Where I bind to XML when extends from ActionBarActivity?

when I extends from Activity, I modify the fragment_main and find controls like this:

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

    MyButton = (Button) findViewById(R.id.button1); 
}

but when I do this when extends from from ActionBarActivity gives me an error, so to do it work I have to comment:

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

and set activity_main to fragment_main

setContentView(R.layout.fragment_main);

but I don't believe this be correct...