title: Download spreadsheet to android studio app with code
body: <p>Hi I've done a lot of research already but I still can't find a good explanation for what callback does in android studio. </p>
  
  <p>For example what function does call back do in the following methods:</p>
  
  <pre><code>     public DownloadWebpageTask(AsyncResult callback) {
      this.callback = callback;
  }
  </code></pre>