Java Programming — MCQ Practice Questions & Answers

Question 1

Which method throws ConcurrentModificationException when collection is modified during iteration?

Accepted Answer

Correct answer: C. Both Iterator and enhanced for loop — Both Iterator and enhanced for loop use fail-fast mechanism and throw ConcurrentModificationException if collection is modified during iteration.

Answer

A. Iterator.next()

Answer

B. Enhanced for loop only

Answer

D. Collection.remove()

Question 2

What is the initial capacity and growth strategy of ArrayList?

Accepted Answer

Correct answer: C. Initial capacity 10, grows by 100% (1.5x from Java 9) — ArrayList starts with capacity 10. Growth factor is 50% + 1 in older versions, and 1.5x from Java 9 onwards.

Answer

A. Initial capacity 10, grows by 50%

Answer

B. Initial capacity 16, grows by 100%

Answer

D. Initial capacity 5, grows by 25%

Question 3

What is the worst-case time complexity of HashMap.get() when hash collisions occur?

Accepted Answer

Correct answer: C. O(n) — In worst case with all collisions stored in linked list, get() becomes O(n). From Java 8, if collisions exceed threshold, linked list converts to balanced tree, making it O(log n).

Answer

A. O(1)

Answer

B. O(log n)

Answer

D. O(n log n)

Question 4

What is the behavior of get() method in LinkedHashMap with accessOrder=true?

Accepted Answer

Correct answer: B. Moves accessed element to end (updates access order) — LinkedHashMap with accessOrder=true maintains access-order. get() moves the element to the end (LRU cache behavior).

Answer

A. Returns element without affecting order

Answer

C. Throws exception

Answer

D. Returns null for accessed elements

Question 5

Consider implementing a Comparator for custom sorting in reverse order. Which approach is correct? List list = Arrays.asList(5, 2, 8, 1);

Accepted Answer

Correct answer: D. All of above — All three approaches work correctly for reverse sorting. Options A, B, and C are all valid Java code.

Answer

A. Collections.sort(list, Collections.reverseOrder());

Answer

B. Collections.sort(list, (a,b) -> b.compareTo(a));

Answer

C. Collections.sort(list, (a,b) -> a.compareTo(b) * -1);

Question 6

In a multi-threaded environment, if you need a thread-safe list that allows concurrent reads, which is optimal?

Accepted Answer

Correct answer: B. new CopyOnWriteArrayList<>() — CopyOnWriteArrayList is optimal for read-heavy concurrent operations. It creates copy on write, allowing safe concurrent reads.

Answer

A. Collections.synchronizedList(new ArrayList<>())

Answer

C. new Vector<>()

Answer

D. new LinkedList<>()

Question 7

In Java 16+, what feature improved collection performance by allowing value-based classes?

Accepted Answer

Correct answer: A. Records with Collections — Records (Java 14+) provide compact syntax for data carriers used with collections, improving performance and readability.

Answer

B. Sealed classes restriction

Answer

C. Pattern matching in iteration

Answer

D. Stream API enhancements

Question 8

What is the behavior when concurrent modification occurs during iteration in ConcurrentHashMap vs HashMap?

Accepted Answer

Correct answer: B. ConcurrentHashMap allows safe iteration, HashMap throws exception — ConcurrentHashMap uses segment-based locking allowing safe iteration during concurrent modifications. HashMap throws ConcurrentModificationException.

Answer

A. Both throw ConcurrentModificationException

Answer

C. Neither throws exception

Answer

D. HashMap allows safe iteration, ConcurrentHashMap throws exception

Java Programming - MCQ Practice Questions