the nitpicker in me wants to say that the hash key can change (and it works if you rehash items whose key changed), but lookup by key will fail because the item

the nitpicker in me wants to say that the hash key can change (and it works if you rehash items whose key changed), but lookup by key will fail because the item will landed in the bucket of its original hash... but yes keys can't change if you want your values to be retrievable by key.