public class LoginActivity extends AppCompatActivity {

//a constant for detecting the login intent result
private static final int RC_SIGN_IN = 1;

//Tag for the logs optional
private static final String TAG = "AndroidMads";

//creating a GoogleSignInClient object
GoogleSignInClient mGoogleSignInClient;

//And also a Firebase Auth object
FirebaseAuth mAuth;

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

    //first we initialized the Firebase Auth object
    mAuth = FirebaseAuth.getInstance();

    //Then we need a GoogleSignInOptions object
    //And we need to build it as below
    GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestIdToken(getString(R.string.default_web_client_id))
            .requestEmail()
            .build();

    //Then we will get the GoogleSignInClient object from GoogleSignIn class
    mGoogleSignInClient = GoogleSignIn.getClient(this, gso);

    //Now we will attach a click listener to the sign_in_button
    //and inside onClick() method we are calling the signIn() method that will open
    //google sign in intent
    findViewById(R.id.sign_in_button).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            //getting the google signin intent
            Intent signInIntent = mGoogleSignInClient.getSignInIntent();
            //starting the activity for result
            startActivityForResult(signInIntent, RC_SIGN_IN);
        }
    });
}

@Override
protected void onStart() {
    super.onStart();
    //if the user is already signed in
    //we will close this activity
    //and take the user to profile activity
    if (mAuth.getCurrentUser() != null) {
        finish();
        startActivity(new Intent(this, ActivitySplash.class));
    }
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    //if the requestCode is the Google Sign In code that we defined at starting
    if (requestCode == RC_SIGN_IN) {

        //Getting the GoogleSignIn Task
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        try {
            //Google Sign In was successful, authenticate with Firebase
            GoogleSignInAccount account = task.getResult(ApiException.class);
            //authenticating with firebase
            firebaseAuthWithGoogle(account);
        } catch (ApiException e) {
            Log.v("API Exception", e.toString());
            Toast.makeText(LoginActivity.this, e.getMessage(), Toast.LENGTH_SHORT).show();
        }
    }
}

private void firebaseAuthWithGoogle(GoogleSignInAccount acct) {
    Log.d(TAG, "firebaseAuthWithGoogle:" + acct.getId());

    //getting the auth credential
    AuthCredential credential = GoogleAuthProvider.getCredential(acct.getIdToken(), null);

    //Now using firebase we are signing in the user here
    mAuth.signInWithCredential(credential)
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    if (task.isSuccessful()) {
                        if (mAuth.getCurrentUser() != null) {
                            startActivity(new Intent(LoginActivity.this, ActivitySplash.class));
                            finish();
                        }
                    } else {
                        // If sign in fails, display a message to the user.
                        Log.w(TAG, "signInWithCredential:failure", task.getException());
                        Toast.makeText(LoginActivity.this, "Authentication failed.",
                                Toast.LENGTH_SHORT).show();

                    }
                }
            });
}}