Android Login and Registration with Firebase Authentication

In this Tutorial, we will see how we can make User Login and Registration in Android by using Firebase. Firebase Authentication provides backend services, easy-to-use SDKs, and ready-made UI libraries to authenticate users to your app. It supports authentication using passwords, popular federated identity providers like Google, Facebook and Twitter, and more.

Before you begin

1. Add Firebase to your Android project.
2. Add the dependency for Firebase Authentication to your app-level build.gradle file:

   compile 'com.google.firebase:firebase-auth:9.6.1'

3. If you haven't yet connected your app to your Firebase project, do so from the Firebase console.
4. Enable Email/Password sign-in:
   1. In the Firebase console, open the Auth section.
   2. On the Sign in method tab, enable the Email/password sign-in method and click Save.

Creating Android Project

1. Create a new project in Android Studio from File ⇒ New Project. When it prompts you to select the default activity, select Blank Activity and proceed.While filling the project details, use the same package name which you gave in firebase console. In my case I am using same org.snowcorp.firebase.auth
2. Open AndroidManifest.xml and add the INTERNET permission as we need to make network calls.

   <uses-permission android:name="android.permission.INTERNET" />

3. Paste the google-services.json file to your project’s app folder. This step is very important as your project won’t build without this file.
4. Now open the build.gradle located in project’s home directory and add firebase dependency.

   dependencies {
       classpath 'com.android.tools.build:gradle:2.2.1'
       classpath 'com.google.gms:google-services:3.0.0'
       // NOTE: Do not place your application dependencies here; they belong
       // in the individual module build.gradle files
   }

5. Add the dependency for Firebase Authentication to your app-level build.gradle file:

   compile 'com.google.firebase:firebase-auth:9.6.1'

6. Open app/build.gradle and add firebase auth dependency. At the very bottom of the file, add apply plugin: ‘com.google.gms.google-services’

   dependencies {
       compile fileTree(dir: 'libs', include: ['*.jar'])
       // ....
       compile 'com.android.support:appcompat-v7:24.2.1'
       compile 'com.android.support:design:24.2.1'
       compile 'com.google.firebase:firebase-auth:9.6.1'
   }
   apply plugin: 'com.google.gms.google-services'

7. Add the below resources to colors.xml and strings.xml. These resources doesn’t require for firebase, but for this article.

   colors.xml
   <?xml version="1.0" encoding="utf-8"?>
   <resources>
       <color name="colorPrimary">#2196F3</color>
       <color name="colorPrimaryDark">#2196F3</color>
       <color name="colorAccent">#03A9F4</color>
       <color name="white">#ffffff</color>
       <color name="lbl_email">#333333</color>
       <color name="input_login">#222222</color>
       <color name="input_register">#222222</color>
       <color name="input_login_hint">#999999</color>
       <color name="input_register_hint">#999999</color>
       <color name="btn_login_register">#2196F3</color>
       <color name="btn_register">#0D47A1</color>
       <color name="btn_logut">#ff6861</color>
   </resources>

   strings.xml
   <?xml version="1.0" encoding="utf-8"?>
   <resources>
       <string name="app_name">Firebase Auth</string>
       <string name="action_sign_in_short">Register</string>
       <string name="email">Email</string>
       <string name="minimum_password">Password too short, enter minimum 6 characters!</string>
       <string name="auth_failed">Authentication failed, check your email and password or sign up</string>
       <string name="title_activity_login">Sign in</string>
       <string name="hint_email">Email</string>
       <string name="hint_password">Password</string>
       <string name="btn_login">LOGIN</string>
       <string name="btn_link_to_register">Don't have an account? Sing up</string>
       <string name="btn_link_to_login">already registered. login me!</string>
       <string name="btn_forgot_password">Forgot your password?</string>
       <string name="btn_reset_password">Reset Password</string>
       <string name="btn_back"><![CDATA[<< Back]]></string>
       <string name="btn_sign_out">Sign Out</string>
       <string name="lbl_forgot_password">Forgot password?</string>
       <string name="forgot_password_msg">We just need your registered Email Id to sent you password reset instructions.</string>
   </resources>

   Sign Up

   Sign up screen contains two EditText fields for email and password. And few buttons to navigate to login and forgot password screens.
8. Create an activity named SignupActivity.java and add the following code to the layout file activity_signup.xml

