Question

When the view is removed, errors occurred, I can not find the cause of the errors：

**java:**

  package com.test;
    import android.app.Activity;
    import android.os.Bundle;
    import android.util.Log;
    import android.view.View;
    import android.view.ViewGroup;
    import android.view.animation.AlphaAnimation;
    import android.view.animation.Animation;
    import android.view.animation.Animation.AnimationListener;
    import android.widget.ImageView;

    public class AnimationActivity extends Activity {
        ImageView imageView = null;
        ViewGroup viewGroup = null;
        /** Called when the activity is first created. */
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.main);
            imageView = (ImageView)findViewById(R.id.imageviewID);
            viewGroup = (ViewGroup)findViewById(R.id.layoutId); 
        }
        public void buttonOnclick(View e){
            int id = e.getId();
            if(id==R.id.alpha){
                AlphaAnimation alphaAnimation = new AlphaAnimation(1.0f, 0f);
                alphaAnimation.setDuration(5000);
                alphaAnimation.setStartOffset(500);
                alphaAnimation.setFillAfter(true);
                alphaAnimation.setFillBefore(false);
                alphaAnimation.setAnimationListener(new RemoveAnimationListener());
                imageView.startAnimation(alphaAnimation);
                Log.d("mydebug","imageView------------>"+imageView);
                Log.d("mydebug","viewGroup------------>"+viewGroup);
            }
        }
        private class RemoveAnimationListener implements AnimationListener{

            @Override
            public void onAnimationEnd(Animation animation) {
                // TODO Auto-generated method stub
                viewGroup.removeView(imageView);

            }
            @Override
            public void onAnimationRepeat(Animation animation) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onAnimationStart(Animation animation) {
                // TODO Auto-generated method stub

            }
        }
    }

Here are errors：

  05-22 10:01:16.367: W/dalvikvm(1839): threadid=1: thread exiting with uncaught exception (group=0x4001d7c8)ss
    05-22 10:01:16.397: E/AndroidRuntime(1839): FATAL EXCEPTION: main
    05-22 10:01:16.397: E/AndroidRuntime(1839): java.lang.NullPointerException
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1366)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewGroup.drawChild(ViewGroup.java:1638)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1367)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.View.draw(View.java:6743)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.widget.FrameLayout.draw(FrameLayout.java:352)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewGroup.drawChild(ViewGroup.java:1640)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1367)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewGroup.drawChild(ViewGroup.java:1638)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1367)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.View.draw(View.java:6743)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.widget.FrameLayout.draw(FrameLayout.java:352)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1857)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewRoot.draw(ViewRoot.java:1407)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewRoot.performTraversals(ViewRoot.java:1163)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1727)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.os.Handler.dispatchMessage(Handler.java:99)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.os.Looper.loop(Looper.java:123)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at android.app.ActivityThread.main(ActivityThread.java:4627)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at java.lang.reflect.Method.invokeNative(Native Method)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at java.lang.reflect.Method.invoke(Method.java:521)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
    05-22 10:01:16.397: E/AndroidRuntime(1839):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

You have use buttononclick method in your AnimationActivity class. So if you have
mentioned which button click in your XML file then good. If you have not mention
in XML which button is click, then you must mention in your activity which button
click so that the corresponding method call is made. One more thing is that if it did not
find view group so that it shows null pointer exception. You are trying to access or
use view group which value is null or not found.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/layoutId"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <ImageView android:id="@+id/imageviewID"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_launcher"
        android:contentDescription="@string/app_name"
        android:paddingBottom="50dip"
        />
    <Button android:id="@+id/alpha"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/alpha"
        android:onClick="buttonOnclick"
        android:layout_below="@id/imageviewID"
        />
</RelativeLayout>