Java Programming — Collections Framework Questions & Answers

Q.1 Hard Collections Framework

In Java 2024, if you need a collection that automatically removes entries based on garbage collection patterns, which would you choose?

A WeakHashMap

B IdentityHashMap

C EnumMap

D ConcurrentHashMap

Correct Answer: A. WeakHashMap

EXPLANATION

WeakHashMap uses weak references for keys. When a key is no longer referenced elsewhere, it becomes eligible for garbage collection and is automatically removed from the map.

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Q.2 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.3 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.4 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.5 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.6 Hard Collections Framework

Consider this code: List list = new CopyOnWriteArrayList(); list.add("A"); list.add("B"); Iterator itr = list.iterator(); list.add("C"); System.out.println(itr.next());

A Throws ConcurrentModificationException

B Prints 'A'

C Prints 'C'

D Throws NoSuchElementException

Correct Answer: B. Prints 'A'

EXPLANATION

CopyOnWriteArrayList creates a snapshot for iterators, so modifications after iterator creation don't affect the iteration. Iterator sees only original elements: A and B.

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Q.7 Easy Collections Framework

What will happen when you execute: Map map = new HashMap(); map.put("key", null); System.out.println(map.get("key"));

A Throws NullPointerException

B Prints 'null'

C Prints 'key'

D Throws IllegalArgumentException

Correct Answer: B. Prints 'null'

EXPLANATION

HashMap allows null values. The code stores null as the value for key 'key' and retrieves it successfully, printing null.

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Q.8 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.9 Easy Collections Framework

What will be the output of the following code? Set set = new TreeSet(); set.add(5); set.add(2); set.add(8); set.add(2); System.out.println(set.size());

A 3

B 4

C 2

D 5

Correct Answer: A. 3

EXPLANATION

TreeSet does not allow duplicates. Adding 2 twice means it is stored only once. Final set contains {2, 5, 8}, so size is 3.

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Q.10 Easy Collections Framework

Which of the following collections maintains insertion order while allowing fast random access?

A ArrayList

B HashSet

C TreeSet

D LinkedHashMap

Correct Answer: A. ArrayList

EXPLANATION

ArrayList maintains insertion order and provides O(1) random access via index. LinkedHashMap maintains insertion order but is a Map, not a List.

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