title: How does UserService class work?
body: <p>I am trying to implement a login system using the Android Studio LoginActivity template. On AppEngine, I try the two lines:</p>
  
  <pre><code>UserService userService = UserServiceFactory.getUserService();
  User user = userService.getCurrentUser();
  </code></pre>
  
  <p>but the user is always null. 
  How is it supposed to function and be implemented? 
  I mean, does UserService class only work when integrating the Login with Google Accounts?</p>
  
  <p>If yes, what is the best and smartest way for me to implement a customized login system?</p>