title: Android: Create unique string for file name
body: <p>I'm doing an android application for image viewer. This app will download the image, and store them in a cache folder. </p>
  
  <p>So, in the cache folder, the image file name must be unique. Currently, I use String.hashCode() to generate the file name.</p>
  
  <p>Are there any other better ways to get unique string?</p>