i have two data structures, one is int(key) and another is list of int's (value).
I am just thinking that out of all the data structures that java has, which one will be more efficient here to use as a key value pair?
Why does it matter? Odds are the difference in speed will be imperceptible for any application you will write. What you should be focusing on is what makes the most sense - what is easiest to write, what is easiest to understand, and what is easiest to maintain.
Picking a data structure based on some ill-defined efficiency need is guaranteed to cause you more headaches than it is ever going to be worth.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors