In ArrayList, elements are accessed by integer index, so o(1) is algorithm complexity in any scenario. In case of best case scenario: HashMap with no collisions, it indeed has o(1) algorithm complexity, BUT: In HashMap, elements are accessed by key which includes hashing. Now in worst-case scenario with n collisions on the keys Key1… KeyN with same “keys hash values” we are trying to retrieve element for, hashmap has o(n) lookup time(since it first finds the bucket which contains a Linked list of n elements in wich we are looking for the KeyT, 1