title: how to change an object that is passed by reference to a mock in Mockito
body: <p>Given the following code</p>
  
  <pre><code>@Mock
  Client client;
  
  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  client.retrieveFile(baos); // client is supposed to fill boas with data
  </code></pre>
  
  <p>how do I instruct Mockito to fill the <code>baos</code> object?</p>