Creating a mutable collection from an immutable one

Question

Let's say I have the following Map which is created using Guava's library: (List<Integer> is also immutable)

Map<String, List<Integer>> map = ImmutableMap.builder()...

I pass this map to a class where I want to create a mutable copy of it and modify it. It is of course possible to do it manually, but is there a way to convert a nested immutable collection back to a mutable one?

Answer 1

As pointed out, I'd use an ImmutableListMultimap<String, Integer> instead of a ImmutableMap<String, ImmutableList<Integer>>.

Then if you want a mutable copy, you can just pass the immutable multimap to the create static factory method on one of the mutable ListMultimap implementations (ArrayListMultimap or LinkedListMultimap).