The error is because the Collections class only supports a List.
public static <T extends Comparable<? super T>> void sort(List<T> list)
To sort your collection, you can try something like:
Collection collection=new HashSet();
collection.add("zebra");
collection.add("frog");
collection.add("bison");
collection.add("puma");
ArrayList<String> temp = new ArrayList<String>(collection);
Collections.sort(temp);
collection = new HashSet(temp);
Hope this helps.