Questions
17 of 17
1What is the average and worst-case time complexity for lookup in a Hash Table?
2What is a Hash Collision?
3What is the difference between HashMap, TreeMap, and LinkedHashMap?
4Why is the worst-case lookup O(n)? How do modern implementations avoid this? (Tree buckets)
5What are the security implications of Hash Collision attacks (HashDoS)?
6What is the Load Factor? How does it trigger a Rehash/Resize?
7What is a Bloom Filter? When is it used? (Cache penetration, Spell check)
8What is a Hash Table? What is a Key-Value pair?
9What is a Cryptographic Hash? How does it differ from a regular hash?
10How do you design a Hash Table from scratch?
11What is a Hash Function? What makes a good hash function?
12What is the significance of Equals() and GetHashCode() in C#/Java when using objects as keys?
13What is Consistent Hashing? How is it used in Distributed Systems?
14Explain Separate Chaining. (Buckets/Linked Lists)
15Explain Open Addressing. (Linear Probing, Quadratic Probing, Double Hashing)
16Why can't we use a Hash Table efficiently for Ordered operations (e.g., find min/max)?
17Why is it bad to use a mutable object as a key in a HashMap?
Data-StructuresData-Structures
Basics
Complexity Analysis
Arrays and Strings
Linked Lists
Stacks and Queues
Hash Tables
17 / 17

Why is it bad to use a mutable object as a key in a HashMap?

Mutable HashMap Keys

javascript
  1. 1

    Hash-based lookup depends on the key's hash value.

  2. 2

    Changing equality-relevant state can change the bucket mapping.

  3. 3

    The entry may remain physically stored in its old bucket.

  4. 4

    Removal using the mutated key may also fail.

  5. 5

    Immutable keys are safer and easier to reason about.

  6. 6

    If mutation is unavoidable, fields participating in equality and hashing should not change while the key is in the map.

Difficulty: 4/10

Follow-up Questions

  • ❓Why are strings commonly good HashMap keys?
  • ❓What happens if only a non-hash field is mutated?