Java: Convert Optional<List<Integer>> to Optional<ArrayList<Integer>>-CodePudding

I try this but it does not work

Optional<List<Integer>> listOne = someAPI.call()
// convert
Optional<ArrayList<Integer>> listTwo = new ArrayList<>(listOne); // does not work
Optional<ArrayList<Integer>> listTwo = Optional.of(new ArrayList(listOne)); // also not work

Note that this API return Optional and I need to send Optional to another API.

Thank you so much

CodePudding user response：

You can convert it like this:

Optional<ArrayList<Integer>> listTwo = listOne.map(ArrayList::new);

CodePudding user response：

If you start with this:

Optional<List<Integer>> optionalList = getFromSomewhere();

You can convert to Optional<ArrayList<Integer>> like this:

Optional<ArrayList<Integer>> optionalArrayList
        = Optional.of(new ArrayList<>(optionalList.get()));

This is close to what you tried - Optional.of(new ArrayList(listOne)) – but your attempt didn't work because listOne is type Optional<List<Integer>>, whereas the constructor to new ArrayList() takes a Collection. If you call .get() then you'll get the list out of the Optional, and that works fine for the ArrayList constructor.