Java Collection: what to convert Collection<List<String>> to ArrayList<List<String>> or List<List<String>>?

Question

I am using Multimap<String, List<String>> in one of my API's. Now to get all the values means list of list I used the .values() method of the multimap. But this method is returning me Collection<List<String>> . Now to play on the index on this collection I want to convert it into List<List<String>> or ArrayList<List<String>> .

How to cast or convert without building new arrayList and explicit add list values from collections to that arraylist.

Answer 1

You can use the appropriate constructor:

List<List<String>> yourList = new ArrayList<>(yourCollection);

The order of the elements in the list is the order of the iterator of the collection.

Answer 2

If you don't want to build a new collection (and don't forget, that may not be costly since you'll create the new collection but not clone the actual members), why not just do

int i = 0;
for (List<String> list : collection) {
   // whatever
   i++;
}

Not hugely nice, I appreciate.

Answer 3

或使用Google Collection API

List<List<String>> = Lists.newArrayList(myMap.values());