Linked lists and arrays allow elements to be arranged in an order of our choice. However, if you want to find a specific element but have forgotten its position, you must visit every element until you find it. This can consume significant time if the collection is large. A data structure that allows for rapidly finding objects is the hash table.

HashMap is an implementation of the Map interface based on a hash table. This implementation provides all optional mapping operations and permits null values and null keys. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.