Android : Cannot print out retrieved Array List object

I cannot print out my array list object.

Here is my first class to return the arraylist object:

public class user_arraylist {
public ArrayList<String> get_user_det()    {
    final ArrayList<String> temp = new ArrayList<String>(); //declare arraylist

    Firebase f_user = new Firebase("https://shareurdaydb.firebaseio.com/User/");
    f_user.addListenerForSingleValueEvent(new ValueEventListener(){
        @Override
        public void onDataChange(DataSnapshot snap_user) {
            // TODO Auto-generated method stub
            Iterable<DataSnapshot> rs = snap_user.getChildren();
            Iterator<DataSnapshot> irs = rs.iterator();
            long allNum2 = snap_user.getChildrenCount();
            int maxNum2 = (int)allNum2;
            int count_user = 1;
            while(irs.hasNext())
            {
                if(count_user <= maxNum2)
                {
                    Firebase user_data = new Firebase("https://myapp.firebaseio.com/"); 
                    AuthData authData = user_data.getAuth();
                    Map<String, Object> nPost = (Map<String, Object>) irs.next().getValue();

                    String db_email = nPost.get("email_addr").toString();
                    //String reply_msg_id = nPost.get("msgid").toString();

                    if (authData != null) {
                        String usr_email = authData.getProviderData().get("email").toString();
                        if(usr_email.equals(db_email))
                        {
                            String disp_name = nPost.get("disp_name").toString();
                            String real_name = nPost.get("real_name").toString();
                            temp.add(disp_name); //add to arraylist
                            temp.add(real_name);

                            for (String str : temp) {
                                            System.out.println("Item is: " + str);
                                        }

                        }
                    } else {
                        System.out.println("Failed");
                    }
                }
                count_user++;
            }                       
        }
        @Override
        public void onCancelled(FirebaseError arg0) {
            // TODO Auto-generated method stub

        }
}); 
return(temp); //return array list

 }

}

Here is my second class to retrieve the array list object and display it:

                    user_arraylist t = new user_arraylist();
                    ArrayList<String> arr = new ArrayList<String>();
                    arr = t.get_user_det(); 

                    //Object user_det = get_user_det();
                    for (String temp : arr) {
                        System.out.println("NAME + " + temp);
                    }

After running this it prints out: nothing

If I print it like this

System.out.println("NAME + " + temp);

It gives me NAME:[];

Can some one explain why it is not working (in the second class it's not printing out the array list)?