title: isEmpty() checking in mockito
body: <p>How to write <code>junit</code> for the statement</p>
  
  <pre><code>if (!messageMap.isEmpty())
  </code></pre>
  
  <p>where <code>messageMap</code> is a <code>TreeMap</code>.
  I wrote as follows,</p>
  
  <pre><code>PowerMockito.when(!messageMap.isEmpty()).thenReturn(true);
  </code></pre>
  
  <p>But this code not working.</p>