int RC_SIGN_IN = 1;
private FirebaseAuth mAuth = FirebaseAuth.getInstance();
GoogleSignInClient mGoogleSignInClient;
GoogleSignInOptions gso;

gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                        .requestIdToken(getString(R.string.default_web_client_id))
                        .requestEmail()
                        .build();

mGoogleSignInClient = GoogleSignIn.getClient(MainActivity.this, gso);
Intent signInIntent = mGoogleSignInClient.getSignInIntent();
startActivityForResult(signInIntent, RC_SIGN_IN);

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (requestCode == RC_SIGN_IN) {
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        try {
            GoogleSignInAccount account = task.getResult(ApiException.class);
            firebaseAuthWithGoogle(account);
        } catch (ApiException e) {
                Log.d("demo", "Google sign in failed: " + e.getMessage());
        }
    }
}

private void firebaseAuthWithGoogle(GoogleSignInAccount acct) {
    AuthCredential credential = GoogleAuthProvider.getCredential(acct.getIdToken(), null);
    mAuth.signInWithCredential(credential).addOnCompleteListener(this, new OnCompleteListener < AuthResult > () {
        @Override
        public void onComplete(@NonNull Task < AuthResult > task) {
            if (task.isSuccessful()) {
                FirebaseUser user = mAuth.getCurrentUser();
                Intent intent = new Intent(MainActivity.this, TripsActivity.class);
                startActivity(intent);
                finish();
            } else {
                Log.d("demo", "Google sign in failed");
            }
        }
    });
}

//Logout

FirebaseAuth.getInstance().signOut();
gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
    .requestIdToken(getString(R.string.default_web_client_id))
    .requestEmail()
    .build();

mGoogleSignInClient = GoogleSignIn.getClient(getActivity(), gso);
mGoogleSignInClient.signOut()
    .addOnCompleteListener(getActivity(), new OnCompleteListener < Void > () {
        @Override
        public void onComplete(@NonNull Task < Void > task) {
            Intent intent = new Intent(getActivity(), MainActivity.class);
            startActivity(intent);
            getActivity().finish();
        }
    });
}