title: fragment placehoder in layout
body: <p>Suppose I have several <strong>fragments</strong>:</p>
  
  <pre><code>public class FirstFragment extends Fragment { ... }
  
  public class SecondFragment extends Fragment { ... }
  
  public class ThirdFragment extends Fragment { ... }
  </code></pre>
  
  <p>I have also a main <strong>activity</strong>, its content is (<code>main.xml</code>):</p>
  
  <pre><code>&lt;LinearLayout ...&gt;
  
    &lt;!-- How to have a fragment placehold without specify which fragment show here? --&gt;
    &lt;fragment&gt;
  
  &lt;/LinearLayout&gt;
  </code></pre>
  
  <p>My question is, in above layout file, how can I define a <strong>fragment placeholder</strong> without specify which fragment show here, and then <strong>in My Activity java code</strong>, inflate a proper fragment to the placeholder ? </p>
  
  <p>Is it possible and how to do it?</p>