HashSet, TreeSet, Comparable and Comparator
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This is an excursion into implementing sets of objects
using java parameterised classes HashSet and TreeSet. We
perform experiments using a class Person, having first and econd names, 
and a class Species, having a name.

Person
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Has a first and second name. It implements the Comparable interface 
and therefore must implement the compareTo method. This then allows
persons to be compared p1.compareTo(p2)

Species
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Has a name, and that is all. It also implements the Comparable interface.

Car
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Has a make and a model. It does NOT implement Comparable. However
we define a class CarComparator, and this can be created and passed
to the TreeSet constructor

Test0
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We produce a TreeSet<Person> and add persons into ths set. 
The duplicate "Prosser, Patrick" is not entered as the 
TreeSet<E> uses the compareTo method within class Person

Test1
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We produce a TreeSet<Species> and add species to the set. As in
Test0 duplicates are not added to the set (species "cat"). This
just showsthat there is nothing peculiar about person and Test0

Test2
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We produce a TreeSet<Car> and when constructing it pass a 
CarComparator. Again, the TreeSet works as advertised with the
duplicate "Ford Mustang" appearing once only.

Test3
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This uses HashSet<Person> and this does not work as advertised.
We cannot pass a Comparator during construction of the HashSet.
Maybe this is so to allow efficient hashing