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

Difficulty: 5/10

hashCode/equals contract, mutability, hash map behavior

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.
6
If mutation is unavoidable, fields participating in equality and hashing should not change while the key is in the map.

Scenario Questions

0-2 years experience

2-5 years experience

1We noticed a memory leak in our service after deploying a feature that caches request objects in a HashMap using the request object itself as the key. The cache started returning null for some entries. Walk me through why this might be happening.

Follow-up Questions