Core Java API Flashcards

What does `Arrays.copyOfRange(arr, 1, 4)` return for `arr = {10,20,30,40,50}`?

Answer: {20,30,40}

`copyOfRange(arr, 1, 4)` copies elements at indices 1, 2, and 3 (exclusive end), yielding {20,30,40}.

Which `String` method checks whether the string begins with a given prefix?

Answer: startsWith()

`String.startsWith(prefix)` returns true if the string starts with the specified prefix.

What is printed by `System.out.println(Double.isNaN(0.0 / 0.0))`?

Answer: true

Dividing 0.0 by 0.0 with floating-point arithmetic produces NaN, so `Double.isNaN()` returns true.

Which method on `Queue` retrieves and removes the head, or returns null if empty?

Answer: poll()

`Queue.poll()` retrieves and removes the head element, returning null if the queue is empty.

What is the effect of calling `Collections.sort(list, Comparator.reverseOrder())`?

Answer: Sorts list in descending order

`Comparator.reverseOrder()` produces a comparator that imposes the reverse of the natural ordering, sorting the list in descending order.

What does `String.join("-", "a", "b", "c")` return?

Answer: "a-b-c"

`String.join(delimiter, elements...)` concatenates the elements with the delimiter between each pair, producing "a-b-c".

Which method converts a `Stream` to an `Optional` containing the first element?

Answer: findFirst()

`Stream.findFirst()` returns an `Optional` describing the first element of the stream, or an empty Optional if the stream is empty.