Difference between a HashMap and a dictionary ADT

Question

What is the difference between a Hash Map and dictionary ADT. And when to prefer one over another. For my programming assignment my instructor has asked to use one of them but I don't see any difference in between both. The program is supposed to work with a huge no. of strings. Any suggestions?

Answer 1

In Java the HashMap implements the Map interface while the Dictionary does not. That makes the Dictionary obsolete (according to the API docs). That is, they both do a similar function so you are right that they seem very similar...a HashMap is a type of dictionary.

You are advised to use the HashMap though.

Answer 2

This Stack Overflow post does a good job explaining the key differences:

Java hashmap vs hashtable

Note that Hashtable is simply an implementation of the Dictionary ADT. Also note that Java considers Dictionary "obsolete".

The fact that Hashtable is synchronized doesn't buy you much for most uses. Use HashMap.

Answer 3

In terms of Java, both the class HashMap and the class Dictionary are implementations of the "Map" abstract data type. Abstract data types are not specific to any one programming language, and the Map ADT can also be known as a Hash, or a Dictionary, or an Associative Array (others at http://en.wikipedia.org/wiki/Associative_array). (Notice we're making a distinction between the Dictionary class and the Dictionary ADT.)

The Dictionary class has been marked as obsolete, so it's best not to use it.