What is the primary difference between fail-fast and fail-safe iterators in Java Collections?
Option B is correct. Fail-fast iterators (e.g. ArrayList iterator) check modCount and throw ConcurrentModificationException if the collection is altered during iteration. Fail-safe iterators (e.g. CopyOnWriteArrayList) iterate over a copy.
Which data structure is internally utilized by java.util.PriorityQueue to order its elements according to natural order or a Comparator?
Option D is correct. PriorityQueue is based on a priority heap (unbounded min-heap), providing O(log n) time for enqueuing (offer) and dequeuing (poll) operations.
In Java HashMap, what structural transformation occurs to a hash bucket when the number of entries in that bucket exceeds TREEIFY_THRESHOLD (default 8)?
Option A is correct. In Java 8+, when a hash bucket's collision chain exceeds 8 entries (and total capacity >= 64), the linked list transitions into a Red-Black Tree, improving lookup from O(n) to O(log n).
What contract must be maintained between equals() and hashCode() methods in Java for objects used as keys in Hash-based collections?
Option D is correct. The general contract dictates that equal objects must have identical hash codes. However, distinct objects may produce identical hash codes (hash collision).
Which Java keyword is used to declare that a variable's value will be read from and written to main memory rather than thread-local CPU caches?
Option B is correct. The volatile keyword guarantees visibility of changes to variables across threads by flushing reads and writes directly to main memory.