code: @Override
  	public void addProperty(final String key, final Object value) {
  		if (containsKey(key)) {
  			String newValue = get(key) + ',' + (value == null ? "" : value);
  			addOrModifyProperty(key, newValue);
  		} else {
  			addOrModifyProperty(key, value == null ? null : value.toString());
  		}
  	}
docs: {@inheritDoc}