Android - onTouch in HorizontalScrollView gives NullPointerException

Question

I'm trying to make a Horizontal Scroll View that snaps to certain places. I've tried doing that with a custom HorizontalScrollView that will "return" it's position using a listener. Then, in the fragment that containts it, it should check for a touch event, and at the end of touch it should check for this position and scroll the view accordingly.

Here's the code:

 public class CustomHorizontalScrollView extends HorizontalScrollView{

    private OnPositionChangeListener listener;

    public CustomHorizontalScrollView(Context context) {
        super(context);

    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt){
        listener.onPosChange(l);
        super.onScrollChanged(l, t, oldl, oldt);

    }

    public interface OnPositionChangeListener {
        public void onPosChange(int l);
      }
}

public class CustomFragment extends Fragment implements ArgonHorizontalScrollView.OnPositionChangeListener{

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        hScV = new CustomHorizontalScrollView(getActivity());


        hScV.setOnTouchListener(new OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                 if(event.getAction() == MotionEvent.ACTION_UP){

                 if(pos>480)
                 hScV.scrollTo(X, 0);

            return true;
        }
                return false;
            }
        });
}

    @Override
    public void onPosChange(int l) {
        pos=l;

    }

This should be the essential code.

The errors appear when i touch the screen, i don't have any idea why. Here's the log(a long one):

java.lang.NullPointerException
03-16 11:48:01.135: E/AndroidRuntime(1118):     at com.ex.musicplayer.CustomHorizontalScrollView.onScrollChanged(CustomHorizontalScrollView.java:18)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.View.scrollTo(View.java:10590)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.widget.HorizontalScrollView.onOverScrolled(HorizontalScrollView.java:751)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.View.overScrollBy(View.java:17501)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.widget.HorizontalScrollView.onTouchEvent(HorizontalScrollView.java:605)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.View.dispatchTouchEvent(View.java:7690)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2210)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1945)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2216)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1959)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2066)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1515)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.app.Activity.dispatchTouchEvent(Activity.java:2457)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.support.v7.app.ActionBarActivityDelegateICS$WindowCallbackWrapper.dispatchTouchEvent(ActionBarActivityDelegateICS.java:260)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2014)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.View.dispatchPointerEvent(View.java:7870)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:3919)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3808)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3406)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3456)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3425)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3510)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3433)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3567)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3406)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3456)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3425)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3433)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3406)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5520)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5500)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5471)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:5594)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:182)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.InputEventReceiver.nativeConsumeBatchedInputEvents(Native Method)
03-16 11:48:01.135: E/AndroidRuntime(1118):     at android.view.InputEventRe
03-16 11:48:09.325: I/Process(1118): Sending signal. PID: 1118 SIG: 9

Answer 1

You didn't initialize listener but then you try using it here:

listener.onPosChange(l);

==> NPE