具有单例侦听器的简单示例上的空指针异常

Question

i have here a vary simple example of using a singleton with a custom listener. i had a simmilar problem with some other code and decided to make this example. it is showing the same null pointer problem as the other more complex code.

null pointer is on this line:

  listener.onPass(sendString);

any idea how to fix the problem, and why it is happening?

three classes shown here, StartSelector.java is used to start the other class, Singleton.java class has a listener interface and a singleton, it sends a short text message to the other class, Singleton2.java, this other class implements the callback method and receives the text message String.

starts the Singleton class

 public class StartSelector {

      public static void main(String args[]) {
      Singleton.getInstance();
      } // end main

 }

sends off String message by using custom listener

 public class Singleton {

 private OnPassStringListener listener;
 private static Singleton instance;

 public static Singleton getInstance(){
      if(instance == null){
               instance = new Singleton();
           }
      }
   return instance;
 }

 private Singleton(){
     doIt();
 }

 private void doIt(){
     transmitString("test string from singleton class");
 }

 public void transmitString(String sendString) {
     listener.onPass(sendString);   <-- NULL POINTER EXCEPTION HERE
 }

 public interface OnPassStringListener {
     public void onPass(String stringSend);
 }    

 } // end class Singleton

receives the String message from the other class and implements the callback method for the interface

public class Singleton2 implements Singleton.OnPassStringListener {

   public Singleton2() {
       System.out.println("singleton2 class started");
   }

    @Override
    public void onPass(String stringSend) {
        System.out.println("message received: " + stringSend);
    }

} // end class singleton2

Answer 1

You haven't initialized the listener in that code. A NullPointerException just means that the object that you're attempting to use doesn't point to a valid object. Either initialize the value when you define it, or in the class constructor.

private OnPassStringListener listener = new Singleton2();

or

private Singleton() {
    listener = new Singleton2();
    doIt();
}

Answer 2

public static Singleton getInstance(){
      if(instance == null){
               instance = new Singleton();
           instance.setListener(new Singleton2());
           }
      }
   return instance;
 }

Answer 3

Two options. Either listener is null, or NullPointerException is thrown by some code INSIDE onPass(). Need to see stack dump to know which one it is. (Edit. Never mind, I missed the text of Singleton2. So yes, it's definitely the former one. Your listener at the first singleton is never initialized. You want to initialize it, and/or check for null.

Technically, it might be a good programming style to decouple Singleton and Singleton2. In the Observer pattern (at which the word 'listener' hints), the listenee does not know its listener (other than the very generic fact that it knows it has a listener). So, if you directly refer to Singleton2 from Singleton1, maybe call it something other than Listener.

Answer 4

Your listener is null. Change this line

private OnPassStringListener listener;

to

private OnPassStringListener listener = new Singleton2();