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

Use Multimap from Google Collections. It allows multiple values for the same key

https://google.github.io/guava/releases/19.0/api/docs/com/google/common/collect/Multimap.html