title: How to cache data in an Android app?
body: <p>In my app, I need some data that I do not want to request every time from the server.
  This dat includes the userId and some array string.
  I think I can store the user id in the <code>SharedPreferences</code>,
  but what about the array?</p>
  
  <p>Is it OK to use static variables?</p>