About Question enthuware.ocpjp.v8.3.1889 :

[henrid]

Why is collect(Collectors.averagingDouble(...)) returning a Double and not an OptionalDouble, just like the DoubleStream.average() method?
collect(Collectors.maxBy(...)) results in an Optional<T> just like Stream.max(), so why this difference?

[admin]

Because Collectors.averagingDouble(...) takes in a ToDoubleFunction, which returns a double instead of OptionalDouble or Optional<T> (as is the case with maxBy).

[henrid]

Okay thanks, and what if the stream is empty? Will it return 0? Or throw an Exception?

[admin]

What happened when you tried it out?
BTW, the Collectors javadoc, clearly says, "If no elements are present, the result is 0.".

[henrid]

I was planning to try it out, and I read a lot of API, but I missed that. It takes a lot of time to study all the thousand details that are not in de books, but Enthuware exam kits are very helpful. Thanks!