How to get Firestore information actually added to a list so the list can passed to other functions later on

I'm really new to programming in general. My question is that I know when reading from Firestore, it is asynchronous. Meaning if I do:

        Log.d("data_base", "got here 1");
        getFeeds();
        Log.d("data_base", "got here 2");

"got here 1" and "got here 2" will be printed simultaneously despite any log statement inside getFeeds()

But what if I do want to store all the information from a Firestore collection into a list and use it later on?

This is what I have now.

    List<String> username = new ArrayList<>();
    List<String> imageUrl = new ArrayList<>();
    getFeeds();
    RecyclerViewAdapter adapter = new RecyclerViewAdapter(username, imageUrl);

    private void getFeeds() {
        db.collection("feeds").addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot value, @Nullable FirebaseFirestoreException error) {
                if (error != null) {
                    Log.d("data_base", "listen:error");
                    return;
                }
                for (QueryDocumentSnapshot doc : value) {
                    if(doc.get("image") != null) {
                        Log.d("data_base", doc.getString("image"));
                        imageUrl.add(doc.getString("image"));
                    }
                    if (doc.get("username") != null) {
                        username.add(doc.getString("username"));
                    }
                }
            }
        });
    }

Turns out I only need one line

    private void getFeeds(final RecyclerViewAdapter adapter) {
        db.collection("feeds").orderBy("username", Query.Direction.ASCENDING).addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot value, @Nullable FirebaseFirestoreException error) {
                if (error != null) {
                    Log.d("data_base", "listen:error");
                    return;
                }
                for (QueryDocumentSnapshot doc : value) {
                    if(doc.get("image") != null) {
                        Log.d("data_base", doc.getString("image"));
                        imageUrl.add(doc.getString("image"));
                    }
                    if (doc.get("username") != null) {
                        username.add(doc.getString("username"));
                    }
                }
                // added line here               
                adapter.notifyDataSetChanged();
            }
        });
    }

My understanding is when onEvent() finish fetching data from Firestore, adapter.notifyDataSetChanged() will be called to update the two lists.

You don't have to use Lists to save data from firestore collection.

1. To store data in firestore collection you can use Hashmap. Code attached below:

    HashMap<String, Object> userData = new HashMap<>();
    userData.put("username", your_username_data);
    userData.put("imageurl", your_imageurl_data);
   
    db.collection("feeds").document().set(userData).addOnSuccessListener(new OnSuccessListener<Void>() {
        @Override
        public void onSuccess(Void aVoid) {
            //This code gets executed if data successfully added to db
            Toast.makeText(getApplicationContext(), "User Data save successfully", Toast.LENGTH_SHORT).show();
        }
    }).addOnFailureListener(new OnFailureListener() {
        @Override
        public void onFailure(@NonNull Exception e) {
            `enter code here`//This code gets executed if data cannot be saved to db
            Toast.makeText(getApplicationContext(), e.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });
   

2. To fetch data from firestore collection you can refer to the code below:

    db.collection("feeds").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            // Code below gets executed if data fetched successfully
            if (task.isSuccessful()) {
                for (QueryDocumentSnapshot documentSnapshot : task.getResult()) {
                    String username = (String) documentSnapshot.get("username"); // this is your username previously saved to db
                    String imageurl = (String) documentSnapshot.get("imageurl"); // this is your imageurl previously saved to db
                    // Bonus: to download images to your imageview you can use Glide library (I will attach link below to Github page)
                    Glide.with(getApplicationContext()).load(imageurl).into(imageview); // imageview is your ImageView that you want to show your images
                }
            }
        }
    });
   

3. If you want to use your data later in your other activity you should use Bundle. Code provided below:

   In your 1st Activity (Activity that you want to pass data from):

    String username = your_username_data;
    String imageurl = your_imageurl_data;
    Intent i = new Intent(Activity1.this, Activity2.class);
    i.putExtra("username", username);
    i.putExtra("imageurl", imageurl);
    startActivity(i);
   

   In your 2nd Activity (Activity that you want to pass data to):

    Bundle extras = getIntent().getExtras();
    if (extras != null) {
        String username = extras.getString("username"); 
        String imageurl = extras.getString("imageurl");
        // inside extras.getString you use key value (value in quote "") which you provided inside i.putExtra
    }     
   

Link: https://github.com/bumptech/glide (this library helps you to download image urls to imageviews)