Firebase Auth (Login)

package com.example.roguex.nyumbaapp;

import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
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;
import com.google.firebase.auth.FirebaseUser;

public class loginActivity extends AppCompatActivity {
    private EditText mEmailField;
    private EditText mPasswordField;
    private Button mLoginBtn;

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;
    private String TAG;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        mAuth = FirebaseAuth.getInstance();
        mEmailField = findViewById(R.id.emailField);
        mPasswordField = findViewById(R.id.passwordField);
        mLoginBtn = findViewById(R.id.loginBtn);


        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                if (firebaseAuth.getCurrentUser() != null){

                    startActivity(new Intent(loginActivity.this, MainActivity.class));
                }
            }
        };

        mLoginBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                startSignIn();
            }
        });


    }

   /**
    @Override
    private void onStart(){
        super.onStart();
        mAuth.addAuthStateListener(mAuthListener);
    }
    **/
    private void startSignIn() {
        String email = mEmailField.getText().toString();
        String password = mPasswordField.getText().toString();

        if (TextUtils.isEmpty(email) || TextUtils.isEmpty(password)) {

            Toast.makeText(loginActivity.this, "Input fields are empty", Toast.LENGTH_SHORT).show();
        } else {
            mAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    if (!task.isSuccessful()) {
                        // Sign in success, update UI with the signed-in user's information
                        Log.d(TAG, "createUserWithEmail:success");
                        FirebaseUser user = mAuth.getCurrentUser();
                        updateUI(user);
                        Toast.makeText(loginActivity.this, "Authentication Failure", Toast.LENGTH_SHORT).show();

                    }else {
                        // If sign in fails, display a message to the user.
                        Log.d(TAG, "createUserWithEmail:failure",  task.getException());
                        Toast.makeText(loginActivity.this, "Authentication Failed.", Toast.LENGTH_SHORT).show();
                        updateUI(null);
                    }
                }
            });
        }
    }

    @Override
    public void onStart() {
        super.onStart();
        // Check if user is signed in (non-null) and update UI accordingly.
        FirebaseUser currentUser = mAuth.getCurrentUser();
        updateUI(currentUser);
    }

    private void updateUI(FirebaseUser currentUser) {
    }
}
//Put these in the build.gradle (Module:app)
//implementation 'com.google.firebase:firebase-auth:11.6.0'
//implementation 'com.google.firebase:firebase-core:11.6.0'