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From Thilo Goetz <twgo...@gmx.de>
Subject Re: generics: getViewIterator
Date Mon, 10 Aug 2009 05:21:19 GMT
Adam Lally wrote:
> On Sat, Aug 8, 2009 at 4:57 PM, Marshall Schor<msa@schor.com> wrote:
>> Jörn Kottmann wrote:
>>> On Aug 7, 2009, at 8:57 PM, Marshall Schor wrote:
>>>
>>>> getViewIterator in CASImpl is written with the signature:
>>>>  Iterator<CAS> getViewIterator()
>>>>
>>>> If you are working with things needing CASImpl objects, you would write:
>>>>
>>>>  Iterator<CAS> s = aCas.getViewIterator();
>>>>  while (s.hasNext()) {
>>>>    CASImpl ci = (CASImpl) s.next();
>>>>      ... code using ci...
>>>>  }
>>>>
>>>> If we changed the signature to:
>>>>  Iterator<T extends CAS> getViewIterator()
>>>> then you would write:
>>>>  Iterator<CASImpl> s = aCas.getViewIterator(); // cast done inside
the
>>>> support code, not here
>>>>  while (s.hasNext()) {
>>>>    CASImpl ci = s.next();  // works OK without casting
>>>>      ... code using ci...
>>>>  }
>>>>
>>>> Would it be better to have that form of the signature?
>>> +1, though did not know that its possible, we should check other
>>> places too
>> +1 to checking other places :-)
>>
>> The tutorial on generics by Gilad Bracha,
>> http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf,  on page 20 near
>> the top says:
>>
>>> In general, if you have an API that only uses a type parameter T as an
>>> argument, its
>>> uses should take advantage of lower bounded wildcards (? super T).
>>> Conversely, if
>>> the API only returns T, you’ll give your clients more flexibility by
>>> using upper bounded
>>> wildcards (? extends T).
>> There are many cases I think in our generification where the 2nd part of
>> this, returning XXX<? extends T> or a variant, is a better choice.
>>
> 
> I must say I don't understand why returning something like List<?
> extends CAS> is an improvement.  Is this supposed to mean that the
> client could specify ANY subtype of CAS and it's supposed to work?  So
> someone could write:
> class MyCas implements CAS {...}
> List<MyCas> result = aCAS.getViewIterator()
> 
> Which couldn't possibly work.  I guess it would compile but generate a
> ClassCastException at runtime.  Is that a good thing?
> 
> I found an expert with a differing point of view from your expert:
> http://www.ibm.com/developerworks/java/library/j-jtp07018.html?S_TACT=105AGX02&S_CMP=EDU
> 
> "Keep bounded wildcards out of return values
> 
> It is sometimes tempting to use a bounded wildcard in the return type
> of a method. But this temptation is best avoided because returning
> bounded wildcards tends to "pollute" client code. If a method were to
> return a Box<? extends T>, then the type of the variable receiving the
> return value would have to be Box<? extends T>, which pushes the
> burden of dealing with bounded wildcards on your callers. Bounded
> wildcards work best when they are used in APIs, not in client code."
> 
>  -Adam

I agree with Adam and Jukka.  Even in our implementation code, it's
most of the time not a good idea to use CASImpl.  We shouldn't encourage
clients to do so.

--Thilo


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