code: public GoogleIdToken verify(String idTokenString) throws GeneralSecurityException, IOException {
      GoogleIdToken idToken = GoogleIdToken.parse(getJsonFactory(), idTokenString);
      return verify(idToken) ? idToken : null;
    }
docs: Verifies that the given ID token is valid using {@link #verify(GoogleIdToken)} and returns the
  ID token if succeeded.
  
  @param idTokenString Google ID token string
  @return Google ID token if verified successfully or {@code null} if failed
  @since 1.9