title: New to android need to understand "?android:attr/actionBarSize"
body: <p>I was going through <a href="http://www.vogella.com/articles/AndroidFragments/article.html" rel="noreferrer">Lars Vogel's tutorial</a> on using Fragments and I came across the following code:</p>
  
  <pre><code>android:layout_marginTop="?android:attr/actionBarSize"
  </code></pre>
  
  <p>I am not sure what <code>?android:attr</code> means, especially the question mark.</p>