How to get list of Firestore users by its uid?

Question

I have a method for get a List of users by its uid:

public void getNeededUsers(List<String> uidList, UsersListCallback usersListCallback) {
        CollectionReference users = db.collection(Consts.USERS_DB);
        for (String uid: uidList) {
            users.whereEqualTo(Consts.UID, uid).get()
                    .addOnCompleteListener(task -> {
                        List<FsUser> fsUserList = new ArrayList<>();
                        for (DocumentSnapshot snapshot : task.getResult().getDocuments()) {
                            fsUserList.add(snapshot.toObject(FsUser.class));
                        }
                        usersListCallback.getUsers(fsUserList);
                    });
        }
    } 

But it seems that this method doesn`t work properly. When I try to iterate through the List of users in another method, I receive users one by one from callback, instead of getting all, so they are duplicated in my list.

Here is a code of another method:

public void createUserChats(UserChatListCallback userChatListCallback) {
        final List<UserChat> userChatList = new ArrayList<>();

        getChatAndUidList((chatList, uidList) -> {
            getRtUsersAndMessages(uidList, (rtUserList, messageList) -> {
                getNeededUsers(uidList, userList -> {

                    if (!userList.isEmpty()) {
                        for (int i = 0; i < userList.size(); i++) {
                            String name = userList.get(i).getName();
                            String image = userList.get(i).getImage();
                            String userId = userList.get(i).getUid();

                            long timeStamp = chatList.get(i).getTimeStamp();
                            boolean seen = chatList.get(i).getSeen();

                            String online = rtUserList.get(i).getOnline();

                            String message = messageList.get(i).getMessage();
                            long messageTimestamp = messageList.get(i).getTime();


                            userChatList.add(new UserChat(name, image, timeStamp, seen, userId, online,
                                    message, messageTimestamp));

                        }
                    }
                    userChatListCallback.getUserChatList(userChatList);
                });
            });
        });
    }

Answer 1

To get a list of users, please use the following code:

List<Task<DocumentSnapshot>> tasks = new ArrayList<>();
for (String uid: uidList) {
    Task<DocumentSnapshot> documentSnapshotTask = users.whereEqualTo(Consts.UID, uid).get();
    tasks.add(documentSnapshotTas);
}
Tasks.whenAllSuccess(tasks).addOnSuccessListener(new OnSuccessListener<List<Object>>() {
    @Override
    public void onSuccess(List<Object> list) {
        //Do what you need to do with your users list
        for (Object object : list) {
            FsUser fsUser = ((DocumentSnapshot) object).toObject(TeacherPojo.class);
            Log.d("TAG", fsUser.getName());
        }
    }
});