如何在Java中对通配符列表进行排序

Question

I am new to Java Streams and i want to sort a list. The list is of type:

List<? extends PositionType> positions

I want to sort this list on the basis of a field that we get on casting each PositionType object to a subclass ConvertedPosition in ascending order.

So far i have tried this:

List<ConvertedPosition> sortedPositions = positions.stream()
                .sorted(Comparator.comparingInt(((ConvertedPosition)PositionType::getPnlSpn()))
                .collect(Collectors.toList());

But i am getting compilation error:

The method comparingInt(ToIntFunction<? super T>) in the type Comparator is not applicable for the arguments (ConvertedPosition)
The target type of this expression must be a functional interface

Answer 1

I would highly discourage the current attempt utilizing type casting. What if there is an item in your list that is not of type ConvertedPosition ?

If it is feasible to discard items that are not of type ConvertedPosition , then I would propose the following extension of the presented code:

List<ConvertedPosition> sortedPositions = positions.stream()
    .filter(e -> e instance ConvertedPosition) // filter out all non-ConvertedPosition items
    .map(e -> (ConvertedPosition) e)           // cast is safe due to the previous filter
    .sorted(Comparator.comparingInt(PositionType::getPnlSpn()))
    .collect(Collectors.toList());

Ideone example (1)

If discarding non- ConvertedPosition items is not an option, there is also the possibility of writing some kind of converter. Yon can find a sample solution in this Ideone example (2) .