Java Programming — Collections Framework Questions & Answers

Q.1 Medium Collections Framework

What will be printed? Queue queue = new LinkedList(); queue.add(10); queue.add(20); queue.add(30); System.out.println(queue.poll()); System.out.println(queue.peek());

A 10 then 20

B 10 then 10

C 30 then 20

D 20 then 30

Correct Answer: A. 10 then 20

EXPLANATION

LinkedList implements Queue with FIFO behavior. poll() removes and returns 10. peek() returns 20 without removing it. Queue now contains [20, 30].

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Q.2 Medium Collections Framework

Which collection interface should be used if you need both key-value pairing and need to iterate in the order of keys' natural ordering?

A HashMap

B TreeMap

C Hashtable

D WeakHashMap

Correct Answer: B. TreeMap

EXPLANATION

TreeMap implements NavigableMap and SortedMap, storing key-value pairs in sorted order of keys using a Red-Black tree structure.

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Q.3 Medium Collections Framework

What is the time complexity of get() operation on a LinkedHashMap with n elements?

A O(1) average case

B O(n)

C O(log n)

D O(n log n)

Correct Answer: A. O(1) average case

EXPLANATION

LinkedHashMap extends HashMap and uses the same hash table for storage, maintaining O(1) average-case get operation. The doubly-linked list only maintains insertion order.

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Q.4 Medium Collections Framework

Which of the following statements is true about PriorityQueue?

A Elements are stored in insertion order

B It implements the sorted set interface

C Elements are ordered based on their natural ordering or a comparator

D It is thread-safe by default

Correct Answer: C. Elements are ordered based on their natural ordering or a comparator

EXPLANATION

PriorityQueue is a heap-based implementation where elements are ordered based on their priority determined by natural ordering or a custom comparator provided during initialization.

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Q.5 Medium Collections Framework

Which collection would you use if you need to maintain a sorted order of elements and also need O(log n) insertion/deletion time complexity?

A ArrayList

B LinkedList

C TreeSet

D Vector

Correct Answer: C. TreeSet

EXPLANATION

TreeSet maintains elements in sorted order using a Red-Black tree internally, providing O(log n) for add, remove, and contains operations.

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Q.6 Medium Collections Framework

Which collection interface guarantees that elements are stored in a sorted order with no duplicates?

A List

B Queue

C SortedSet

D Collection

Correct Answer: C. SortedSet

EXPLANATION

SortedSet interface guarantees sorted order and no duplicates. TreeSet is the primary implementation of SortedSet.

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Q.7 Medium Collections Framework

Consider a TreeMap with Integer keys. What will headMap(10) return?

A All entries with keys greater than 10

B All entries with keys less than 10

C All entries with keys less than or equal to 10

D All entries in reverse order

Correct Answer: B. All entries with keys less than 10

EXPLANATION

TreeMap.headMap(10) returns a view of all entries with keys strictly less than 10. It's exclusive of the provided key.

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Q.8 Medium Collections Framework

What is the purpose of the fail-fast iterator in Collections?

A To speed up iteration

B To detect concurrent modifications during iteration

C To remove elements faster

D To sort elements during iteration

Correct Answer: B. To detect concurrent modifications during iteration

EXPLANATION

Fail-fast iterators throw ConcurrentModificationException if the collection is modified during iteration (except via iterator.remove()).

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Q.9 Medium Collections Framework

Which collection implementation is best for frequent insertions and deletions at both ends?

A ArrayList

B LinkedList

C Vector

D Stack

Correct Answer: B. LinkedList

EXPLANATION

LinkedList provides O(1) operations for adding/removing at both ends via addFirst(), addLast(), removeFirst(), removeLast().

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Q.10 Medium Collections Framework

What happens when you add a null key to a HashMap?

A NullPointerException is thrown

B The null key is stored at index 0

C The null key is ignored

D The insertion fails silently

Correct Answer: B. The null key is stored at index 0

EXPLANATION

HashMap allows one null key which is handled specially and stored at index 0. TreeMap does not allow null keys.

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