How is the efficiency of key-based search in a HashMap data structure evaluated?

The efficiency of searching for a value by key in a HashMap is evaluated based on access time, which on average is O(1) — constant time.

HashMap uses a hash function to compute an index in an array where a linked list or a tree (starting from Java 8) of elements with the same hash (collisions) is stored.

• Average search time: O(1), since the hash function allows direct access to the needed bucket.
• In the worst case: O(n), if all keys fall into one bucket (rare, due to poor hash function).

Example:

Map<String, Integer> map = new HashMap<>();
map.put("key1", 100);
Integer value = map.get("key1"); // Fast access by key

Thus, HashMap provides very fast key-based search thanks to hashing, making it efficient for many tasks.