code: public boolean isSet(String key) {
      if (key == null) throw new IllegalArgumentException(CLASS + ": key may not be null");
      if (!keys.containsKey(key)) throw new IllegalArgumentException(CLASS + ": unknown key: " + key);
      return keys.get(key).getResultCount() > 0 ? true : false;
    }
docs: Check whether a specific option is set, i. e. whether it was specified at least once on the command line.
  <p>
  @param key The key for the option
  <p>
  @return <code>true</code> or <code>false</code>, depending on the outcome of the check
  <p>
  @throws IllegalArgumentException If the <code>key</code> is <code>null</code> or unknown in this set