   <?xml version="1.0" encoding="utf-8"?>
   <android.support.design.widget.CoordinatorLayout
       xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:tools="http://schemas.android.com/tools"
       xmlns:app="http://schemas.android.com/apk/res-auto"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:fitsSystemWindows="true"
       tools:context=".SignupActivity">
       <android.support.design.widget.AppBarLayout
           android:id="@+id/appbar"
           android:layout_width="match_parent"
           android:layout_height="wrap_content"
           android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">
           <include layout="@layout/toolbar" />
       </android.support.design.widget.AppBarLayout>
       <LinearLayout
           app:layout_behavior="@string/appbar_scrolling_view_behavior"
           android:layout_width="fill_parent"
           android:layout_height="fill_parent"
           android:gravity="center"
           android:orientation="vertical"
           android:padding="@dimen/activity_horizontal_margin">
           <TextView
               android:layout_width="match_parent"
               android:layout_height="wrap_content"
               android:gravity="center"
               android:layout_marginBottom="30dp"
               android:text="Sign up"
               android:textSize="25sp"/>
           <android.support.design.widget.TextInputLayout
               android:layout_width="match_parent"
               android:layout_height="wrap_content">
               <EditText
                   android:id="@+id/email"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:hint="@string/email"
                   android:inputType="textEmailAddress"
                   android:imeOptions="actionNext"
                   android:maxLines="1"
                   android:textColor="@color/input_register" />
           </android.support.design.widget.TextInputLayout>
           <android.support.design.widget.TextInputLayout
               android:layout_width="match_parent"
               android:layout_height="wrap_content">
               <EditText
                   android:id="@+id/password"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:focusableInTouchMode="true"
                   android:hint="@string/hint_password"
                   android:imeOptions="actionUnspecified"
                   android:inputType="textPassword"
                   android:maxLines="1"
                   android:textColor="@color/input_register" />
           </android.support.design.widget.TextInputLayout>
           <!-- Sign up Button -->
           <Button
               android:id="@+id/sign_up_button"
               style="?android:textAppearanceSmall"
               android:layout_width="match_parent"
               android:layout_height="wrap_content"
               android:layout_marginTop="16dp"
               android:background="@color/btn_login_register"
               android:text="@string/action_sign_in_short"
               android:textColor="@color/white"
               android:textStyle="bold" />
           <Button
               android:id="@+id/btn_reset_password"
               android:layout_width="fill_parent"
               android:layout_height="wrap_content"
               android:layout_marginTop="20dip"
               android:background="@null"
               android:text="@string/btn_forgot_password"
               android:textAllCaps="false"
               android:textColor="@color/btn_login_register" />
           <!-- Link to Login Screen -->
           <Button
               android:id="@+id/sign_in_button"
               android:layout_width="fill_parent"
               android:layout_height="wrap_content"
               android:layout_marginTop="10dip"
               android:background="@null"
               android:text="@string/btn_link_to_login"
               android:textAllCaps="false"
               android:textColor="@color/btn_register"
               android:textSize="15sp" />
       </LinearLayout>
       <ProgressBar
           android:id="@+id/progressBar"
           android:layout_width="30dp"
           android:layout_height="30dp"
           android:layout_gravity="center|bottom"
           android:layout_marginBottom="10dp"
           android:visibility="gone" />
   </android.support.design.widget.CoordinatorLayout>

9. Open SignupActivity.java and add the following. Firebase provides createUserWithEmailAndPassword()method to create a new user with email and password data.

