title: NullPointerException while working with GestureDetector
body: <p>Below is the code of loading frameanimation for two different images on the occurrence of different events.First event is while the activity start.Others are the <code>onTouch()</code> where i making use of <code>GestureDetector</code> for the <code>onDown()</code> and <code>onScroll()</code> The problem is that i get <code>NullPointerException</code> while its in the <code>OnScroll()</code>.I found that it is not getting the Y coordinates what can be the issue.</p>
  
  <pre><code>package com.example.animationtry;
  
  import android.os.Bundle;
  import android.app.Activity;
  import android.graphics.drawable.AnimationDrawable;
  import android.util.Log;
  import android.view.GestureDetector;
  import android.view.MotionEvent;
  import android.view.View;
  import android.view.GestureDetector.SimpleOnGestureListener;
  import android.widget.ImageView;
  
  public class MainActivity extends Activity
  {
      ImageView img,img1;
      AnimationDrawable animation;
      private GestureDetector gestureDetector;
  
  
      @Override
      protected void onCreate(Bundle savedInstanceState) 
      {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
  
          img=(ImageView)findViewById(R.id.image);
          img.post(new Runnable()
          {
              public void run()
              {
                  img.setBackgroundResource(R.anim.frameanimation);
                  AnimationDrawable frameAnimation =  (AnimationDrawable) img .getBackground();
                  frameAnimation.setVisible(false, true);
                  frameAnimation.start();
              }
          });
          img.setOnTouchListener(new View.OnTouchListener() 
          {
              public boolean onTouch(View v, MotionEvent event) 
              {
                  String img="img";
                  gestureDetector = new GestureDetector(new MyGestureDetector(img));
                  if (gestureDetector.onTouchEvent(event)) 
                  {
                      return true;
                  }   
                  return false;
              }
          });
  
          img1=(ImageView)findViewById(R.id.image1);
          img.post(new Runnable()
          {
              public void run()
              {
                  img1.setBackgroundResource(R.anim.frameanimation);
                  AnimationDrawable frameAnimation =  (AnimationDrawable) img1.getBackground();
                  frameAnimation.setVisible(false, true);
                  frameAnimation.start();
              }
          });
          img1.setOnTouchListener(new View.OnTouchListener() 
          {
              public boolean onTouch(View v, MotionEvent event) 
              {
                  String img1="img1";
                  gestureDetector = new GestureDetector(new MyGestureDetector(img1));
                  if (gestureDetector.onTouchEvent(event)) 
                  {
                      return true;
                  }   
                  return false;
              }
          });
      }
  
      class MyGestureDetector extends SimpleOnGestureListener
      {
          private static final int SWIPE_MAX_OFF_PATH = 40;
          private static final int SWIPE_THRESHOLD_VELOCITY = 30;
          String viewname;
          public MyGestureDetector(String view)
          {
              viewname=view;
          }
  
          @Override
          public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,float velocityY) 
          {
              return true;
          }
          @Override
          public boolean onDown(MotionEvent e) 
          {
              Log.i("View Name",viewname);
              if(viewname.equalsIgnoreCase("img"))
              {
                  img.setBackgroundResource(R.anim.clickanimation);
                  animation = (AnimationDrawable)img.getBackground();
                  animation.setVisible(false, true);
                  animation.start();
              }
              else if(viewname.equalsIgnoreCase("img1"))
              {
                  img1.setBackgroundResource(R.anim.clickanimation);
                  animation = (AnimationDrawable)img1.getBackground();
                  animation.setVisible(false, true);
                  animation.start();
              }
              return true;
          }
  
          @Override
          public boolean onScroll(MotionEvent e1, MotionEvent e2,float distanceX, float distanceY) 
          {
              Log.i("y1",Float.toString(e1.getY()));
              Log.i("y2",Float.toString(e2.getY()));
              Log.i("Drag View Name",viewname);
              if (Math.abs(e1.getY() - e2.getY()) &gt; SWIPE_MAX_OFF_PATH &amp;&amp; Math.abs(distanceY) &gt; SWIPE_THRESHOLD_VELOCITY) 
              {
                  if(viewname.equalsIgnoreCase("img"))
                  {
                      img.setBackgroundResource(R.anim.draganimaton);
                      animation = (AnimationDrawable)img.getBackground();
                      animation.setVisible(false, true);
                      animation.start();
                  }
                  else if(viewname.equalsIgnoreCase("img1"))
                  {
                      img1.setBackgroundResource(R.anim.draganimaton);
                      animation = (AnimationDrawable)img1.getBackground();
                      animation.setVisible(false, true);
                      animation.start();
                  }
                  return true;
              }
              return true;
          }
      }
  }
  </code></pre>
  
  <p>and below is the logcat:</p>
  
  <pre><code>01-12 06:12:22.637: E/AndroidRuntime(1062): FATAL EXCEPTION: main
  01-12 06:12:22.637: E/AndroidRuntime(1062): java.lang.NullPointerException
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at com.example.animationtry.MainActivity$MyGestureDetector.onScroll(MainActivity.java:117)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.GestureDetector.onTouchEvent(GestureDetector.java:541)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at com.example.animationtry.MainActivity$4.onTouch(MainActivity.java:69)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.View.dispatchTouchEvent(View.java:3881)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:942)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:942)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:942)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:942)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1691)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1125)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.app.Activity.dispatchTouchEvent(Activity.java:2096)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1675)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.ViewRoot.deliverPointerEvent(ViewRoot.java:2194)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1878)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.os.Handler.dispatchMessage(Handler.java:99)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.os.Looper.loop(Looper.java:123)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at android.app.ActivityThread.main(ActivityThread.java:3683)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at java.lang.reflect.Method.invokeNative(Native Method)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at java.lang.reflect.Method.invoke(Method.java:507)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
  01-12 06:12:22.637: E/AndroidRuntime(1062):     at dalvik.system.NativeStart.main(Native Method)
  </code></pre>