How can I know if a user already created an account while using firebase-ui on my android application?

Question

And here's the long version:

I'm totally new to Firebase and Android Studio, and I have already written up some parts like signing in with google and FB in my app. I followed this tutorial.

Everything works fine but what I want to do now is collect some info from user and store it in Firebase Database(which I think I've a basic idea of). But the problem is that I've no idea on how I can know if its the first time a user is creating the account, because I only want to ask these details once per account. So I thought I'd use the 'User UID' from Auth. But then, the problem I encountered was, how /where am I supposed to write the code to check if the user already exists when using the above mentioned tutorial. Am I supposed to do it in the rules section in the Database or in the code itself?

I'll post the code I made by following the tutorial. I'm sorry if this is a noob question, but none of the developers I know have experience with Firebase UI. So I don't really have anyone to ask about his. Any help is appreciated.

MainActivity.java

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import com.firebase.ui.auth.AuthUI;
import com.google.firebase.auth.FirebaseAuth;

public class MainActivity extends AppCompatActivity {

private static final int RC_SIGN_IN=0;
private FirebaseAuth auth;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    auth = FirebaseAuth.getInstance();
    if(auth.getCurrentUser() != null) {
        //user already signed in

        Log.d("AUTH", auth.getCurrentUser().getEmail());
        Intent intent = new Intent(this, com.jacob.appdevitae.HomeActivity.class);
        startActivity(intent);
        finish();
    }
    else {
        startActivityForResult(AuthUI.getInstance()
                .createSignInIntentBuilder()
                .setProviders(
                        AuthUI.FACEBOOK_PROVIDER,
                        AuthUI.GOOGLE_PROVIDER)
                .build(),RC_SIGN_IN);
    }
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if(requestCode == RC_SIGN_IN) {
        if(resultCode == RESULT_OK) {
            //User logged in
            Log.d("AUTH", auth.getCurrentUser().getEmail());
            Intent intent = new Intent(this, com.jacob.appdevitae.HomeActivity.class);
            startActivity(intent);
            finish();
        }
        else {
            //User not authenticated
            Log.d("AUTH", "NOT AUTHENTICATED");
        }
    }
}}

Answer 1

First add the dependencie in the app level of gradle
com.google.firebase:firebase-database:9.6.1

The user only sign un once so you dont have to check if user exists, just add the information after signing up.

For example:

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if(requestCode == RC_SIGN_IN) {
    if(resultCode == RESULT_OK) {
        //User logged in
        Log.d("AUTH", auth.getCurrentUser().getEmail());
//create the database
        DatabaseReference mdatabase=FirebaseDatabase.getInstance.getReference.child("User-Information");

        //set the information, you can put how much as you want
        mdatabase.child("user-information1").setValue("User-information1");
        mdatabase.child("user-information2").setValue("User-information2")
             .addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    Log.d(LOG_TAG, "signIn:onComplete:" + task.isSuccessful());

                    if (task.isSuccessful()) {
                        //success
                    } else {
                        //Error

                    }
                }
            });;

        Intent intent = new Intent(this, com.jacob.appdevitae.HomeActivity.class);
        startActivity(intent);
        finish();
    }
    else {
        //User not authenticated
        Log.d("AUTH", "NOT AUTHENTICATED");
    }
}