findViewById in onCreate returns NULL when using Fragment

    private SensorManager mSensorManager;
    private PowerManager mPowerManager;
    private WindowManager mWindowManager;
    private Display mDisplay;

// onCreate
   @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

       // Get an instance of the SensorManager
        mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);

        // Get an instance of the PowerManager
        mPowerManager = (PowerManager) getSystemService(POWER_SERVICE);

        // Get an instance of the WindowManager
        mWindowManager = (WindowManager) getSystemService(WINDOW_SERVICE);
        mDisplay = mWindowManager.getDefaultDisplay();

        setContentView(R.layout.main); // main.xml or your xml file name

}

// main.xml
    <Button
        android:layout_width="150dip" 
        android:layout_height="wrap_content"
        android:text="My button with a long text for marquee as a example source code"
        android:singleLine="true"
        android:ellipsize="marquee"
        android:marqueeRepeatLimit="1"/>

import android.content.SharedPreferences;

public class MainClass extends Activity {
  public static final String PREFERENCES_NAME = "MyPrefsFile";
  String myNewString;
  int myNewInt;
  Boolean myNewBool;


   @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // read old settings if exist
      SharedPreferences settings = getSharedPreferences(PREFERENCES_NAME, 0);
			
     String sDefault = "Hello!";
      String sStringFromPrefFile = settings.getString("myString", sDefault); // new text  or default Hello!
          
      int nDefaultIndex = 2; //
      int nIndexFromPrefFile = settings.getInt("myInt", nDefaultIndex); // 4 or default 2
			
      Boolean bDefault = false;
      Boolean bFromPrefFile = settings.getBoolean("silentMode", bDefault); // true or default false
        
        
        // new settings will saved in onStop or onPause
        myNewString = "new text";
        myNewInt = 4;
        myNewBool = true;
     }   

    @Override
    protected void onPause() {
    	super.onPause();
        Toast.makeText(getApplicationContext(), "onPause "
        		, Toast.LENGTH_SHORT).show();

    	savePreferences();
    }
    
    @Override
    protected void onStop() {
    	super.onStop();
    	savePreferences();
    }
    /**save settings*/	
    public void savePreferences(){
    	try {
    		SharedPreferences settings = getSharedPreferences(PREFERENCES_NAME, 0);
    		SharedPreferences.Editor editor = settings.edit();
 
    		editor.putString("myString", myNewString);
     		editor.putInt("nMyInt", myNewInt);
    		editor.putBoolean("silentMode", myNewBool); // 
    		
    		editor.commit();
    	} catch (Exception e) {
    		// TODO Auto-generated catch block
    		e.printStackTrace();
    	}
    	
    }	
}