Optionals erase types, even with casting

OptionalErasure.java

List<Map> resultTemplates = new ArrayList();

// Will not compile
List<String> descriptors =
    Optional.of(resultTemplates.get(0))
        .map(m -> m.get("descriptors"))
        .map(List.class::cast)
        .orElseThrow(() -> new AssertionError("Result template data was malformed"))
        .stream()
        .map(String.class::cast) // Explicitly ignoring this cast
        .collect(Collectors.toList()); // Returning an Object, not a List

Need to tell the compiler that the data in the optional is a List<?> so raw types are not used automatically.

Method references cannot transport the type information on their own. Either of these would work:

outsideList
        .stream()
        .findFirst()
        .map((Function<Object, List<?>>) List.class::cast)
        .orElseThrow(RuntimeException::new)
        .stream()
        .map(String.class::cast)
        .collect(Collectors.toList());

Or:

outsideList
        .stream()
        .findFirst()
        .map(l -> (List<?>) l)
        .orElseThrow(RuntimeException::new)
        .stream()
        .map(String.class::cast)
        .collect(Collectors.toList());