Hash Maps A hashtable is a data structure that allows the user to look up things by a key value. Java implements a hashtable called "HashMap". The way it works is to specify two types of classes. One is the "key", and another is the "value". You can put data values into the HashMap at an "index" specified by the key Like Array List, HashMap is a generic class, so you declare it using angle brackets. For HashMap, you must include both the key type and the value type in this declaration, so the declaration looks like HashMap