Java: Sort an unmodifiable list

Question

How would one do this?

I have tried creating a new, empty list, then copying unmodifiable list's elements to it, but I'm getting unsupported operation error.

Any help is appreciated.

Answer 1

    List unmodifiableList = Collections.unmodifiableList(list);      List newList = new ArrayList(unmodifiableList);      Collections.sort(newList); 

The constructor of ArrayList takes an existing list, reads its elements (without modifying them!), and adds them to the new List.

Answer 2

FWIW, with google-collections it's a one-liner:

List<Foo> sorted = Ordering.natural().sortedCopy(unmodifiableList);

Answer 3

Are you creating an empty list using Collections.emptyList()? If so, that is an unmodifiable list too and that may be the source of your problem.

Create a new list using the constructor for your List implementation, such as new ArrayList(). You can pass the original list into the constructor or else use the addAll() method.