1Z0-819 Stream API 3

Question 1

What does Stream.of("a","b","c").reduce("", (x,y) -> x + y) return?

Accepted Answer

abc

Answer

The two-argument reduce() with an identity value returns the result directly (not wrapped in Optional).

Question 2

Which of the following is a stateful intermediate operation?

Accepted Answer

sorted()

Answer

sorted() must process all elements before emitting any, making it stateful; the others can process elements one at a time.

Question 3

What is the output of Stream.of(3,1,4,1,5).distinct().sorted().findFirst().orElse(-1)?

Accepted Answer

1

Answer

distinct() gives 3,1,4,5; sorted() gives 1,3,4,5; findFirst() returns Optional[1], orElse(-1) returns 1.

Question 4

Which Collectors method produces a Map<K, Long> counting elements per group?

Accepted Answer

Collectors.groupingBy(classifier, Collectors.counting())

Answer

Collectors.groupingBy with a downstream Collectors.counting() produces a Map where each key maps to the count of matching elements.

Question 5

What does Stream.empty() return when collect(Collectors.toList()) is called?

Accepted Answer

An empty List

Answer

Stream.empty() has no elements, so collecting to a list produces an empty (non-null) List.

(1Z0-819) Oracle Java SE 11 Developer Practice Test