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

As already mentioned, MultiMap is your best option.

Depending on your business requirements or constraints on the data file, you may want to consider doing a one-off sorting of it, to make it more optimised for loading.