Not sure if I am doing something wrong but using this api https://www.firebase.com/docs/security/simple-login-email-password.html I can successfully create a user - accordin
i will answer it simply as much as possible just add the registered user to your data base by using the following code
you can also use shared prefernces to save the data locally but it won't be able for other user.
once you save the list of user in the database simply retrieve it from there using adapters
FirebaseDatabase.getInstance().getReference().child("my_user")
.child(task.getResult().getUser().getUid())
.child("username").setValue(autoCompleteTextView1.getText().toString());