Check the answer to this stackoverflow.  It says it better than I can.
http://stackoverflow.com/questions/3009745/what-does-the-question-mark-in-java-generics-type-parameter-mean

On Sat, May 21, 2016 at 1:08 PM, trent shipley <trent.shipley@gmail.com>
wrote:

> Can anyone tell me what the keyword "super" does in this parameter list
> for the constructor below? I can only find documentation for the more
> mundane use when a method references an ancestor.
>
> public WeightedCollection(Comparator<? super E> comp)
>
>
> package pcgen.base.util;
>
> <</ deleted stuff>>
>
> /**
>
> * Constructs an empty WeightedCollection with the given Comparator used to
>
> * establish equality and order in the WeightedCollection.
>
> *
>
> * @param comp
>
> *            The Comparator this Set will use to determine equality and
>
> *            order of the WeightedCollection
>
> */
>
> public WeightedCollection(Comparator<? super E> comp)
>
> {
>
>      if (comp == null)
>
>      {
>
>           theData = new ListSet<WeightedItem<E>>();
>
>      }
>
>      else
>
>      {
>
>           theData = new TreeSet<WeightedItem<E>>(
>
>                 new WeightedItemComparator<E>(comp));
>
>      }
>
> }
>
>
>
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-- 
James McPhee
jmcphe@gmail.com