Question

我已经使用Android的听众并且从来没有遇到任何问题，但是今天当它试图用Java创建一个简单的监听器时我得到NullPointerException，原因是什么以及如何解决？

这包括3个类，ListenerMainStart.java，SendMessageClass.java和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);

}   
}

Answer 1

在打开定时器之前，您似乎需要调用“SetOnStringRequestedListener”。实际上，该方法永远不会被调用，并且onStringListener永远不会被设置。你确实在接收器中调用了那行代码，但当然在那里为时已晚。你的主要应该立即同步接收者和发送者，SetOnStringRequestedListener，然后设置定时器。

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

编辑：然后取出接收器中引用发件人的任何代码。使用监听器的想法是这两个类不直接相互了解。

自定义侦听器为null

1 个答案: