title: setContentView(R.layout.activity_main) vs getMenuInflater().inflate(R.menu.activity_main, menu)
body: <p>Why do I have to tell my activity what its layout should be twice?</p>
  
  <pre><code>protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main); // &lt;--
  }
  
  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
      // Inflate the menu; this adds items to the action bar if it is present.
      getMenuInflater().inflate(R.menu.activity_main, menu); // &lt;--
      return true;
  }
  </code></pre>
  
  <p>What's the difference between these two methods?. when should I use one, and when the other?</p>