Does key-based lookup in HashMap guarantee constant time complexity in all cases?

Searching by key in a HashMap (or similar hash-based structures) typically has an amortized constant time complexity of O(1). This is achieved through key hashing and quick access to the corresponding bucket.

However, in the worst case, if many keys hash to the same bucket (collisions), the search can degrade to linear complexity O(n), where n is the number of elements in that bucket.

Modern implementations, such as in Java starting from version 8, use balanced trees (e.g., red-black trees) within buckets when there are many collisions, reducing the worst case to O(log n).

Thus, constant time complexity is not guaranteed in all cases, but in practice, due to improvements and good hashing, the search remains very fast.