Java Optional orElseThrow with empty collection

Question

I\'m implementing a stream in which I use a collection listOfFoo to get ids of all items in that list and use them to get values of Bar instances.

Answer 1

The book Effective Java mentions the following:

Container types, including collections, maps, streams, arrays, and optionals should not be wrapped in optionals. (P.252)

Answer 2

I don't really see the benefit of using Optional, it would be more readable without it :

List<Bar> bars = listOfFoos.stream()
   .map(Foo::getId)       
   .map(service::getBars)                    
   .collect(Collectors.toList());

if (bars.isEmpty()) {
   throw new ResourceNotFoundException(Bar.class, OBJECT_NULL);
}

Answer 3

Just add an Optional.filter for it then. You could do it as :

List<Bar> bars = Optional.ofNullable(
        listOfFoos.stream().map(fooId -> service.getBars(fooId))
                .filter(Objects::nonNull).collect(Collectors.toList()))
        .filter(a -> !a.isEmpty())
        .orElseThrow(() -> new ResourceNotFoundException(Bar.class, OBJECT_NULL));

Aside: By the implementation shared in the code, the list returned by the stream could not be null, so Optional.ofNullable could possibly be replaced by Optional.of.