   package org.snowcorp.firebase.auth;
   import android.content.Intent;
   import android.os.Bundle;
   import android.support.annotation.NonNull;
   import android.support.v7.app.AppCompatActivity;
   import android.support.v7.widget.Toolbar;
   import android.text.TextUtils;
   import android.view.View;
   import android.widget.Button;
   import android.widget.EditText;
   import android.widget.ProgressBar;
   import android.widget.Toast;
   import com.google.android.gms.tasks.OnCompleteListener;
   import com.google.android.gms.tasks.Task;
   import com.google.firebase.auth.AuthResult;
   import com.google.firebase.auth.FirebaseAuth;
   /**
    * Created by Akshay Raj on 10/17/2016.
    * Snow Corporation Inc.
    * www.snowcorp.org
    */
   public class SignupActivity extends AppCompatActivity {
       private EditText inputEmail, inputPassword;
       private Button btnSignIn, btnSignUp, btnResetPassword;
       private ProgressBar progressBar;
       private FirebaseAuth auth;
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_signup);
           Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
           setSupportActionBar(toolbar);
           getSupportActionBar().setDisplayHomeAsUpEnabled(true);
           //Get Firebase auth instance
           auth = FirebaseAuth.getInstance();
           btnSignIn = (Button) findViewById(R.id.sign_in_button);
           btnSignUp = (Button) findViewById(R.id.sign_up_button);
           inputEmail = (EditText) findViewById(R.id.email);
           inputPassword = (EditText) findViewById(R.id.password);
           progressBar = (ProgressBar) findViewById(R.id.progressBar);
           btnResetPassword = (Button) findViewById(R.id.btn_reset_password);
           btnResetPassword.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {
                   resetPassword();
                   //startActivity(new Intent(SignupActivity.this, ResetPasswordActivity.class));
               }
           });
           btnSignIn.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {
                   finish();
               }
           });
           btnSignUp.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {
                   String email = inputEmail.getText().toString().trim();
                   String password = inputPassword.getText().toString().trim();
                   if (TextUtils.isEmpty(email)) {
                       Toast.makeText(getApplicationContext(), "Enter email address!", Toast.LENGTH_SHORT).show();
                       return;
                   }
                   if (TextUtils.isEmpty(password)) {
                       Toast.makeText(getApplicationContext(), "Enter password!", Toast.LENGTH_SHORT).show();
                       return;
                   }
                   if (password.length() < 6) {
                       Toast.makeText(getApplicationContext(), "Password too short, enter minimum 6 characters!", Toast.LENGTH_SHORT).show();
                       return;
                   }
                   progressBar.setVisibility(View.VISIBLE);
                   //create user
                   auth.createUserWithEmailAndPassword(email, password)
                           .addOnCompleteListener(SignupActivity.this, new OnCompleteListener<AuthResult>() {
                               @Override
                               public void onComplete(@NonNull Task<AuthResult> task) {
                                   Toast.makeText(SignupActivity.this, "createUserWithEmail:onComplete:" + task.isSuccessful(), Toast.LENGTH_SHORT).show();
                                   progressBar.setVisibility(View.GONE);
                                   // If sign in fails, display a message to the user. If sign in succeeds
                                   // the auth state listener will be notified and logic to handle the
                                   // signed in user can be handled in the listener.
                                   if (!task.isSuccessful()) {
                                       Toast.makeText(SignupActivity.this, "Authentication failed." + task.getException(),
                                               Toast.LENGTH_SHORT).show();
                                   } else {
                                       startActivity(new Intent(SignupActivity.this, MainActivity.class));
                                       finish();
                                   }
                               }
                           });
               }
           });
       }
       @Override
       protected void onResume() {
           super.onResume();
           progressBar.setVisibility(View.GONE);
       }
   }

   If you login to Firebase console, you can see the user created with the email id you have given the android app.

   Log In

   Now we’ll add the login screen and check the credentials we have created on sign up screen.
10. Create another activity named LoginActivity.java and add the below code to its layout file activity_login.xml.

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.design.widget.CoordinatorLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true"
        tools:context=".LoginActivity">
        <android.support.design.widget.AppBarLayout
            android:id="@+id/appbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">
            <include layout="@layout/toolbar" />
        </android.support.design.widget.AppBarLayout>
        <LinearLayout
            app:layout_behavior="@string/appbar_scrolling_view_behavior"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:gravity="center"
            android:orientation="vertical"
            android:padding="@dimen/activity_horizontal_margin">
            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:gravity="center"
                android:layout_marginBottom="30dp"
                android:text="Login"
                android:textSize="25sp"/>
            <android.support.design.widget.TextInputLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content">
                <EditText
                    android:id="@+id/email"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:layout_marginBottom="10dp"
                    android:hint="@string/hint_email"
                    android:inputType="textEmailAddress"
                    android:imeOptions="actionNext"
                    android:textColor="@color/input_login"
                    android:textColorHint="@color/input_login_hint" />
            </android.support.design.widget.TextInputLayout>
            <android.support.design.widget.TextInputLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content">
                <EditText
                    android:id="@+id/password"
                    android:layout_width="fill_parent"
                    android:layout_height="wrap_content"
                    android:layout_marginBottom="10dp"
                    android:hint="@string/hint_password"
                    android:inputType="textPassword"
                    android:imeOptions="actionDone"
                    android:textColor="@color/input_login"
                    android:textColorHint="@color/input_login_hint" />
            </android.support.design.widget.TextInputLayout>
            <!-- Login Button -->
            <Button
                android:id="@+id/btn_login"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="20dip"
                android:background="@color/btn_login_register"
                android:text="@string/btn_login"
                android:textColor="@color/white" />
            <Button
                android:id="@+id/btn_reset_password"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="20dip"
                android:background="@null"
                android:text="@string/btn_forgot_password"
                android:textAllCaps="false"
                android:textColor="@color/btn_login_register" />
            <!-- Link to Sign up Screen -->
            <Button
                android:id="@+id/btn_signup"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="10dip"
                android:background="@null"
                android:text="@string/btn_link_to_register"
                android:textAllCaps="false"
                android:textColor="@color/btn_register"
                android:textSize="15sp" />
        </LinearLayout>
        <ProgressBar
            android:id="@+id/progressBar"
            android:layout_width="30dp"
            android:layout_height="30dp"
            android:layout_gravity="center|bottom"
            android:layout_marginBottom="10dp"
            android:visibility="gone" />
    </android.support.design.widget.CoordinatorLayout>

