Decorates another Collection to ensure it can't be altered.
This class is Serializable from Commons Collections 3.1.
since: Commons Collections 3.0 version: $Revision: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $ author: Stephen Colebourne
Factory method to create an unmodifiable collection.
If the collection passed in is already unmodifiable, it is returned.
Parameters: coll - the collection to decorate, must not be null an unmodifiable collection throws: IllegalArgumentException - if collection is null