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Collections Interview Questions
Last Updated : 26 May, 2026
title: Collections Framework Interview Questions
Interview Questions239 words10 headingsTables includedExamples included
title: Collections Framework Interview Questions description: Java Collections interview questions with answers
Q1. Collection aur Collections mein difference?
Collection— Interface hai (List, Set, Queue ka parent)Collections— Utility class hai (sort, shuffle, reverse methods)
Q2. ArrayList vs LinkedList?
| ArrayList | LinkedList | |
|---|---|---|
| -- | -- | -- |
| Internal | Dynamic Array | Doubly Linked List |
| Access | Fast (O(1)) | Slow (O(n)) |
| Insert/Delete | Slow (O(n)) | Fast (O(1)) |
Q3. HashMap vs Hashtable?
| HashMap | Hashtable | |
|---|---|---|
| -- | -- | -- |
| Thread Safe | No | Yes |
| Null Key | 1 allowed | Not allowed |
| Speed | Fast | Slow |
Q4. HashSet vs TreeSet vs LinkedHashSet?
HashSet— No order, fastestLinkedHashSet— Insertion order maintainTreeSet— Sorted order, slower
Q5. HashMap kaam kaise karta hai?
hashCode()se bucket index calculate hota hai- Same bucket mein collision: LinkedList (Java 8+ mein TreeNode)
equals()se exact key dhundha jaata hai
Q6. ConcurrentModificationException kab aati hai?
Jab Iterator use karte time collection modify kar do.
java exampleWoHoTech
// Wrong
for (String s : list) {
list.remove(s); // ConcurrentModificationException
}
// Correct
Iterator<String> it = list.iterator();
while (it.hasNext()) {
it.next();
it.remove(); // Safe
}Q7. Comparable vs Comparator?
Comparable— Class khud sorting define karta hai (compareTo())Comparator— Bahar se sorting logic define karo (compare())
Q8. PriorityQueue kya hai?
Min-heap based queue. Smallest element pehle nikalta hai. O(log n) insert/delete.
Q9. Iterator vs ListIterator?
Iterator— Sirf forward, read + removeListIterator— Forward + backward, read + write + remove
Q10. fail-fast vs fail-safe iterators?
- fail-fast — Modification pe
ConcurrentModificationException(ArrayList, HashMap) - fail-safe — Copy pe iterate karta hai, exception nahi (CopyOnWriteArrayList)
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