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

The ArrayList values in your HashMap are references. You don't need to "put it back to HashMap". You're operating on the object that already exists as a value in the HashMap.