How to get all documents from a firestore collection and return them in an array list?

Question

I have a static method that gets all the documents in a collection. I am having trouble creating an array of these documents to return. How could I do that? Let us call the Array list 'allItems'. My approach was to add each item that is created inside the for loop to an the allItems array list, but this list is null when I return it and android studio is prompting me to make the array list final.

public static ArrayList<Item> geAllFromFireStore() {

        FirebaseFirestore db = FirebaseFirestore.getInstance();
        ArrayList<Item> allItems = new ArrayList<>();

        db.collection("Items")
                .get()
                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        if (task.isSuccessful()) {

                            for (QueryDocumentSnapshot document : task.getResult()) {
                                Log.d(TAG, document.getId() + " => " + document.getData());

                                String name = document.get("name") + "";
                                String categoryName = document.get("category") + "";

                                Item item = new Item(name, category);

                                // I would like to add these items to an array list and return that array list

                            }
                        } else {
                            Log.w(TAG, "Error getting documents.", task.getException());
                        }
                    }
                });

        return allItems;
    }

check it out. https://stackoverflow.com/questions/48594059/firestore-fetch-array-from-collection — Gourango Sutradhar, Nov 26 '18 at 04:32
This is going to return an empty list to the calling method, just FYI — OneCricketeer, Nov 26 '18 at 05:14

Hello World · Answer 1 · 2018-11-26T05:27:05.667

1

I write it in Kotlin, so please adapt this in your code.

// Declare variable that use to collect DocumentSnapshot
private lateinit var list: MutableList<DocumentSnapshot>

// In addOnCompleteListener…
if(task.isSuccessful){
    lstMAP = mutableListOf<DocumentSnapshot>()
    lstMAP.clear()
    lstMAP.addAll(task.result!!.documents)
} else {
    Log.d("doc", "0 doc")
}

edited Nov 26 '18 at 05:27

answered Nov 26 '18 at 04:42

Hello World

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Thank you, I am having trouble understanding this though. How will this allow me to return an array list with the documents? – Oamar Kanji Nov 26 '18 at 05:01
I already edit my code, so please check it again. If you still use for-loop then return it after for-loop. – Hello World Nov 26 '18 at 05:05
By the way, please careful when you use your allItems. Because it can cause IndexOutOfBoundsException when you get value from empty arrayList if your Collection have no documents. – Hello World Nov 26 '18 at 05:09
If it works, please accept my answer. – Hello World Nov 26 '18 at 05:11
Thanks, I already forgot that his code already has return statement. – Hello World Nov 26 '18 at 05:28

OneCricketeer · Accepted Answer · 2018-11-26T05:29:22.167

but this list is null when I return it and android studio is prompting me to make the array list final.

There's nothing wrong with making it final, and given that code, it's impossible for it to return null, but very possibly might be empty, because it's actually returning before Firebase even attempts to make a remote database query.

That's the problem with being asynchronous - you need to wait for the result before you can return it.

For example, parameterize the listener that gets the snapshot

public static void getAllFromFireStore(OnCompleteListener<QuerySnapshot> listener) {
    FirebaseFirestore db = FirebaseFirestore.getInstance();
    db.collection("Items").get().addOnCompleteListener(listener);
}

And that's it.

From there, refactor the rest of the code to pass in the listener you've already written rather than actually expect an object to be returned from the method call

API.getAllFromFireStore(new OnCompleteListener<QuerySnapshot>() {
    @Override 
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
            // TODO: get data 
        } 
});

From inside of that, you can populate a RecyclerView Adapter, for example

How to get all documents from a firestore collection and return them in an array list?

2 Answers2