code: public void add(int index, Double value) {
  		if (value == null) {
  			stateList.add(index, Type.NULL);
  		} else {
  			stateList.add(index, Type.DOUBLE);
  		}
  
  		super.add(index, value);
  	}
docs: see {@link List#add(int, Object)}
  this method is alternative of {@link #add(int, Object, Type)} call with {@link Type#DOUBLE}.
  @param index
  @param value
  @since 1.4.12
  @author vvakame