MainActivity.java

package com.example.nabeel.postandcommenttutorial.ui.activities;

import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageView;
import android.widget.TextView;

import com.bumptech.glide.Glide;

import com.example.nabeel.postandcommenttutorial.R;
import com.example.nabeel.postandcommenttutorial.models.User;
import com.example.nabeel.postandcommenttutorial.ui.fragments.homeFragment;
import com.example.nabeel.postandcommenttutorial.utils.BaseActivity;
import com.example.nabeel.postandcommenttutorial.utils.FirebaseUtils;
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.ValueEventListener;

public class MainActivity extends BaseActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    private FirebaseAuth.AuthStateListener mAuthStateListener;
    private ImageView mDisplayImageView;
    private TextView mNameTextView;
    private TextView mEmailTextView;
    private ValueEventListener mUserValueEventListener;
    private DatabaseReference mUserRef;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        mAuthStateListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                if (firebaseAuth.getCurrentUser() == null) {
                    startActivity(new Intent(MainActivity.this, RegisterActivity.class));
                }
            }
        };

        init();
        getSupportFragmentManager().beginTransaction().replace(R.id.container, new HomeFragment())
                .commit();


        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

        View navHeaderView = navigationView.getHeaderView(0);
        initNavHeader(navHeaderView);
    }

    private void init() {
        if (mFirebaseUser != null) {
            mUserRef = FirebaseUtils.getUserRef(mFirebaseUser.getEmail().replace(".", ","));
        }
    }

    private void initNavHeader(View view) {
        mDisplayImageView = (ImageView) view.findViewById(R.id.imageView_display);
        mNameTextView = (TextView) view.findViewById(R.id.textview_name);
        mEmailTextView = (TextView) view.findViewById(R.id.textView_email);

        mUserValueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if (dataSnapshot.getValue() != null) {
                    User users = dataSnapshot.getValue(User.class);
                    Glide.with(MainActivity.this)
                            .load(users.getPhotoUrl())
                            .into(mDisplayImageView);

                    mNameTextView.setText(users.getUser());
                    mEmailTextView.setText(users.getEmail());
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        };
    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

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

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_camera) {
            // Handle the camera action
        } else if (id == R.id.nav_gallery) {

        } else if (id == R.id.nav_slideshow) {

        } else if (id == R.id.nav_manage) {

        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_send) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }

    @Override
    protected void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthStateListener);
        if (mUserRef != null) {
            mUserRef.addValueEventListener(mUserValueEventListener);
        }
    }

    @Override
    protected void onStop() {
        super.onStop();
        if (mAuthStateListener != null)
            mAuth.removeAuthStateListener(mAuthStateListener);
        if (mUserRef != null)
            mUserRef.removeEventListener(mUserValueEventListener);
    }
}

nav_header_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="@dimen/nav_header_height"
    android:background="@drawable/side_nav_bar"
    android:gravity="bottom"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:theme="@style/ThemeOverlay.AppCompat.Dark">

    <de.hdodenhof.circleimageview.CircleImageView
        android:id="@+id/imageView_display"
        android:layout_width="60dp"
        android:layout_height="60dp"
        android:paddingTop="@dimen/nav_header_vertical_spacing"
        android:src="@drawable/facebook" />

    <TextView
        android:id="@+id/textview_name"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/nav_header_vertical_spacing"
        android:text="Android Studio"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1" />

    <TextView
        android:id="@+id/textView_email"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="[email protected]" />

</LinearLayout>