MainActivity Tutorial Firebase

package info.rekayasa.reinfirebase;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

import com.google.firebase.auth.FirebaseAuth;
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;

import java.util.HashMap;

public class MainActivity extends AppCompatActivity {

    FirebaseAuth mAuth;
    // Firebase Database
    DatabaseReference mRef, mRoot;
    FirebaseDatabase mDatabase;
    // Declare views
    TextView tvFullName, tvEmail;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        mAuth = FirebaseAuth.getInstance();
        mDatabase = FirebaseDatabase.getInstance();

        // Check authentication
        if (mAuth.getCurrentUser() == null){
            startActivity(new Intent(getApplicationContext(), Authentication.class));
            finish();
        }

        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        tvFullName = (TextView) findViewById(R.id.tvFullName);
        tvEmail = (TextView) findViewById(R.id.tvEmail);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        mRoot = mDatabase.getReference();
        String userId = mAuth.getCurrentUser().getUid();
        mRef = mRoot.child("users").child(userId);
        mRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                User user = dataSnapshot.getValue(User.class);
                tvFullName.setText(user.getFullName());
                tvEmail.setText(user.getEmail());
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.signOut) {
            mAuth.signOut();
            finish();
            startActivity(new Intent(getApplicationContext(), Authentication.class));
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}