Android, crossfading between 2 views that are different images using a button

Question

I am having an issue crossfading between 2 images using a button. The button isn't implemented but the problem is that I can't even load the image view. Eclipse doesn't give an error in the java code, but when I try to run the application, it crashes. I've included the MainActivity.java, the LogCat, and the XML file below.

public class MainActivity extends ActionBarActivity {

    private View imageView1;
    private View imageView2;
    private int mShortAnimationDuration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        }

        imageView1 = findViewById(R.id.imageView1);
        imageView2 = findViewById(R.id.imageView2);

        // Initially hide the content view.
        imageView2.setVisibility(View.GONE);

        // Retrieve and cache the system's default "short" animation time.
        mShortAnimationDuration = getResources().getInteger(
        android.R.integer.config_shortAnimTime);
    }

    private void crossfade() {

        // Set the content view to 0% opacity but visible, so that it is visible
        // (but fully transparent) during the animation.
        imageView1.setAlpha(0f);
        imageView1.setVisibility(View.VISIBLE);

        // Animate the content view to 100% opacity, and clear any animation
        // listener set on the view.
        imageView1.animate()
                .alpha(1f)
                .setDuration(mShortAnimationDuration)
                .setListener(null);

        // Animate the loading view to 0% opacity. After the animation ends,
        // set its visibility to GONE as an optimization step (it won't
        // participate in layout passes, etc.)
        imageView2.animate()
                .alpha(0f)
                .setDuration(mShortAnimationDuration)
                .setListener(new AnimatorListenerAdapter() {
                    @Override
                    public void onAnimationEnd(Animator animation) {
                        imageView2.setVisibility(View.GONE);
                    }
                });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }
    }

}

My xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="ca.mohawk.you.lab6a.MainActivity$PlaceholderFragment" >

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:src="@drawable/desert" />

    <ImageView
        android:id="@+id/imageView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:src="@drawable/desert" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="42dp"
        android:text="@string/button_click" />

</RelativeLayout>

Logcat:

04-07 21:51:13.463: D/AndroidRuntime(1099): Shutting down VM
04-07 21:51:13.473: W/dalvikvm(1099): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 21:51:13.483: E/AndroidRuntime(1099): FATAL EXCEPTION: main
04-07 21:51:13.483: E/AndroidRuntime(1099): Process: ca.mohawk.you.lab6a, PID: 1099
04-07 21:51:13.483: E/AndroidRuntime(1099): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.os.Looper.loop(Looper.java:136)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at dalvik.system.NativeStart.main(Native Method)
04-07 21:51:13.483: E/AndroidRuntime(1099): Caused by: java.lang.NullPointerException
04-07 21:51:13.483: E/AndroidRuntime(1099):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 21:51:13.483: E/AndroidRuntime(1099):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 21:51:13.483: E/AndroidRuntime(1099):     ... 11 more
04-07 21:54:14.453: D/AndroidRuntime(1164): Shutting down VM
04-07 21:54:14.453: W/dalvikvm(1164): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 21:54:14.463: E/AndroidRuntime(1164): FATAL EXCEPTION: main
04-07 21:54:14.463: E/AndroidRuntime(1164): Process: ca.mohawk.you.lab6a, PID: 1164
04-07 21:54:14.463: E/AndroidRuntime(1164): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.os.Looper.loop(Looper.java:136)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at dalvik.system.NativeStart.main(Native Method)
04-07 21:54:14.463: E/AndroidRuntime(1164): Caused by: java.lang.NullPointerException
04-07 21:54:14.463: E/AndroidRuntime(1164):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 21:54:14.463: E/AndroidRuntime(1164):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 21:54:14.463: E/AndroidRuntime(1164):     ... 11 more
04-07 21:54:17.973: I/Process(1164): Sending signal. PID: 1164 SIG: 9
04-07 22:01:16.333: D/AndroidRuntime(1184): Shutting down VM
04-07 22:01:16.333: W/dalvikvm(1184): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 22:01:16.353: E/AndroidRuntime(1184): FATAL EXCEPTION: main
04-07 22:01:16.353: E/AndroidRuntime(1184): Process: ca.mohawk.you.lab6a, PID: 1184
04-07 22:01:16.353: E/AndroidRuntime(1184): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.os.Looper.loop(Looper.java:136)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at dalvik.system.NativeStart.main(Native Method)
04-07 22:01:16.353: E/AndroidRuntime(1184): Caused by: java.lang.NullPointerException
04-07 22:01:16.353: E/AndroidRuntime(1184):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 22:01:16.353: E/AndroidRuntime(1184):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 22:01:16.353: E/AndroidRuntime(1184):     ... 11 more
04-07 22:01:59.343: I/Process(1184): Sending signal. PID: 1184 SIG: 9
04-07 22:02:01.843: D/AndroidRuntime(1205): Shutting down VM
04-07 22:02:01.843: W/dalvikvm(1205): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 22:02:01.853: E/AndroidRuntime(1205): FATAL EXCEPTION: main
04-07 22:02:01.853: E/AndroidRuntime(1205): Process: ca.mohawk.you.lab6a, PID: 1205
04-07 22:02:01.853: E/AndroidRuntime(1205): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.os.Looper.loop(Looper.java:136)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at dalvik.system.NativeStart.main(Native Method)
04-07 22:02:01.853: E/AndroidRuntime(1205): Caused by: java.lang.NullPointerException
04-07 22:02:01.853: E/AndroidRuntime(1205):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 22:02:01.853: E/AndroidRuntime(1205):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 22:02:01.853: E/AndroidRuntime(1205):     ... 11 more
04-07 22:02:51.663: D/AndroidRuntime(1251): Shutting down VM
04-07 22:02:51.663: W/dalvikvm(1251): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
04-07 22:02:51.673: E/AndroidRuntime(1251): FATAL EXCEPTION: main
04-07 22:02:51.673: E/AndroidRuntime(1251): Process: ca.mohawk.you.lab6a, PID: 1251
04-07 22:02:51.673: E/AndroidRuntime(1251): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab6a/ca.mohawk.you.lab6a.MainActivity}: java.lang.NullPointerException
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.os.Looper.loop(Looper.java:136)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at java.lang.reflect.Method.invoke(Method.java:515)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at dalvik.system.NativeStart.main(Native Method)
04-07 22:02:51.673: E/AndroidRuntime(1251): Caused by: java.lang.NullPointerException
04-07 22:02:51.673: E/AndroidRuntime(1251):     at ca.mohawk.you.lab6a.MainActivity.onCreate(MainActivity.java:36)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.Activity.performCreate(Activity.java:5231)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 22:02:51.673: E/AndroidRuntime(1251):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 22:02:51.673: E/AndroidRuntime(1251):     ... 11 more

Answer 1

It appears that the xml you've posted is from the file fragment_main.xml file. If this is the case, then the problem you're having is that findViewById() is searching for your Views in the Activity's View hierarchy before the Fragment's ViewGroup has been attached to the Activity. If you don't want/need to use Fragments, then simply move the posted xml to activity_main.xml, and remove the relevant Fragment blocks of code. If you want to use Fragments, then move your View initializations and functionalities to PlaceholderFragment.