Java Collections List : Converting from List '<Column <String1, String2>>' to 'List <String1>'

[Raj]

I have a function that returns a list like this:-

List <Column <String1, String2>>

Next I want to pass this list to a 2nd function, but 2nd function just needs a list which contains only 1st part (string1) of the Column(s) of the above list.

So I want pass just this list to 2nd function:-

List <String1>

What would be the best way to do this ??

[my use case: Both the functions are from a library that I use to access database(Cassandra) for a web application. 1st function gives me a list of all columns which has two parts name(String1) and value(String2). So 1st function gives me a list of all columns(each of which has two strings) then I just need to use the list of column names to supply it to 2nd function that'll query the DB for those columns.]