Firebase Authentication

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              xmlns:tools="http://schemas.android.com/tools"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:gravity="center_horizontal"
              android:orientation="vertical"

              tools:context="com.tabian.firebaseauthentication.MainActivity">

    <ScrollView
        android:id="@+id/login_form"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <LinearLayout
            android:id="@+id/email_login_form"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">



            <EditText
                android:id="@+id/email"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="@string/prompt_email"
                android:inputType="textEmailAddress"
                android:maxLines="1"/>



            <EditText
                android:id="@+id/password"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="Password"
                android:imeActionId="@+id/login"
                android:imeActionLabel="@string/action_sign_in_short"
                android:imeOptions="actionUnspecified"
                android:inputType="textPassword"
                android:maxLines="1"
                />

            <Button
                android:id="@+id/email_sign_in_button"
                style="?android:textAppearanceSmall"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:text="Sign In"
                android:textStyle="bold"/>

            <Button
                android:id="@+id/email_sign_out_button"
                style="?android:textAppearanceSmall"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:text="Sign out"
                android:textStyle="bold"/>

            <Button
                android:id="@+id/add_item_screen"
                style="?android:textAppearanceSmall"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:text="Add Items to Database"
                android:textStyle="bold"/>

        </LinearLayout>
    </ScrollView>
</LinearLayout>

MainActivity.java

package com.tabian.firebaseauthentication;

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

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

import static android.R.id.message;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;

    // UI references.
    private EditText mEmail, mPassword;
    private Button btnSignIn,btnSignOut;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //declare buttons and edit texts in oncreate
        mEmail = (EditText) findViewById(R.id.email);
        mPassword = (EditText) findViewById(R.id.password);
        btnSignIn = (Button) findViewById(R.id.email_sign_in_button);
        btnSignOut = (Button) findViewById(R.id.email_sign_out_button);
        

        mAuth = FirebaseAuth.getInstance();

        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    // User is signed in
                    Log.d(TAG, "onAuthStateChanged:signed_in:" + user.getUid());
                    toastMessage("Successfully signed in with: " + user.getEmail());
                } else {
                    // User is signed out
                    Log.d(TAG, "onAuthStateChanged:signed_out");
                    toastMessage("Successfully signed out.");
                }
                // ...
            }
        };

        btnSignIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String email = mEmail.getText().toString();
                String pass = mPassword.getText().toString();
                if(!email.equals("") && !pass.equals("")){
                    mAuth.signInWithEmailAndPassword(email,pass);
                }else{
                    toastMessage("You didn't fill in all the fields.");
                }
            }
        });

        btnSignOut.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                mAuth.signOut();
                toastMessage("Signing Out...");
            }
        });


    @Override
    public void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthListener);
    }

    @Override
    public void onStop() {
        super.onStop();
        if (mAuthListener != null) {
            mAuth.removeAuthStateListener(mAuthListener);
        }
    }

    //add a toast to show when successfully signed in
    /**
     * customizable toast
     * @param message
     */
    private void toastMessage(String message){
        Toast.makeText(this,message,Toast.LENGTH_SHORT).show();
    }
}