java - 自定义监听器为空

Question

我在 Android 中使用了监听器，从未遇到过任何问题，但是今天当它尝试在 Java 中创建一个简单的监听器时，我收到 NullPointerException，这是什么原因以及如何修复它？

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

看来您需要在打开计时器之前调用“SetOnStringRequestedListener”。事实上，该方法永远不会被调用，并且 onStringListener 永远不会被设置。您确实在接收器中调用了该行代码，但当然已经太晚了。您的 main 应该实例化接收者和发送者 SetOnStringRequestedListener，然后关闭计时器。

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

编辑:然后取出接收者中引用发送者的所有代码。使用监听器背后的想法是，这两个类并不直接了解彼此。