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Re: Email proposal to the core-libs-dev@openjdk.java.net
- From: Werner Keil <
>
- To:
- Subject: Re: Email proposal to the
- Date: Mon, 20 Oct 2014 02:04:34 +0200
I asked them, and they should come back during office hours, maybe got the
question Thu or Fri.
Please especially Martin/Jean-Marie, would you be happy to "split" the API?
Both on Quantity AND Unit of course, because the "crux" meaning all <?>
methods, Otavio feels are inappropriate result from Unit;-)
Ideally a simple Yes/No, similar to what I asked the IoT guys.
The question, if you prefer cast and where applicable asType() was
answered, and except a minor issue with SE8 and asType() I am happy with
that, too.
Now what about removing every <?> method:
divide() and multiply(<?>) invert() and potentially pow() from both
Quantity and Unit?
Again, doing it just on Quantity won't solve the problem, the Unit ones
operate like that and those change the type.
WDYT?
Am 20.10.2014 01:12 schrieb "Leonardo Lima"
<
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Werner,
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What Otávio meant was an issue to discuss how to deal with it, because he
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(still) thinks <?> is not ok.
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You keep saying that "most IoT projects at Eclipse would want them
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removed either", name one, two, three persons in these projects, let them
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join our group (if not as experts, at least in the other mailing lists) and
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let them discuss with us, if /how should we deal with it so we all (EG and
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users) can play along...
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>
Regards,
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Leonardo.
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>
On Sun, Oct 19, 2014 at 9:02 PM, Werner Keil
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<
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wrote:
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> Nobody wants to remove them;-O
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> Neither Martin nor Jean-Marie, they want them to stay with <?> wildcard.
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> I doubt, most IoT projects at Eclipse would want them removed either.
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>
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> If something like asType() can be found for Quantity, that would be a
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> good thing to add;-) It works for Unit and in most cases it should also
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> work for Quantity. A pure Dimension check as currently done with Unit may
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> be a bit tricky here. Many custom types like BMI can be seen as
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> Dimensionless, but then if you declared such new quantities, will telling
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> BMI.class from CO2Footprint.class or HeartBeat.class all work?
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> Expressing them purely via Quantity<?> seems fine, taking care of proper
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> formatting is far trickier. I told you we must focus on that, at least for
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> SI types, so e.g. ThePerfectStorm dealing just with standard quantities
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> produces adequate results.
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>
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> Regards,
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> Werner
>
> Am 20.10.2014 00:43 schrieb "Otávio Gonçalves de Santana" <
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> Ok Leonardo, waiting the issue to talk about remove these methods.
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>> On Oct 19, 2014 7:34 PM, "Leonardo Lima"
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>> <
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>> wrote:
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>>
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>>> I think that a nice round up is needed again :)
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>>>
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>>> We all but Werner would NOT like the "new signature", we (Leo, Otávio,
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>>> Martin and Jean-Marie) understood it breaks some rules and logic. Still,
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>>> that doesn't solve some other problems and raises other questions like
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>>> "should we remove the operation or not", but *that's for another mail
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>>> thread and JIRA issue*. JIRA #62 (and that mail thread) is about
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>>> changing the signature method.
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>>>
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>>> I really don't know if Werner is pushing for or against the new
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>>> signature (the new signature is the one proposed by Otávio).
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>>>
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>>> Werner, a simple yes and no question: do you want the new signature?
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>>>
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>>>
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>>> On Sun, Oct 19, 2014 at 6:51 PM, Werner Keil
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>>> <
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>>> wrote:
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>>>
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>>>>
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>>>> Am 19.10.2014 22:22 schrieb "Martin Desruisseaux" <
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>>>>
>:
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>>>> >
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>>>> > Bellow is my email proposal for the
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>>>> >
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>>>> list (I may edit again tomorrow). Is there any objection or change
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>>>> proposal?
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>>>> >
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>>>> > Note the following risk: if we get an answer, I'm 100% sure that the
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>>>> answer will be that such break of parameterized type safety in the core
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>>>> JDK
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>>>> is a big no-no, with no excuse tolerated. Some peoples may be surprised
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>>>> that a JSR group considered such idea seriously enough for a debate, to
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>>>> the
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>>>> point of becoming reluctant to include anything coming from such group
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>>>> in
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>>>> the JDK.
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>>>> >
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>>>>
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>>>> That is out of discussion, since we propose a standalone JSR.
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>>>> If we succeed going 1.0 maybe some day a platform part could find it
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>>>> interesting, but I see that more likely on ME, than SE.
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>>>>
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>>>> > ________________________________
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>>>> >
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>>>> > Subject: Interpretation of parameterized type usage in a JSR
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>>>> >
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>>>> > Hello all
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>>>> >
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>>>> > My apologize for resorting to this mailing list for a question about
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>>>> an external API. I'm doing that, with agreement from other members of
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>>>> the
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>>>> JSR group, for a JSR which could possibly be proposed for inclusion in
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>>>> the
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>>>> JDK in some future version. I'm seeking confirmation about whether the
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>>>> following rule shall suffers no exception. In a nutshell (more detailed
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>>>> explanation below), given the following method signature where X, Y and
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>>>> Z
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>>>> are interfaces:
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>>>> >>
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>>>> >> X<Y> foo();
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>>>> >
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>>>> > I claim that it would be unacceptable to have the foo() method
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>>>> implementation actually returning instances of X<Z> through
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>>>> parameterized
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>>>> type erasure (ignoring cases like Collections.emptyFoo() where object
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>>>> properties do not depend on the parameter type). I claim that no
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>>>> "developer
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>>>> convenience" consideration can justify such breaking of Java parameter
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>>>> type
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>>>> safety. Some other peoples have the point of view that the developer
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>>>> should
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>>>> know what he is doing.
