I am trying to unzip a folder using Android's AsyncTask. The class (called Decompress) is an inner class of Unzip where Unzip itself is a non-Activity class. The pseudo-code is:

public class Unzip {  
  private String index;  
  private String unzipDest;    //destination file for storing folder.
  private Activity activity;
  private boolean result;      //result of decompress.

  public void unzip(String loc) {

    Decompress workThread = new Decompress(loc, activity);
    if(unzip operation was successful) {

  //Class Decompress:
class Decompress extends AsyncTask<Void, Integer, Boolean> {

        private ProgressDialog pd = null;
        private Context mContext;
                private String loc;
        private int nEntries;
        private int entriesUnzipped;

        public Decompress(String location, Context c) {
                        loc = location;
            mContext = c;
            nEntries = 0;
            entriesUnzipped = 0;
            Log.v(this.toString(), "Exiting decompress constructor.");

        protected void onPreExecute() {
            Log.v(this.toString(), "Inside onPreExecute.");
            pd = new ProgressDialog(mContext);
            pd.setTitle("Unzipping folder.");
            pd.setMessage("Unzip in progress.");
            Log.v(this.toString(), "Showing dialog and exiting.");

        protected Boolean doInBackground(Void... params) {
                       //unzip operation goes here.
                       unzipDest = something;  //unzip destination is set here.

                       if(unzip operation is successful) {
                          result = true;
                          index = url pointing to location of unzipped folder.
                       } else {
                         result = false;

        protected void onPostExecute(Boolean result) {
            if(result) {
                if(pd != null) {
                    pd.setMessage("folder is now ready for use.");
                    pd = null;
                    Log.v(this.toString(), "Unzipped.");

                    index = unzipDest + "/someURL";
                    Log.v(this.toString(), "index present in: " + index);
            } else {
                pd =, "Failure", "Cannot unzip.");

Problems I am facing:
1. The value of unzipDest and index, updated in doInBackground, remain null to Unzip and all its objects. How can I ensure that the values remain updated?
2. I know that doInBackground occurs in a thread separate from the main UI thread. Does that mean that any values updated in the new thread will be lost once that thread returns?

Solution 1

How can I ensure that the values remain updated?

They will be updated since they are member variables. However, since AsyncTask is asynchrounous, they might not be updated yet when you check them. You can use an interface to create a callback when these values are updated. This SO answer covers how to do this

Does that mean that any values updated in the new thread will be lost once that thread returns?

No they shouldn't be "lost". They probably just haven't been changed in the AsyncTask when you check them.

Since this isn't your actual code I can't see when you are trying to access them but you can use the interface method or call the functions that need these values in onPostExecute(). You also can do a null check before trying to access them. It just depends on the functionality and flow that you need as to which is the best way. Hope that helps.


In the answer I linked to, you tell the Activity that you will use that interface and override its method(s) with implements AsyncResponse in your Activity declaration after creating the separate interface class

public class MainActivity implements AsyncResponse{

then, in your Activity still, you override the method you declared in that class (void processFinish(String output);)

 void processFinish(String output){  // using same params as onPostExecute()
 //this you will received result fired from async class of onPostExecute(result) method.

then this is called in onPostExecute() when the listener sees that it is done with delegate.processFinish(result); delegate is an instance of AsyncResponse (your interface class)

    public class AasyncTask extends AsyncTask{
public AsyncResponse delegate=null;

   protected void onPostExecute(String result) {

Interface example taken from linked answer above and adjusted/commented for clarity. So be sure to upvote that answer if it helps anyone.