11. Open LoginActivity.java and do the below changes. Firebase provides signInWithEmailAndPassword()method to sign in the user.

    package org.snowcorp.firebase.auth;
    import android.content.Intent;
    import android.os.Bundle;
    import android.support.annotation.NonNull;
    import android.support.v7.app.AppCompatActivity;
    import android.support.v7.widget.Toolbar;
    import android.text.TextUtils;
    import android.view.View;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.ProgressBar;
    import android.widget.Toast;
    import com.google.android.gms.tasks.OnCompleteListener;
    import com.google.android.gms.tasks.Task;
    import com.google.firebase.auth.AuthResult;
    import com.google.firebase.auth.FirebaseAuth;
    /**
     * Created by Akshay Raj on 10/17/2016.
     * Snow Corporation Inc.
     * www.snowcorp.org
     */
    public class LoginActivity extends AppCompatActivity {
        private EditText inputEmail, inputPassword;
        private FirebaseAuth auth;
        private ProgressBar progressBar;
        private Button btnSignup, btnLogin, btnReset;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            //Get Firebase auth instance
            auth = FirebaseAuth.getInstance();
            if (auth.getCurrentUser() != null) {
                startActivity(new Intent(LoginActivity.this, MainActivity.class));
                finish();
            }
            // set the view now
            setContentView(R.layout.activity_login);
            Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
            setSupportActionBar(toolbar);
            inputEmail = (EditText) findViewById(R.id.email);
            inputPassword = (EditText) findViewById(R.id.password);
            progressBar = (ProgressBar) findViewById(R.id.progressBar);
            btnSignup = (Button) findViewById(R.id.btn_signup);
            btnLogin = (Button) findViewById(R.id.btn_login);
            btnReset = (Button) findViewById(R.id.btn_reset_password);
            //Get Firebase auth instance
            auth = FirebaseAuth.getInstance();
            btnSignup.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    startActivity(new Intent(LoginActivity.this, SignupActivity.class));
                }
            });
            btnReset.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    resetPassword();
                    //startActivity(new Intent(LoginActivity.this, ResetPasswordActivity.class));
                }
            });
            btnLogin.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    String email = inputEmail.getText().toString();
                    final String password = inputPassword.getText().toString();
                    if (TextUtils.isEmpty(email)) {
                        Toast.makeText(getApplicationContext(), "Enter email address!", Toast.LENGTH_SHORT).show();
                        return;
                    }
                    if (TextUtils.isEmpty(password)) {
                        Toast.makeText(getApplicationContext(), "Enter password!", Toast.LENGTH_SHORT).show();
                        return;
                    }
                    progressBar.setVisibility(View.VISIBLE);
                    //authenticate user
                    auth.signInWithEmailAndPassword(email, password)
                            .addOnCompleteListener(LoginActivity.this, new OnCompleteListener<AuthResult>() {
                                @Override
                                public void onComplete(@NonNull Task<AuthResult> task) {
                                    // If sign in fails, display a message to the user. If sign in succeeds
                                    // the auth state listener will be notified and logic to handle the
                                    // signed in user can be handled in the listener.
                                    progressBar.setVisibility(View.GONE);
                                    if (!task.isSuccessful()) {
                                        // there was an error
                                        if (password.length() < 6) {
                                            inputPassword.setError(getString(R.string.minimum_password));
                                        } else {
                                            Toast.makeText(LoginActivity.this, getString(R.string.auth_failed), Toast.LENGTH_LONG).show();
                                        }
                                    } else {
                                        Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                                        startActivity(intent);
                                        finish();
                                    }
                                }
                            });
                }
            });
        }
    }

    Run the project and login with the credentials which you used while signing up.

