Justification for using HashMap and not ArrayList

Question

Hi, I've recently completed a project for my course and now I'm doing the write up for it. Part of my project was to load a dictionary into a data structure. I went with a HashMap because I had code from a previous project to help implement it, the dictionary was used to check possible decrypted words and return possible words that the half decrypted word could be.
Now I should justify my use of HashMap v ArrayList. Time complexity of ArrayList is better than the hashmap if im correct, so are there any 'technical' reasons I could say why I chose the HashMap:
ArrayList O(1)
HashMap O(n)
So searching 50,000 + words would be best using an arraylist? Could anyone possibly give a good reason why hashmaps would be better than arraylists?

Answer 1

Linear searching is (generally) the slowest way to find something.
If you know the exact location of what you're looking for, then obviously just accessing it using an ArrayList would be fast. Using a HashMap you can, by using the input key, figure the index of the bucket which contains your data, so you don't have to linearly search through the whole map just to find what you're looking for, your key gives you it's location.
I'm sure these links can explain stuff better than I did:
http://java.sun.com/j2se/1.4.2/docs/api/java/util/ArrayList.html
http://java.sun.com/j2se/1.4.2/docs/api/java/util/HashMap.html