Populate Gridview With Firebase Using Hashmap

I'm trying to populate my gridview using Hashmap but i'm facing this problem:

using this code to get data (books info) from firebase and populate my gridview I get an empty grid view:

 mDatabasee = FirebaseDatabase.getInstance().getReference();

    mDatabasee.addValueEventListener(new ValueEventListener() {

        @Override

        public void onDataChange(DataSnapshot snapshot) {

            for (DataSnapshot postSnapshot : snapshot.child("Items").getChildren()) {



                Item person = postSnapshot.getValue(Item.class);

                Log.d("picture",person.getBookImage());

                hm.put(postSnapshot.getKey().toString(),person.getBookImage());

                String number = String.valueOf(hm.size());

                Log.d("count",number);

this Log.d return count : 1 exactly like I have in firebase

 }

            for(Map.Entry m:hm.entrySet()){

                System.out.println(m.getKey()+" "+m.getValue());



            }



        }



        public void onCancelled(DatabaseError firebaseError) {

            System.out.println("The read failed: " + firebaseError.getMessage());

        }

    });



    String number = String.valueOf(hm.size());

    Log.d("css css css",number);

    gridView.setAdapter(new ImageAdapter(this, hm));

the last Log.d return
"css css css 0"
so it's normal that I'm receiving an empty grid view, to make sure i have added manually 1 book so the new code become:

hm.put("-KWyZJM9GhhTe1PzuJMh","http://books.google.com/books/content?id=aJeTCgAAQBAJ&printsec=frontcover&img=1&zoom=1&source=gbs_api");

    String number = String.valueOf(hm.size());

    Log.d("css css css",number);

    gridView.setAdapter(new ImageAdapter(this, hm));