title: FirebaseAuth.getInstance().getUid() - How does it work?
body: <p>I have 3 questions about this:</p>
  
  <pre><code>FirebaseAuth.getInstance().getUid()
  </code></pre>
  
  <ol>
  <li><p>According to spec, getInstance() returns instance of the FirebaseAuth
  class, but the getUid not function in this class. But it works. How ?</p></li>
  <li><p>It seems that getUid generates unique ID if this is a new user but it
  is not written in the spec. How does this function work?</p></li>
  <li>When does it throw exception or return null ?</li>
  </ol>
  
  <p>Thanks,
  Alex</p>