void writeMcastPort(int mcastPort) {
    if (optionalFields != null) {
      if (mcastPort != 0) {
        optionalFields.setProperty(MCAST_PORT, Integer.toString(mcastPort));
      }
    }
  }
Write the value for the optional field {@link #HOSTED_LOCATORS}.
geode-core/src/main/java/org/apache/geode/distributed/internal/StartupMessageData.java