1Z0-819 Stream API 4

Question 1

Which collector creates an unmodifiable list in Java 10+?

Accepted Answer

Collectors.toUnmodifiableList()

Answer

Collectors.toUnmodifiableList() was introduced in Java 10 and returns an unmodifiable List.

Question 2

What happens when you call a terminal operation on a stream that has already been consumed?

Accepted Answer

It throws IllegalStateException

Answer

Streams can only be consumed once; calling a second terminal operation throws IllegalStateException: stream has already been operated upon or closed.

Question 3

Which of the following correctly creates a parallel stream from a List?

Accepted Answer

Both A and B

Answer

Both list.stream().parallel() and list.parallelStream() produce an equivalent parallel stream.

Question 4

What does flatMap() do differently from map()?

Accepted Answer

flatMap() flattens a stream of streams into a single stream

Answer

flatMap() applies a function that returns a Stream for each element and then flattens all resulting streams into one.

Question 5

What is the return type of Stream<T>.collect(Collectors.toMap(k, v))?

Accepted Answer

Map<K,V>

Answer

Collectors.toMap() returns a Map<K,V> (the concrete type is unspecified but typically HashMap).

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