Java 自定义侦听器为空

我在Android上使用过监听器，从来没有遇到过任何问题，但是今天当它试图用Java创建一个简单的监听器时，我得到了NullPointerException，这是什么原因，我如何修复它

这包括3个类：ListenerMainStart.java、SendMessageClass.java和ReceiveMessageClass.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");
    }

}

}

}

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);

}   
}

ListenerMartart.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");
    }

}

}

}

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);

}   
}

ReceiveMessageClass.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");
    }

}

}

}

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);

}   
}

---

在打开计时器之前，似乎需要调用“setOnStringRequestListener”。实际上，该方法从未被调用，onStringListener也从未被设置。你确实调用了接收器中的那一行代码，但当然在那里太晚了。您的main应该实例化接收方和发送方，setOnStringRequestListener，然后设置计时器

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

---

编辑：然后取出接收方中引用发送方的任何代码。使用监听器背后的想法是，这两个类彼此并不直接了解。

用Java实例化监听器的正确或标准方法是什么？使用android时，我会在一个片段的onAttach方法中这样做，但不确定最好的方法是什么。在新的SendMessageClass（）之后，程序不会立即死亡？我没有死，但它打印出消息“onStringListener为null”并停止，我说的是main（）除了new SendMessageClass（）之外什么都没有……谢谢，它现在可以工作了。我还从main方法启动了计时器。Java和Android必须以不同的方式进行。