code: @Override
  	public String put(String key, String value) throws PropertiesException {
  		if(isLocked()) {
  			throw new PropertiesException("properties map is locked, its contents cannot be altered.");
  		}		
  		return super.put(key, value);
  	}
docs: Puts a new value into the properties map.
  
  @param key
  the key of the property to set.
  @param value
  the value of the property to set.
  @return
  the previous value associated with the key, or null if no such value
  was present in the map.
  @throws PropertiesException
  if the map is in read-only mode (note: it's a runtime exception).