The class java.util.Collections (with an "s") has a private constructor, so you cannot extend it.
But if you want to define a "MyCollection" class, I think you really mean java.util.Collection (no "s"), which is an interface that you should be able to implement. Better yet, check the API for some abstract classes that already implement this interface. That will at least give you a running start.
"We're kind of on the level of crossword puzzle writers... And no one ever goes to them and gives them an award." ~Joe Strummer sscce.org