Botón button = findViewById (R.id.button) siempre resuelve null en Android Studio

Soy nuevo en el desarrollo de Android y Android Studio, así que perdón mi ignorancia.

findViewById de un botón que agregué siempre resuelve a null . Por lo tanto, si intento setonClickListener falla toda la actividad.

• ¿Por qué Android Studio me dice que utilice getSupportActionBar () en lugar de getActionBar ()?
• Creación de un proyecto Android Jar del proyecto Uso de gradle con dependencias
• Error: Causa: buildToolsVersion no se especifica
• Prueba ortográfica android studio / IntelliJ Idea
• Android Studio está atascado en gradle resolver dependencias

Actividad principal:

 public class MainActivity extends ActionBarActivity { @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(); } Button buttonClick = (Button) findViewById(R.id.button); buttonClick.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { onButtonClick((Button) view); } }); } public void onButtonClick(Button view) { TextView textHello = (TextView) findViewById(R.id.textView); textHello.setText("Clicked !!!"); } @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; } } } 

Activity_main.xml

 <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/container" android:layout_width="match_parent" android:layout_height="match_parent" tools:context="com.pluralsight.activitylifecycle.MainActivity" tools:ignore="MergeRootFrame" /> 

Fragment_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="com.pluralsight.activitylifecycle.MainActivity$PlaceholderFragment"> <TextView android:text="@string/hello_world" android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/textView" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/buttonClick" android:id="@+id/button" android:layout_below="@+id/textView" android:layout_alignParentRight="true" android:layout_alignParentEnd="true" android:layout_marginRight="54dp" /> </RelativeLayout> 

 //... View rootView = inflater.inflate(R.layout.fragment_main, container, false); Button buttonClick = (Button)rootView.findViewById(R.id.button); buttonClick.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { onButtonClick((Button) view); } }); 

 Button buttonClick = (Button)rootView.findViewById(R.id.button); 

 @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View view = inflater.inflate(R.layout.fragment_main, container, false); Button buttonClick = (Button) view.findViewById(R.id.button); buttonClick.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { onButtonClick((Button) view); } }); return view; }