public class MyNewActivity extends AppCompatActivity {

    private static final int RC_SIGN_IN = 9001;

    private GoogleSignInClient mSignInClient;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        GoogleSignInOptions options =
               new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                        .requestScopes(Drive.SCOPE_FILE)
                        .build();

        mSignInClient = GoogleSignIn.getClient(this, options);
    }

    private void signIn() {
        // Launches the sign in flow, the result is returned in onActivityResult
        Intent intent = mSignInClient.getSignInIntent();
        startActivityForResult(intent, RC_SIGN_IN);
    }

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

        if (requestCode == RC_SIGN_IN) {
            Task<GoogleSignInAccount> task =
                    GoogleSignIn.getSignedInAccountFromIntent(data);
            if (task.isSuccessful()) {
                // Sign in succeeded, proceed with account
                GoogleSignInAccount acct = task.getResult();
            } else {
                // Sign in failed, handle failure and update UI
                // ...
            }
        }
    }
}