I tried as you said .and i got my Exception
Exception in thread "main" java.lang.UnsupportedOperationException at java.util.Collections$UnmodifiableCollection.add(Unknown Source)
The return value is specifically designed not to allow additions or removals from the List. So it throws Exceptions if you try to use the unimplemented methods.
If the original List changes, the "unmodifiable" copy changes to match, however.