Return value from valueEventListener java

Question

I am trying to return a boolean but the value returned is always false, i.e. 0 in this case.

The check variable is an instance variable and the below function is getting called when a button is pressed.

private boolean checkAnswerSubmission() {

    DatabaseReference answerDatabase = FirebaseDatabase.getInstance().getReference().child("userPuzzleHistory").child(uid);
    answerDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.hasChild(String.valueOf(imagename))) {
                check = 1;
            } else {
                check = 0;
            }
     //here the value check is working fine but the value cannot be returned from here.
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    //here the value returned is always 0
    if (check == 1)
        return true;
    else
        return false;

}

Answer 1

As with all async operations I often do it in a callback manner.

Your solution could work like this:

1. Create a simple callback interface (I have mine in a library that I use in almost every app)

   public interface SimpleCallback {
       void callback(Object data);
   }
   
   // You could do it as well generic, that's what I do in my lib:
   public interface SimpleCallback<T> {
       void callback(T data);
   }
   

Then redesign the signature of your method like this:

private void checkAnswerSubmission(@NonNull SimpleCallback<boolean> finishedCallback) {

    DatabaseReference answerDatabase = FirebaseDatabase.getInstance().getReference().child("userPuzzleHistory").child(uid);
    answerDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // This will simple call your callback containing a boolean true/false
            finishedCallback.callback(dataSnapshot.hasChild(String.valueOf(imagename)));
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

You call this with:

checkAnswerSubmission(new SimpleCallback<boolean>() {
   @Override
   void callback(boolean data) {
       if (data) {
          // true was returned
       } else {
          // false was returned
       }
   }
});

Answer 2

The database reference is not done on the same thread as that running your function. It just adds a listener for the async call that is only made when the data is received from server.

So your program initializes a listener and moves on to the next statement in this line.

answerDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.hasChild(String.valueOf(imagename))) {
                check = 1;
            } else {
                check = 0;
            }
     //here the value check is working fine but the value cannot be returned from here.
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

Now as the program doesn't wait for response to be received from the server it executes :

if (check == 1)
        return true;
    else
        return false;

So if your check value was initialized it will most likely take 0 as value and always return false.