    Forgot Password (Send Reset Email)

    Another cool feature firebase provides is, sending reset password email when required. This is a very difficult task if you want to have your own email server.
12. Create a layout file activity_reset_password.xml and add the below code

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.design.widget.CoordinatorLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:fitsSystemWindows="true">
        <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:orientation="vertical"
            android:paddingLeft="@dimen/activity_horizontal_margin"
            android:paddingRight="@dimen/activity_horizontal_margin">
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center_horizontal"
                android:padding="10dp"
                android:text="@string/lbl_forgot_password"
                android:textColor="@color/colorAccent"
                android:textSize="20dp" />
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginBottom="10dp"
                android:gravity="center_horizontal"
                android:padding="@dimen/activity_horizontal_margin"
                android:text="@string/forgot_password_msg"
                android:textColor="@color/colorAccent"
                android:textSize="14dp" />
            <android.support.design.widget.TextInputLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content">
                <EditText
                    android:id="@+id/email"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:layout_marginBottom="10dp"
                    android:layout_marginTop="20dp"
                    android:hint="@string/hint_email"
                    android:inputType="textEmailAddress"
                    android:textColor="@color/input_login"
                    android:textColorHint="@color/input_login_hint" />
            </android.support.design.widget.TextInputLayout>
            <!-- Login Button -->
            <Button
                android:id="@+id/btn_reset_password"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="20dip"
                android:background="@color/btn_login_register"
                android:text="@string/btn_reset_password"
                android:textColor="@android:color/white" />
            <Button
                android:id="@+id/btn_back"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginTop="10dp"
                android:background="@null"
                android:text="@string/btn_back"
                android:textColor="@color/btn_register" />
            <ProgressBar
                android:id="@+id/progressBar"
                android:layout_width="30dp"
                android:layout_height="30dp"
                android:layout_marginBottom="10dp"
                android:visibility="gone" />
        </LinearLayout>
    </android.support.design.widget.CoordinatorLayout>

13. Open your SignupActivity.java and LoginActivity.java and add below code

    private void resetPassword() {
        final AlertDialog.Builder dialogBuilder = new AlertDialog.Builder(this);
        LayoutInflater inflater = getLayoutInflater();
        final View dialogView = inflater.inflate(R.layout.activity_reset_password, null);
        dialogBuilder.setView(dialogView);
        final EditText editEmail = (EditText) dialogView.findViewById(R.id.email);
        final Button btnReset = (Button) dialogView.findViewById(R.id.btn_reset_password);
        final ProgressBar progressBar1 = (ProgressBar) dialogView.findViewById(R.id.progressBar);
        //dialogBuilder.setTitle("Send Photos");
        final AlertDialog dialog = dialogBuilder.create();
        btnReset.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                String email = editEmail.getText().toString().trim();
                if (TextUtils.isEmpty(email)) {
                    Toast.makeText(getApplication(), "Enter your registered email id", Toast.LENGTH_SHORT).show();
                    return;
                }
                progressBar1.setVisibility(View.VISIBLE);
                auth.sendPasswordResetEmail(email)
                        .addOnCompleteListener(new OnCompleteListener<Void>() {
                            @Override
                            public void onComplete(@NonNull Task<Void> task) {
                                if (task.isSuccessful()) {
                                    Toast.makeText(LoginActivity.this, "We have sent you instructions to reset your password!", Toast.LENGTH_SHORT).show();
                                } else {
                                    Toast.makeText(LoginActivity.this, "Failed to send reset email!", Toast.LENGTH_SHORT).show();
                                }
                                progressBar1.setVisibility(View.GONE);
                                dialog.dismiss();
                            }
                        });
            }
        });
        dialog.show();
    }

    Here is the password reset email user will receive.

