Collections Framework Flashcards

Which collection type in Java guarantees that no duplicate elements are stored AND maintains elements in sorted order?

Answer: TreeSet

`TreeSet` implements `NavigableSet` backed by a `TreeMap`, which stores elements with no duplicates in their natural or comparator-defined sorted order.

What does `Collections.disjoint(Collection c1, Collection c2)` return when the two collections share at least one common element?

Answer: false

`Collections.disjoint()` returns `true` if the collections have NO elements in common, and `false` if they share at least one element.

Which method of `PriorityQueue` removes and returns the head element, or returns null if the queue is empty?

Answer: poll()

`poll()` retrieves and removes the head, returning null if empty, while `remove()` also removes the head but throws `NoSuchElementException` if empty.

What is the time complexity of `HashMap.get()` in the average case?

Answer: O(1)

`HashMap.get()` has O(1) average-case performance because it computes the hash and directly indexes into the backing array.

Which statement about `CopyOnWriteArrayList` is TRUE?

Answer: Writes create a fresh copy of the underlying array

Every mutating operation on `CopyOnWriteArrayList` creates a new copy of the array, making iterators snapshot-safe but memory-intensive.

Which interface does `ArrayDeque` implement that `ArrayList` does NOT?

Answer: Deque

`ArrayDeque` implements `Deque` (double-ended queue) allowing efficient insertion and removal at both ends, which `ArrayList` does not support via that interface.

After calling `List.of(1, 2, 3)`, which operation will succeed without throwing an exception?

Answer: list.contains(2)

`List.of()` returns an unmodifiable list, so structural mutations like `add`, `set`, and `remove` throw `UnsupportedOperationException`, but read operations like `contains()` work fine.