Streams and Lambda Expressions Flashcards

Which terminal operation returns an OptionalDouble representing the average of a DoubleStream?

Answer: average()

DoubleStream.average() is a terminal operation that returns OptionalDouble containing the arithmetic mean of all elements.

What does the flatMap() operation do that map() does not?

Answer: It flattens a stream of streams into a single stream

flatMap() maps each element to a stream and then flattens all those streams into a single stream, whereas map() returns one element per input element.

Which functional interface is used by Stream.filter()?

Answer: Predicate

Stream.filter() accepts a Predicate, which is a functional interface with a single abstract method test(T t) returning boolean.

What is the result of calling Stream.of(1, 2, 3).reduce(0, Integer::sum)?

Answer: 6

reduce(identity, accumulator) returns the result directly (not wrapped in Optional) because the identity element guarantees a value; 0+1+2+3=6.

Which of the following creates a lazy infinite stream of random doubles?

Answer: Stream.generate(Math::random)

Stream.generate(Supplier) produces an infinite sequential unordered stream where each element is generated by the provided Supplier.

What does Collectors.joining(", ") do when used with Stream.collect()?

Answer: Concatenates stream elements into a single String separated by ", "

Collectors.joining(delimiter) concatenates all CharSequence elements of a stream into a single String with the given delimiter between each element.

Which method on Optional executes an action only if a value is present?

Answer: ifPresent(Consumer)

Optional.ifPresent(Consumer) invokes the given consumer with the value if present, and does nothing if the Optional is empty.