SendMessageclass");
    } else {
        System.out.println("onStringListener is null");
    }

}

}

}

ReceiveMessageClass.java

public class ReceiveMessageClass implements SendMessageClass.OnStringRequestedListener {

SendMessageClass senderClass;

public ReceiveMessageClass() {

// instantiate class
senderClass = new SendMessageClass();

   // set the listener to the class
senderClass.SetOnStringRequestedListener(this);

}

@Override
public void passString(String sendString) {

     // do action when string is sent from other class
     System.out.println("message recevied from other class is " + sendString);

}   
}

Upvotes: 1

Views: 199

Answers (1)

MikeHelland
MikeHelland

Reputation: 1159

It looks like you need to call "SetOnStringRequestedListener" before you turn on your timers. As it is, that method never gets called and onStringListener never gets set. You do call that line of code in the receiver, but of course its far too late there. Your main should instantantiate both the receiver and the sender, SetOnStringRequestedListener, and then set off the timers.

SendMessageClass send = new SendMessageClass();
ReceiveMessageClass recv = new ReceiveMessageClass()
send.SetOnStringRequestedListener(recv)

EDIT: Then take out any code in the receiver that references the sender. The idea behind using the listener is that the two classes don't know directly about each other.

Upvotes: 2

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