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

it would be faster if you used a LinkedList instead of an ArrayList, as the ArrayList will need to resize when it nears capacity.

you will also want to appropriately estimate the capacity of the wrapping collection (HashMap or Multimap) you are creating to avoid repetitive rehashing.