How to add and retrieve data into Firebase using Lists/ArrayLists?

Question

I've been trying to figure out how to add data to my firebase real time database using Lists or ArrayLists. For example I want to create a list of users who have liked a post and add them in a list/arraylist then upload that list/ArrayLists in my real time database. After that I also want to retrieve the data from firebase.

Solution

To achieve this please use the following code:

List<String> friends = new ArrayList<>();
friends.add("John");
friends.add("Steve");
friends.add("Anna");

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
for(String friend : friends) {
    rootRef.child("friends").child(friend).setValue(true);
}

Your database will look like this:

Firebase-root
    |
    --- friends
           |
           --- John: true
           |
           --- Steve: true
           |
           --- Anna: true

To get all those names into a List, please use the following code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference friendsRef = rootRef.child("friends");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<String> friends = new ArrayList<>();
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String friend = ds.getKey();
            friends.add(friend);
        }
        Log.d("TAG", friends);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
friendsRef.addListenerForSingleValueEvent(eventListener);

Your output will be:

[John, Steve, Anna]

Comments

[Marlon]

How can you make this so that you don't have to have a boolean value. The friends list would only contain the names.

[Alex Mamo]

@Marlon You can use an array but is more convenient to use a map.