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I've been putting together a page with FAQs regarding Java Generics at
http://www.langer.camelot.de/Generic...nericsFAQ.html.

It was intially intended as companion material to my seminars, but it
may well be of interest to a broader audience. It should at least
answer many of the beginner's questions. If you're interested, feel
free to use it. Comments are welcome.

Angelika Langer

On 5 Oct 2004 11:27:15 -0700, Angelika Langer wrote:

> I've been putting together a page with FAQs regarding Java Generics at
> http://www.langer.camelot.de/Generic...nericsFAQ.html.

Nice work. I've linked from my more general FAQ.
<http://www.physci.org/codes/javafaq.jsp#faq>

--
Andrew Thompson
http://www.PhySci.org/codes/ Web & IT Help
http://www.PhySci.org/ Open-source software suite
http://www.1point1C.org/ Science & Technology
http://www.lensescapes.com/ Images that escape the mundane

Thanks for the excellent FAQ! I have another question for it (although
whether it is frequently asked remains to be seen!

I have the following interface:

public interface DataConverter<I,O> {
public O convert(I obj);
}

It appears that it should be legal to define the following method:

/**
* Returns a Map<Class,DataConverter> that maps Java classes
* to DataConverter objects that know how to operate on
* data of the type specified by the map's keys.
*/
public <C extends Class> Map<C,DataConverter<? extends C,?>>
getDataConverters();

I cannot, however, figure out exactly how to instantiate a Map that I
could actually return from an implementation of this method!

Any help would be greatly appreciated.

Kris


Angelika Langer wrote:
> I've been putting together a page with FAQs regarding Java Generics at
> http://www.langer.camelot.de/Generic...nericsFAQ.html.
>
> It was intially intended as companion material to my seminars, but it
> may well be of interest to a broader audience. It should at least
> answer many of the beginner's questions. If you're interested, feel
> free to use it. Comments are welcome.
>
> Angelika Langer

On Tue, 05 Oct 2004 15:59:05 -0600, Kris Nuttycombe wrote:

> I have another question for it ..

(chuckles) Usually people who post FAQ's demand that
you also provide the answer to your 'Q'..

--
Andrew Thompson
http://www.PhySci.org/codes/ Web & IT Help
http://www.PhySci.org/ Open-source software suite
http://www.1point1C.org/ Science & Technology
http://www.lensescapes.com/ Images that escape the mundane

Kris Nuttycombe < > wrote in message news:<cjv5fa$8pg$>...
> Thanks for the excellent FAQ! I have another question for it (although
> whether it is frequently asked remains to be seen!
>
> I have the following interface:
>
> public interface DataConverter<I,O> {
> public O convert(I obj);
> }
>
> It appears that it should be legal to define the following method:
>
> /**
> * Returns a Map<Class,DataConverter> that maps Java classes
> * to DataConverter objects that know how to operate on
> * data of the type specified by the map's keys.
> */
> public <C extends Class> Map<C,DataConverter<? extends C,?>>
> getDataConverters();
>
> I cannot, however, figure out exactly how to instantiate a Map that I
> could actually return from an implementation of this method!

What are you trying to do? <C extends Class> is pointless since Class
is final and there can't be any class that extends it. So your
declaration can be reduced to:

Map<Class,DataConverter<Class,?>> getDataConverters();

So you return a Map which maps from an instance of Class to a
DataConverter that takes an instance of Class (not necessarily the
same instance used as the key) and returns anything. Doesn't make much
sense.

Are you trying to create a Map which maps from a Class instance to a
DataConverter that only takes instances of that Class as argument and
that this should be checked during compile time? It's impossible to
create a Map that does that.

/Jesper Nordenberg

Really? I havn't done anything with generics, but in the1.5 API docs
define Class as
public final class Class<T>extends Objectimplements Serializable,
GenericDeclaration, Type, AnnotatedElementI suppose the parameter specifies
the type of object that is represented by
the Class instance? So would not a Map which maps from a Class instance to a
DataConverter that only takes instances of that Class as argument be
declared
as:

Map<Class<T>,DataConverter<Class<T>,?>>

Sorry, can't test this for lack of a JDK.

