How to compare the phone contacts numbers with the Firebase registered numbers?

Question

I am trying to display the phonebook contacts' numbers in a RecyclerView but only those are registered in the Firebase database where the contacts numbers are the key.

For that, I am trying to compare an ArrayList from the phone book and one from Firebase database and then saving the results in a third ArrayList that I aim to use it in the RecyclerViewAdapter.

The two problems:

• I can only access Firebase database data inside the listeners of the references, so the size of the third ArrayList outside these listeners is always 0.
• I can not use FirebaseRecyclerAdapter to display the custom third ArrayList.

So, how can I compare these values to get a third accessible ArrayList?

Answer 1

Assuming that you have declared 2 lists that look like this:

List<String> phoneBookList = new ArrayList<>();
List<String> finalList = new ArrayList<>();

And you have a database structure that looks like this:

Firebase-root
  |
  --- firebaseDatabasePhoneNumbers
            |
            --- firebaseDatabasePhoneNumberKeyOne
            |       |
            |       --- //details
            |
            --- firebaseDatabasePhoneNumberKeyTwo
                    |
                    --- //details

To achieve what you want, please use the following code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference firebaseDatabasePhoneNumbersRef = rootRef.child("firebaseDatabasePhoneNumbers");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<String> firebaseDatabaseList = new ArrayList<>();
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String firebaseDatabasePhoneNumber = ds.getKey();
            firebaseDatabaseList.add(firebaseDatabasePhoneNumber);
        }

        for(String phoneBookNumber : phoneBookList) {
            if (firebaseDatabaseList.contains(phoneBookNumber)) {
                finalList.add(phoneBookNumber);
            }
        }

        //Do what you need to do with the finalList
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
firebaseDatabasePhoneNumbersRef.addListenerForSingleValueEvent(valueEventListener);