title: How to store a HashMap in Firebase for offline use
body: <p>I have a Map:   </p>
  
  <pre><code>  Map&lt;String, String&gt; noidung = new HashMap&lt;&gt;();
  </code></pre>
  
  <p>The <code>Map</code> receives data from Firebase and I show them in a <code>Gridview</code>. Everything worked well.
  But when the phone goes offline, I want the data I have loaded to remain on the Gridview.
  My idea is to store the <code>Map noidung</code> to a file (I think SharedPreference) to use offline. I have tried very much solutions but failed.</p>