code: protected void addToDefaults(String key, String value) {
  		final String currentValue = defaultData.get(key);
  		if (currentValue == null) {
  			if (value == null) {
  				value = DEFAULT_UNDEFINED;
  			}
  			defaultData.put(key, value);
  		} else {
  			// there is already an entry for this key. Check if the value is the undefined
  			if (currentValue.equals(DEFAULT_UNDEFINED) && value != null) {
  				// update key with better default value
  				defaultData.put(key, value);
  			}
  		}
  	}
docs: --------------- Internals