title: No PlaceholderFragment class in MainActivity.java
body: <p>I am trying to learn android programming by google's android programming course (android studio) in udacity.
  The problem is I do not have "PlaceholderFragment" class in my MainActivity.java.
  I only have MainActivity class in this file.
  Could anyone help me please?</p>
  
  <p>Edit: 
  my MainActivity.java is:</p>
  
  <pre><code>public class MainActivity extends ActionBarActivity {
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
      }
  
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
          // Inflate the menu; this adds items to the action bar if it is present.
          getMenuInflater().inflate(R.menu.menu_main, menu);
          return true;
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          // Handle action bar item clicks here. The action bar will
          // automatically handle clicks on the Home/Up button, so long
          // as you specify a parent activity in AndroidManifest.xml.
          int id = item.getItemId();
  
          //noinspection SimplifiableIfStatement
          if (id == R.id.action_settings) {
              return true;
          }
  
          return super.onOptionsItemSelected(item);
      }
  }
  </code></pre>