java - ViewGroup is not resolved in Android Studio

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I'm creating a small animation app using Android studio. When a button press images should work according to the particular animation.
There's an error with line viewGroup = (viewGroup)findViewById(R.id.ViewGroup);
It shows " Cannot resolve symbol ViewGroup ". I typed Alt+Enter and selected a solution. But nothing happened.
public class MainActivity extends Activity {
private ViewGroup viewGroup;
private ImageView imageView, imageView2, imageView3,imageView4,imageView5,imageView6,imageView7,imageView8,imageView9,imageView10,imageView11,imageView12;
private Button button,button1;
@Override
protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    viewGroup=(viewGroup)findViewById(R.id.ViewGroup);
    imageView= (ImageView) findViewById(R.id.imageView);
    imageView2= (ImageView) findViewById(R.id.imageView2);
    imageView3= (ImageView) findViewById(R.id.imageView3);
    imageView4= (ImageView) findViewById(R.id.imageView4);
    imageView5= (ImageView) findViewById(R.id.imageView5);
    imageView6=(ImageView) findViewById(R.id.imageView6);
    imageView7=(ImageView) findViewById(R.id.imageView7);
    imageView8=(ImageView) findViewById(R.id.imageView8);
    imageView9=(ImageView) findViewById(R.id.imageView9);
    imageView10=(ImageView) findViewById(R.id.imageView10);
    imageView11=(ImageView) findViewById(R.id.imageView11);
    imageView12=(ImageView) findViewById(R.id.imageView12);
    button = (Button)findViewById(R.id.button);
    button1=(Button)findViewById(R.id.button1);
    button1.setOnClickListener(new View.OnClickListener(){
        public void onClick(View v){
            TransitionManager.beginDelayedTransition(viewGroup, new Fade());
            fade(imageView, imageView2, imageView3,imageView4,imageView5,imageView6);
    button.setOnClickListener(new View.OnClickListener(){
        public void onClick(View v) {
            TransitionManager.beginDelayedTransition(viewGroup, new Slide());
            toggle(imageView7,imageView8,imageView9,imageView10,imageView11,imageView12);
private static void toggle(View... views) {
    for (View v : views) {
        boolean isVisible = v.getVisibility() == View.VISIBLE;
        v.setVisibility(isVisible ? View.INVISIBLE :
                View.VISIBLE);
private static void fade(View... views) {
    for (View v : views) {
        boolean isVisible = v.getVisibility() == View.VISIBLE;
        v.setVisibility(isVisible ? View.INVISIBLE :
                View.VISIBLE);
Here's my activity_main 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:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity"
android:id="@+id/myAniLayout">
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="New Button"
    android:id="@+id/button"
    android:layout_alignParentBottom="true"
    android:layout_alignParentStart="true"
    android:layout_marginBottom="44dp" />
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="New Button"
    android:id="@+id/button1"
    android:layout_alignBottom="@+id/button"
    android:layout_alignParentEnd="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:src="@mipmap/ic_launcher" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView2"
    android:src="@mipmap/ic_launcher"
    android:layout_below="@+id/imageView"
    android:layout_alignParentStart="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView3"
    android:src="@mipmap/ic_launcher"
    android:layout_alignTop="@+id/imageView2"
    android:layout_alignParentEnd="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView4"
    android:src="@mipmap/ic_launcher"
    android:layout_below="@+id/imageView2"
    android:layout_centerHorizontal="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView5"
    android:src="@mipmap/ic_launcher"
    android:layout_below="@+id/imageView4"
    android:layout_alignParentStart="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView6"
    android:src="@mipmap/ic_launcher"
    android:layout_below="@+id/imageView4"
    android:layout_alignParentEnd="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView7"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true"
    android:src="@android:drawable/star_big_on" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView8"
    android:src="@android:drawable/star_big_on"
    android:layout_below="@+id/imageView7"
    android:layout_alignParentStart="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView9"
    android:src="@android:drawable/star_big_on"
    android:layout_below="@+id/imageView7"
    android:layout_alignParentEnd="true" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView10"
    android:src="@android:drawable/star_big_on"
    android:layout_below="@+id/imageView9"
    android:layout_alignStart="@+id/button1" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView11"
    android:src="@android:drawable/star_big_on"
    android:layout_below="@+id/imageView8"
    android:layout_alignEnd="@+id/button" />
<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView12"
    android:src="@android:drawable/star_big_on"
    android:layout_above="@+id/button"
    android:layout_alignStart="@+id/imageView4" />
  The ViewGroup class is a subclass of the View class. ViewGroup
  instances work as containers for View instances to group View
  instances together.
http://developer.android.com/intl/es/reference/android/view/ViewGroup.html
As per your xml there is not an any id with ViewGroup in your xml code. please give id to your layout with ViewGroup and cast it as below,
viewGroup=(ViewGroup)findViewById(R.id.ViewGroup);
there was also a spelling mistake in your line,
viewGroup=(viewGroup)findViewById(R.id.ViewGroup);
As they pointed out, you have no R.id.ViewGroup in your layout.
If you really need the view group try this:
viewGroup = (ViewGroup)findViewById(android.R.id.content)
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