SparseArray really ought to have a Bidirectional iterator or better
still a random access iterator when the index set has a total order.

Make the structure of all equals methods:

  public boolean equals ( final Object o ) {
    if ( this == o ) { return true ; }
    if ( ( o == null ) || ! ( o instanceof TYPE ) ) { return false ; }
    // test for value equality
  }

Make count sort deal with negative indexes.  Do this as an exercise?

Implement a Red-Black Tree.

Ensure all the Mapping correctly sort out uniqueness of the items in the domain.  Currently they
are just implementing relations.