1Z0-819 Collections Framework 3

Question 1

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

Accepted Answer

TreeSet

Answer

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

Question 2

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

Accepted Answer

false

Answer

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

Question 3

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

Accepted Answer

poll()

Answer

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

Question 4

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

Accepted Answer

O(1)

Answer

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

Question 5

Which statement about `CopyOnWriteArrayList` is TRUE?

Accepted Answer

Writes create a fresh copy of the underlying array

Answer

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

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

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

1Z0-819 Collections Framework 3