How to authenticate users using fire base authentication

FirebaseAuthentication.java

FirebaseAuth authUser = FirebaseAuth.getInstance();
authUser.signInWithEmailAndPassword(userEmail, userPassword).addOnCompleteListener(new OnCompleteListener() {
        @Override
        public void onComplete(Task task) {
            if (task.isSuccessful()) {
                FirebaseUser user = task.getResult().getUser();
                String userEmail = user.getEmail();
            }
        }
    });