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>>>> >
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>>>> > Details
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>>>> >
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>>>> > In this discussion I use the standard Comparable<?> interface and
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>>>> its Short and Integer implementations for familiarity, but the actual
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>>>> JSR
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>>>> objects are different. Consider the following method:
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>>>> >>
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>>>> >> <X,Y> Comparable<?> multiply(Comparable<X> x, Comparable<Y> y);
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>>>> >
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>>>> > Let assume that the type of the return value is not necessarily the
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>>>> type, or a subtype, or a super-type of any of the arguments. It may be
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>>>> for
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>>>> example because the multiply method applies the Java rules of Binary
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>>>> Numeric Promotion (JLS §5.6.2). Consequently, the return type use the
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>>>> <?>
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>>>> wildcard since we can not express the actual return type as a function
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>>>> of X
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>>>> and Y with the parameterized type syntax. This force the users to cast
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>>>> as
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>>>> in the following example:
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>>>> >>
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>>>> >> Short x = 3;
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>>>> >> Short y = 4;
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>>>> >> Integer r = (Integer) multiply(x, y);
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>>>> >
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>>>> > For reasons out of scope of this email, some peoples want to use
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>>>> only the base interface:
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>>>> >>
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>>>> >> Comparable<Short> x = (short) 3;
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>>>> >> Comparable<Short> y = (short) 4;
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>>>> >> Comparable<Integer> r = (Comparable<Integer>) multiply(x, y); //
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>>>> Unsafe cast
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>>>> >
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>>>> > Some peoples in our JSR group feel very strongly against the <?>
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>>>> wildcard in return value. They consider it as a sign of bad design (note
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>>>> that I disagree - for me in this particular situation, it means "can
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>>>> not be
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>>>> expressed by the current Java parameterized type syntax". A JDK example
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>>>> of
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>>>> such case is Class.getEnclosingClass()). Some peoples are pushing for
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>>>> the
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>>>> following method declaration:
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>>>> >>
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>>>> >> <X,Y,R> Comparable<R> multiply(Comparable<X> x, Comparable<Y> y);
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>>>> >
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>>>> > In the above method signature X, Y and R appear independents - our
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>>>> relationship between (X,Y) and R is not expressed. Of course this
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>>>> method is
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>>>> impossible to implement without "unsafe cast" warnings, and those casts
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>>>> can
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>>>> not be proved to be safe. But that method signature allows users to
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>>>> write:
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>>>> >>
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>>>> >> Comparable<Short> x = (short) 3;
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>>>> >> Comparable<Short> y = (short) 4;
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>>>> >> Comparable<Integer> r = multiply(x, y);
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>>>> >
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>>>>
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>>>> What are you trying to explain here, the <?> version, or the proposed
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>>>> alternative?
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>>>> There's no need to cast required, so except the multiply() line the
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>>>> others seem unrelated and make no real sense.
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>>>>
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>>>> > The cast and the warning disappeared, which was the effect desired
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>>>> by the promoters. But that method signature also let users write:
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>>>> >>
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>>>> >> Comparable<Byte> r1 = multiply(x, y); // Wrong!
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>>>> >> Comparable<Float> r2 = multiply(x, y); // Wrong!
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>>>> >> Comparable<String> r3 = multiply(x, y); // Wrong!
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>>>> >> etc...
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>>>> >
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>>>> > Above code compile without warnings and can run, but will fail
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>>>> randomly at some later point when the assigned variables will be used.
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>>>> This
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>>>> is understood, but partisans of the later method signature said that
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>>>> developers should know what they are doing. My point of view is that no
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>>>> argument can justify to break the Java parameterized type safety that
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>>>> way.
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>>>> >
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>>>> > Alternative exists (e.g. not using parameterized types at all, do
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>>>> something similar to Class.cast(Object), etc.), but I would like to
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>>>> have a
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>>>> confirmation - or a correction if I'm wrong - of the following:
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>>>> >
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>>>> > Public parameterized method which are impossible to implement
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>>>> without unprovable "unsafe cast" warnings, and which are known to break
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>>>> parameterized type safety, are unacceptable.
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>>>> > There is nothing wrong in using the <?> wildcard in a return value
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>>>> if the actual type can not be expressed by the Java parameterized type
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>>>> syntax.
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>>>> >
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>>>> >
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>>>> > Regards,
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>>>> >
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>>>> > Martin
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>>>> >
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>>>>
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>>>> It could be worth adding, some JSR members who considered the wildcard
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>>>> a bad or outdated practice already work with the JDK team, at least via
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>>>> AdoptOpenJDK (@Otavio, what have you done there so far if anything?;-)
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>>>>
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>>>> Regards,
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>>>> Werner
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>>>>
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>>>
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>>>
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