Questions
17 of 17
Why is it bad to use a mutable object as a key in a HashMap?
Difficulty: 4/10
Mutable HashMap Keys
A mutable object is dangerous as a HashMap key when its fields used by equality or hashing can change after insertion. If the object's hash code changes, the HashMap may search for it in a different bucket than the one where the entry was originally stored, making the value difficult or impossible to retrieve.
javascript
- 1
Hash-based lookup depends on the key's hash value.
- 2
Changing equality-relevant state can change the bucket mapping.
- 3
The entry may remain physically stored in its old bucket.
- 4
Removal using the mutated key may also fail.
- 5
Immutable keys are safer and easier to reason about.
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If mutation is unavoidable, fields participating in equality and hashing should not change while the key is in the map.
Follow-up Questions
- Why are strings commonly good HashMap keys?
- What happens if only a non-hash field is mutated?