    Welcome Screen

    
14. Open the layout file of main activity activity_main.xml and add the below layout code.

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.design.widget.CoordinatorLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true"
        tools:context=".MainActivity">
        <android.support.design.widget.AppBarLayout
            android:id="@+id/appbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">
            <include layout="@layout/toolbar" />
        </android.support.design.widget.AppBarLayout>
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            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"
            app:layout_behavior="@string/appbar_scrolling_view_behavior">
            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:id="@+id/email"
                android:gravity="center"
                android:textAppearance="@style/TextAppearance.AppCompat.Medium"/>
            <Button
                android:id="@+id/sign_out"
                style="?android:textAppearanceSmall"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:background="@color/colorPrimary"
                android:text="@string/btn_sign_out"
                android:textColor="@android:color/white"
                android:textStyle="bold" />
        </LinearLayout>
    </android.support.design.widget.CoordinatorLayout>

15. Open the MainActivity.java and the following code.

    package org.snowcorp.firebase.auth;
    import android.content.Intent;
    import android.os.Bundle;
    import android.support.annotation.NonNull;
    import android.support.v7.app.AppCompatActivity;
    import android.support.v7.widget.Toolbar;
    import android.view.View;
    import android.widget.Button;
    import android.widget.TextView;
    import com.google.firebase.auth.FirebaseAuth;
    import com.google.firebase.auth.FirebaseUser;
    public class MainActivity extends AppCompatActivity {
        private TextView email;
        private Button signOut;
        private FirebaseAuth.AuthStateListener authListener;
        private FirebaseAuth auth;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
            toolbar.setTitle(getString(R.string.app_name));
            setSupportActionBar(toolbar);
            //get firebase auth instance
            auth = FirebaseAuth.getInstance();
            //get current user
            final FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
            authListener = new FirebaseAuth.AuthStateListener() {
                @Override
                public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                    FirebaseUser user = firebaseAuth.getCurrentUser();
                    if (user == null) {
                        // user auth state is changed - user is null
                        // launch login activity
                        startActivity(new Intent(MainActivity.this, LoginActivity.class));
                        finish();
                    }
                }
            };
            signOut = (Button) findViewById(R.id.sign_out);
            email = (TextView) findViewById(R.id.email);
            email.setText(user.getEmail());
            signOut.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    signOut();
                }
            });
        }
        //sign out method
        public void signOut() {
            auth.signOut();
        }
        @Override
        public void onStart() {
            super.onStart();
            auth.addAuthStateListener(authListener);
        }
        @Override
        public void onStop() {
            super.onStop();
            if (authListener != null) {
                auth.removeAuthStateListener(authListener);
            }
        }
    }

Now you have the basics for creating a simple login / register app with firebase. Below are the quick code snippets to other user functionalities.

Checking User Session

auth = FirebaseAuth.getInstance();
if (auth.getCurrentUser() != null) {
    // User is logged in
}

Change Password

FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
user.updatePassword(newPassword.getText().toString().trim())
        .addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {
                if (task.isSuccessful()) {
                    Toast.makeText(MainActivity.this, "Password is updated!", Toast.LENGTH_SHORT).show();
                } else {
                    Toast.makeText(MainActivity.this, "Failed to update password!", Toast.LENGTH_SHORT).show();
                    progressBar.setVisibility(View.GONE);
                }
            }
        });

Change Email

FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
user.updateEmail(newEmail.getText().toString().trim())
        .addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {
                if (task.isSuccessful()) {
                    Toast.makeText(MainActivity.this, "Email address is updated.", Toast.LENGTH_LONG).show();
                } else {
                    Toast.makeText(MainActivity.this, "Failed to update email!", Toast.LENGTH_LONG).show();
                }
            }
        });

Deleting Account / User

FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
if (user != null) {
    user.delete()
            .addOnCompleteListener(new OnCompleteListener<Void>() {
                @Override
                public void onComplete(@NonNull Task<Void> task) {
                    if (task.isSuccessful()) {
                        Toast.makeText(MainActivity.this, "Your profile is deleted:( Create a account now!", Toast.LENGTH_SHORT).show();
                    } else {
                        Toast.makeText(MainActivity.this, "Failed to delete your account!", Toast.LENGTH_SHORT).show();
                    }
                }
            });
}

Sign Out

auth.signOut();
// this listener will be called when there is change in firebase user session
FirebaseAuth.AuthStateListener authListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser user = firebaseAuth.getCurrentUser();
        if (user == null) {
            // user auth state is changed - user is null
            // launch login activity
            startActivity(new Intent(MainActivity.this, LoginActivity.class));
            finish();
        }
    }
};

Download Firebase Authentication Source Code