I havent read the entire tutorial, but they use the DMV example a couple
of times in the tutorial, its just a generic example.

In a real life DMV situation you would not get access to their actual
database, you would get a copy only. But in the generics example you
would get direct access and thats a problem because then you can start
inserting data into the list that does not conform to the original
intent of the list.

So when reading the rest of the tutorial, they explain how you can avoid
this problem with generics.

regards

tommy

On Sun, 22 Jun 2008 21:39:55 -0700 (PDT), Thufir
<hawat.thufir@gmail.com> wrote, quoted or indirectly quoted someone
who said :

You might make two versions of a Class, one that is immutable. If you
want read-only access, you use the immutable version to avoid
inadvertently modifying the master copy.  This would make more sense
with a POD where changes to RAM object result in permanent changes.

See http://mindprod.com/jgloss/pod.html

Driver is likely a subclass of Person. It has more fields.  However
there would be fewer Driver records in the universe than Person
records. Every Driver would also be a Person.  Either Driver extends
Person (isa) or Person has an optional field to point to the Driver
extension fields (hasa).

see http://mindprod.com/jgloss/isa.html

The "registry of drivers" is the List<Driver>.

I don't find the source you cite to be the most clearly worded.

The point they're trying to make is that List<Driver> is not a subtype
of List<Person> even though Driver might be a subtype of Person.

If List<Driver> were a subtype of List<Person>, it would be possible
to add a Person to the List<Driver>.  That would cause weirdness if
that Person instance were not also a Driver.

Don't let the confusion of the example obscure the point.

Let's use 'C' (meant to look like a "subset" symbol) to mean 'is a
subtype of'.

The point the tutorial wants to make is:

(Driver 'C' Person) DOES NOT IMPLY (List<Driver> 'C' List<Person>)

--
Lew