Firebase Firestore get data from collection

I want to get data from my Firebase Firestore database. I have a collection called user and every user has collection of some objects of the same type (My Java custom object). I want to fill my ArrayList with these objects when my Activity is created.

private static ArrayList<Type> mArrayList = new ArrayList<>();;

In onCreate():

getListItems();
Log.d(TAG, "onCreate: LIST IN ONCREATE = " + mArrayList);
*// it logs empty list here

Method called to get items to list:

private void getListItems() {
    mFirebaseFirestore.collection("some collection").get()
            .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(QuerySnapshot documentSnapshots) {
                    if (documentSnapshots.isEmpty()) {
                        Log.d(TAG, "onSuccess: LIST EMPTY");
                        return;
                    } else {
                        for (DocumentSnapshot documentSnapshot : documentSnapshots) {
                            if (documentSnapshot.exists()) {
                                Log.d(TAG, "onSuccess: DOCUMENT" + documentSnapshot.getId() + " ; " + documentSnapshot.getData());
                                DocumentReference documentReference1 = FirebaseFirestore.getInstance().document("some path");
                                documentReference1.get().addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
                                    @Override
                                    public void onSuccess(DocumentSnapshot documentSnapshot) {
                                        Type type= documentSnapshot.toObject(Type.class);
                                        Log.d(TAG, "onSuccess: " + type.toString());
                                        mArrayList.add(type);
                                        Log.d(TAG, "onSuccess: " + mArrayList);
                                        /* these logs here display correct data but when
                                         I log it in onCreate() method it's empty*/
                                    }
                                });
                            }
                        }
                    }
                }
            }).addOnFailureListener(new OnFailureListener() {
        @Override
        public void onFailure(@NonNull Exception e) {
            Toast.makeText(getApplicationContext(), "Error getting data!!!", Toast.LENGTH_LONG).show();
        }
    });
}

The get() operation returns a Task<> which means it is an asynchronous operation. Calling getListItems() only starts the operation, it does not wait for it to complete, that's why you have to add success and failure listeners.

Although there's not much you can do about the async nature of the operation, you can simplify your code as follows:

private void getListItems() {
    mFirebaseFirestore.collection("some collection").get()
            .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(QuerySnapshot documentSnapshots) {
                    if (documentSnapshots.isEmpty()) {
                        Log.d(TAG, "onSuccess: LIST EMPTY");
                        return;
                    } else {
                        // Convert the whole Query Snapshot to a list
                        // of objects directly! No need to fetch each
                        // document.
                        List<Type> types = documentSnapshots.toObjects(Type.class);   

                        // Add all to your list
                        mArrayList.addAll(types);
                        Log.d(TAG, "onSuccess: " + mArrayList);
                    }
            })
            .addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {
                    Toast.makeText(getApplicationContext(), "Error getting data!!!", Toast.LENGTH_LONG).show();
                }
            });
}

Try this..Working fine.Below function will get Realtime Updates from firebse as well..

db = FirebaseFirestore.getInstance();


        db.collection("dynamic_menu").addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(QuerySnapshot documentSnapshots, FirebaseFirestoreException e) {

                if (e !=null)
                {

                }

                for (DocumentChange documentChange : documentSnapshots.getDocumentChanges())
                {
                 String   isAttendance =  documentChange.getDocument().getData().get("Attendance").toString();
                 String  isCalender   =  documentChange.getDocument().getData().get("Calender").toString();
                 String isEnablelocation = documentChange.getDocument().getData().get("Enable Location").toString();

                   }
                }
        });

More reference :https://firebase.google.com/docs/firestore/query-data/listen

If You do not want realtime updates refer Below Document

https://firebase.google.com/docs/firestore/query-data/get-data

    db.collection("users").get().then((querySnapshot) => {
    querySnapshot.forEach((doc) => {
        console.log(`${doc.id} => ${doc.data()}`);
    });

source:- https://firebase.google.com/docs/firestore/quickstart

Here is a simplified example:

Create a collection "DownloadInfo" in Firebase.

And add a few documents with these fields inside it:

file_name (string), id (string), size (number)

Create your class:

public class DownloadInfo {
    public String file_name;
    public String id;
    public Integer size;
}

Code to get list of objects:

FirebaseFirestore db = FirebaseFirestore.getInstance();

db.collection("DownloadInfo")
        .get()
        .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                     if (task.getResult() != null) {
                            List<DownloadInfo> downloadInfoList = task.getResult().toObjects(DownloadInfo.class);
                            for (DownloadInfo downloadInfo : downloadInfoList) {
                                doSomething(downloadInfo.file_name, downloadInfo.id, downloadInfo.size);
                            }
                        }
                    }
                } else {
                    Log.w(TAG, "Error getting documents.", task.getException());
                }
            }
        });