Best way to create a hashmap of arraylist

Question

I have one million rows of data in .txt format. the format is very simple. For each row:

user1,value1
user2,value2
user3,value3
user1,value4
...

You k

Answer 1

i think what you want is the Multimap. You can get it from apache's commons collection, or google-collections.

http://commons.apache.org/collections/

http://code.google.com/p/google-collections/

"collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values."