test.java

     for (final String groupId: memberList){
                        Query searchQuery3 = database.child("userGroups").child(groupId);
                        ValueEventListener valueEventListener = new ValueEventListener() {
                            public void onDataChange(DataSnapshot dataSnapshot) {
                                if (dataSnapshot.exists()) {

                                    System.out.println("**i am here");
                                    final UserGroup userGroup = dataSnapshot.getValue(UserGroup.class);
                                    final List<String> usersList = userGroup.getUsersList();

                                    for (int i=0; i<usersList.size(); i++) {

                                        final String userId = usersList.get(i);

                                        Query searchQuery2 = database.child("users").child(userId);
                                        searchQuery2.addListenerForSingleValueEvent(new ValueEventListener() {
                                            @Override
                                            public void onDataChange(DataSnapshot dataSnapshot) {

                                                if (dataSnapshot.exists()) {
                                                    System.out.println("user exists: " + dataSnapshot.getValue(User.class).toString());
                                                    final User getUser = dataSnapshot.getValue(User.class);

                                                    HashMap<String, Boolean> newChatGroupList = getUser.chatGroupsHashMap;
                                                    newChatGroupList.put(newChatGroup.getId(), true);

                                                    getUser.chatGroupsHashMap = newChatGroupList;
                                                    database.child("users").child(userId).setValue(getUser);


                                                }
                                            }

                                            @Override
                                            public void onCancelled(DatabaseError databaseError) {
                                                System.out.println(databaseError.getMessage());
                                                deferredResult.setResult("pages/chat_group_edit");
                                            }
                                        });
                                    }


                                }
                            }
                            public void onCancelled(DatabaseError databaseError) {
                                System.out.println(databaseError.getMessage());
                            }
                        };
                        searchQuery3.addListenerForSingleValueEvent(valueEventListener);
                    }