Home.java

package com.example.twostepauthentication;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;

import android.content.Intent;
import android.os.Bundle;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

public class MainActivity extends AppCompatActivity {
    //firebase auth to check if user is authenticated.
    private FirebaseAuth mAuth;
    private DatabaseReference userRef;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.main_toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("Home");

        mAuth = FirebaseAuth.getInstance();
        userRef = FirebaseDatabase.getInstance().getReference().child("Users");
    }
//onstart method used to check if the user is registered or not
    @Override
    protected void onStart()
    {
        super.onStart();

        FirebaseUser currentUser = mAuth.getCurrentUser();
        if (currentUser ==null){
            SendUserToLoginActivity();
        }
        else{
            //checking if the user exists in the firebase database
            CheckUserExistence();
        }
    }

    private void CheckUserExistence()
    {
        //get the user id
        final String currentUserId = mAuth.getCurrentUser().getUid();
        userRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot)
            {
                if (!dataSnapshot.hasChild(currentUserId)){
                    //user is authenticated but but his record is not present in real time firebase database
                    SendUserToStepTwoAuthentication();
                }

            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    private void SendUserToStepTwoAuthentication()
    {
        Intent steptwoIntent = new Intent(MainActivity.this, StepTwoAuthentication.class);
        steptwoIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(steptwoIntent);
        finish();
    }

    private void SendUserToLoginActivity()
    {
        Intent loginIntent = new Intent(MainActivity.this, Login.class);
        loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(loginIntent);
        finish();
    }
}