Trigger event when AsyncTask is complete using Listener

Question

I'm having some confusion on how to use Listeners to pass information between classes and activities.

The scenario is this:
I have a task that reads data from an Sqlite database and puts this data into an ArrayList that is passed to an Adapter which then displays it on the screen.

The question:
My app crashes when i attempt to update the listener interface with a value. It doesnt show anything in the logcat...it just hangs the app. Could someone kindly point out where i'm going wrong. - Thanks

Details:
I have the database processing done by a class called DatbaseRoutines and within it an inner method doing the heavy lifting. the method is using AsyncTask so as not to hog the main thread.

I have created and Interface with a single method

 public interface DatabaseProcessListener {
  public void ProcessingIsDone(boolean blnProcessIsFinished);
} 

In my DatabaseRoutines class i am creating a private variable

private DatabaseProcessListener dbProcesslistener;

along with a setter for it

   public void setDbProcesslistener(DatabaseProcessListener dbProcesslistener) {
    this.dbProcesslistener = dbProcesslistener;
    }

When processing is done, i use the PostExecute method to update the Listener so that this can change the value in the listener to True (ie that db processing is done) below is the code

            protected void onPostExecute(Boolean aBoolean) {
                super.onPostExecute(aBoolean);
                dialog.dismiss(); //i have progress dialog during processing and its                        
                //closed here

                if (blnUpdateSucceededStatus) 
                 {
                     dbProcesslistener.ProcessingIsDone(true);
                  } 
                   else 
                  {
                    Toast.makeText(ctx, "Transactions Processing   
                     failed",Toast.LENGTH_SHORT);
                   } ;

Finally in the activity displaying the data gathered from the database I Implement the listener. see below.

public class MyActivity extends Activity implements DatabaseProcessListener 
{

@Override
public void ProcessingIsDone(boolean blnProcessIsFinished) {
    if(blnProcessIsFinished){
        PopulateExpandableListView();
    }
   public void PopulateExpandableListView() {
   //code for populating the listview goes here;
     }
}

after some digging i found the logcat error..its a nullpointer exception

05-05 11:11:18.590: ERROR/AndroidRuntime(22339): FATAL EXCEPTION: main
    java.lang.NullPointerException
    at com.example.mledger.DatabaseRoutines$1.onPostExecute(DatabaseRoutines.java:258)
    at com.example.mledger.DatabaseRoutines$1.onPostExecute(DatabaseRoutines.java:141)
    at android.os.AsyncTask.finish(AsyncTask.java:631)
    at android.os.AsyncTask.access$600(AsyncTask.java:177)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4921)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1038)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805)
    at dalvik.system.NativeStart.main(Native Method)

Unfortunately the app is hanging and the interface never updates the Activity that displays data with true (ie processing is done)

Do point me in the right direction. - Thanks

Answer 1

Sounds like the dbProcesslistener variable is NULL. Do you call your setter and pass it an initialized Listener object some time before you invoke dbProcesslistener.ProcessingIsDone(true);?

Answer 2

Got the Answer! I was not initializing the listener in my calling class. all i needed to do was put in this line

db.setDbProcesslistener(this);

ie

DatabaseRoutines db = new DatabaseRoutines(MyActivity.this);
db.setDbProcesslistener(MyActivity.this);

and it started working.

Thanks to everyone who had a look and tried to think of the solution :)