Java Programming — Collections Framework Questions & Answers

Q.41 Medium Collections Framework

What exception is thrown when you try to add a null value to a TreeSet?

A NullPointerException

B IllegalArgumentException

C ClassCastException

D UnsupportedOperationException

Correct Answer: A. NullPointerException

EXPLANATION

TreeSet uses comparator or natural ordering, which cannot handle null values, throwing NullPointerException during insertion.

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Q.42 Medium Collections Framework

Which method of PriorityQueue returns the element without removing it?

A poll()

B remove()

C peek()

D pop()

Correct Answer: C. peek()

EXPLANATION

peek() returns the head element without removing it. poll() removes and returns, remove() throws exception if empty, and pop() is not a PriorityQueue method.

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Q.43 Medium Collections Framework

What is the difference between ArrayList and Vector?

A ArrayList is synchronized, Vector is not

B Vector is synchronized, ArrayList is not

C Both are synchronized

D Neither is synchronized

Correct Answer: B. Vector is synchronized, ArrayList is not

EXPLANATION

Vector is a legacy synchronized collection, while ArrayList is unsynchronized but faster. For thread-safety with ArrayList, use Collections.synchronizedList().

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Q.44 Medium Collections Framework

What is the time complexity of get() operation in TreeMap?

A O(1)

B O(log n)

C O(n)

D O(n log n)

Correct Answer: B. O(log n)

EXPLANATION

TreeMap is based on Red-Black tree, so get(), put(), and remove() operations have O(log n) time complexity due to tree traversal.

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Q.45 Easy Collections Framework

Which of the following maintains insertion order in Java?

A HashSet

B TreeSet

C LinkedHashSet

D ConcurrentHashMap

Correct Answer: C. LinkedHashSet

EXPLANATION

LinkedHashSet maintains insertion order using a doubly-linked list. HashSet does not maintain order, TreeSet maintains sorted order, and ConcurrentHashMap doesn't guarantee insertion order.

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Q.46 Easy Collections Framework

What is the underlying data structure of HashMap in Java?

A Balanced Binary Tree

B Hash Table with Buckets

C Linked List

D Array

Correct Answer: B. Hash Table with Buckets

EXPLANATION

HashMap uses a hash table with buckets (arrays of linked lists or red-black trees) to store key-value pairs for efficient O(1) average lookup time.

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Q.47 Easy Collections Framework

Which interface in Java Collections Framework does not allow duplicate elements?

A List

B Set

C Queue

D Map

Correct Answer: B. Set

EXPLANATION

Set interface explicitly does not allow duplicate elements. List allows duplicates, Queue is for ordered collections, and Map stores key-value pairs.

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Q.48 Hard Collections Framework

What does the spliterator() method introduced in Java 8 provide over Iterator?

A Better support for parallel processing

B Faster sequential iteration

C Backward iteration capability

D Automatic synchronization

Correct Answer: A. Better support for parallel processing

EXPLANATION

Spliterator provides split() capability enabling efficient parallel stream processing with characteristics advertisement.

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Q.49 Hard Collections Framework

In a scenario where you need to maintain a mapping with LRU (Least Recently Used) eviction policy, which class should be extended?

A LinkedHashMap with override of removeEldestEntry()

B TreeMap

C WeakHashMap

D ConcurrentHashMap

Correct Answer: A. LinkedHashMap with override of removeEldestEntry()

EXPLANATION

LinkedHashMap can be extended and removeEldestEntry() overridden to implement LRU cache eviction policy.

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Q.50 Hard Collections Framework

What is the worst-case time complexity of QuickSort when used with Collections.sort()?

A O(n log n)

B O(n²)

C O(n)

D O(log n)

Correct Answer: B. O(n²)

EXPLANATION

Collections.sort() uses TimSort (hybrid of merge and insertion sort) with O(n log n) worst-case, but QuickSort's worst-case is O(n²).

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