Android custom listener for an event

Question

I'm trying to fire an event when an integer value is updated, but it's failing. Here's the code I'm using:

Declaring The Custom Listener

public class fieldactivity extends AppCompatActivity implements View.OnClickListener {
    OnModeUpdate modeupdate; //Create custom listener for mode update

    int mode = 1;

Mode Update Code

 protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fieldsignals);
        Button button = (Button) findViewById(R.id.mode_rotate_button);
        button.setOnClickListener(this);
    }

 @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case(R.id.rotate_button):
                mode += 1;
                modeupdate.onEvent(); //Fire Custom Lisentener - Fails On This Line
        }

    }

Interface Code

public interface OnModeUpdate {
//BreakPoint here, but is never reached
        void onEvent();
    }

    public void setModeupdate(OnModeUpdate eventListener) {
        modeupdate = eventListener;
    }

The error I am getting is:

java.lang.NullPointerException: Attempt to invoke interface method 'void alveare.com.plcsignalconverter.fieldactivity$OnModeUpdate.onEvent()' on a null object reference

Is there something I am missing?

Answer 1

The NullPointerException is caused by calling methods on a null referenc object, which is an object that has not been initialized.

In your case, the null object is modeUpdate. Try to initialize it in the onCreate() of your activity.

modeupdate = new OnModeUpdate() {
    @Override
    public void onEvent() {
      /**
      * Write the code to handle the case
      */
    }
};

Answer 2

More general name for listener is observer * design pattern. Speaking the pattern terminology what you're trying to achieve is to make an observer, i.e modeupdate, get notified when an event (integer value change) occurrs within an observable object, i.e.fieldactivity interacting with a user.

The observer, modeupdate, may be of any type and must implement the OnModeUpdate interface in order to receive notifications from the observable object. E.g.**

public class CustomListener implements OnModeUpdate {
    @Override
    public void onEvent() {
        ...
    }
}

Now you can assign the observer (modeupdate) within the observable object (fieldactivity) by one of the following options (according to your business logic):

• initialize modeupdate with an "external" class instance

  OnModeUpdate modeupdate = new CustomListener();
  

• initialize modeupdate with an anonymous inner class instance within fieldactivity class

  modeupdate = new OnModeUpdate() {
      @Override
      public void onEvent() {
          // Do stuff regarding the mode change
      }
  };
  

• assign modeupdate from outside of the fieldactivity class by calling setModeupdate(...) on the reference to fieldactivity instance ***.

Above said explains the cause of NullPointerException. You never initialized modeupdate. After the initialization of modeupdate the onEvent() method will be triggered on a button click.

Note: you wrote in the comment to your code: "... BreakPoint here, but is never reached." It cannot be reached within the interface declaration, only on an instance of the object that implements the interface.

---

^{* Another name of the pattern is Subscriber-Publisher pattern.
** If OnModeUpdate interface is declared within fieldactivity use public class CustomListener implements fieldactivity.OnModeUpdate
*** Same as you did with the button variable by calling setOnClickListener() outside of the Button class and assigning a listener, i.e. an object (fieldactivity) that implements View.OnClickListener interface}