code: @Override
  	public Color put(Double position, Color color) {
  		if( position == null ) {
  			throw new NullPointerException("Null endpoint position");
  		}
  		if( color == null ){
  			throw new NullPointerException("Null colors are not allowed.");
  		}
  		return mapping.put(position, color);
  	}
docs: Adds a gradient endpoint at the specified position.
  @param position The endpoint position. May be Double.POSITIVE_INFINITY or Double.NEGATIVE_INFINITY for endpoints.
  @param color
  @return
  @see java.util.Map#put(java.lang.Object, java.lang.Object)