-- Sebastian

"Jesper Nordenberg" <> schrieb im Newsbeitrag
news: m...
> Kris Nuttycombe < > wrote
in message news:<cjv5fa$8pg$>...
> > Thanks for the excellent FAQ! I have another question for it (although
> > whether it is frequently asked remains to be seen!
> >
> > I have the following interface:
> >
> > public interface DataConverter<I,O> {
> > public O convert(I obj);
> > }
> >
> > It appears that it should be legal to define the following method:
> >
> > /**
> > * Returns a Map<Class,DataConverter> that maps Java classes
> > * to DataConverter objects that know how to operate on
> > * data of the type specified by the map's keys.
> > */
> > public <C extends Class> Map<C,DataConverter<? extends C,?>>
> > getDataConverters();
> >
> > I cannot, however, figure out exactly how to instantiate a Map that I
> > could actually return from an implementation of this method!
>
> What are you trying to do? <C extends Class> is pointless since Class
> is final and there can't be any class that extends it. So your
> declaration can be reduced to:
>
> Map<Class,DataConverter<Class,?>> getDataConverters();
>
> So you return a Map which maps from an instance of Class to a
> DataConverter that takes an instance of Class (not necessarily the
> same instance used as the key) and returns anything. Doesn't make much
> sense.
>
> Are you trying to create a Map which maps from a Class instance to a
> DataConverter that only takes instances of that Class as argument and
> that this should be checked during compile time? It's impossible to
> create a Map that does that.
>
> /Jesper Nordenberg

short addition: re-reading the OP's requuirement, I guess that
should really be:

Map<Class<T>,DataConverter<T,?>>

It would even be easy to add additional contraints on T,
perhaps in terms of an interface ConvertibleTo<E>, and
specify DataConverter as

public interface DataConverter<I extends ConvertibleTo<E>,E>

Does this make sense?

-- Sebastian

"Sebastian Millies" <> schrieb im
Newsbeitrag news:4163cbac$0$287$...
> Really? I havn't done anything with generics, but in the1.5 API docs
> define Class as
> public final class Class<T>extends Objectimplements Serializable,
> GenericDeclaration, Type, AnnotatedElementI suppose the parameter
specifies
> the type of object that is represented by
> the Class instance? So would not a Map which maps from a Class instance to
a
> DataConverter that only takes instances of that Class as argument be
> declared
> as:
>
> Map<Class<T>,DataConverter<Class<T>,?>>
>
> Sorry, can't test this for lack of a JDK.
>
> -- Sebastian
>
>

That's exactly what I was looking for, thanks! I like the addition of
the ConvertibleTo<E> interface as well.

Kris

Sebastian Millies wrote:
> short addition: re-reading the OP's requuirement, I guess that
> should really be:
>
> Map<Class<T>,DataConverter<T,?>>
>
> It would even be easy to add additional contraints on T,
> perhaps in terms of an interface ConvertibleTo<E>, and
> specify DataConverter as
>
> public interface DataConverter<I extends ConvertibleTo<E>,E>
>
> Does this make sense?
>
> -- Sebastian
>
> "Sebastian Millies" <> schrieb im
> Newsbeitrag news:4163cbac$0$287$...
>
>>Really? I havn't done anything with generics, but in the1.5 API docs
>>define Class as
>>public final class Class<T>extends Objectimplements Serializable,
>>GenericDeclaration, Type, AnnotatedElementI suppose the parameter
>
> specifies
>
>>the type of object that is represented by
>>the Class instance? So would not a Map which maps from a Class instance to
>
> a
>
>>DataConverter that only takes instances of that Class as argument be
>>declared
>>as:
>>
>>Map<Class<T>,DataConverter<Class<T>,?>>
>>
>>Sorry, can't test this for lack of a JDK.
>>
>>-- Sebastian
>>
>>
>
>
>

"Sebastian Millies" <> wrote in message news:<4163d061$0$286$>...
> short addition: re-reading the OP's requuirement, I guess that
> should really be:
>
> Map<Class<T>,DataConverter<T,?>>

This is not what I meant, but sure, this can be done, but it doesn't
make any sense. There is only one key you can use for such a map
(since there is only one instance of Class<T> for any T), so why are
you even using a Map in the first place?

I think the OP wants a map where you given a class can lookup a
DataConverter for a objects of that class. Something like this:

interface ObjectConverterMap<O> {
<T> void put(Class<T> key, DataConverter<? super T, O> value);
<T> DataConverter<? super T, O> get(Class<T> key);
}

Or maybe he wants a DataConverter for the class object:

interface ClassConverterMap<O> {
<T> void put(Class<T> key, DataConverter<Class<T>, O> value);
<T> DataConverter<Class<T>, O> get(Class<T> key);
}

Neither of these interfaces are compatible with the java.util.Map
interface.

/Jesper Nordenberg

Aha, Now I understand what I was doing wrong. My confusion was between
the runtime and compile time types. Thanks!

Kris

Jesper Nordenberg wrote:
> "Sebastian Millies" <> wrote in message news:<4163d061$0$286$>...
>
>>short addition: re-reading the OP's requuirement, I guess that
>>should really be:
>>
>>Map<Class<T>,DataConverter<T,?>>
>
>
> This is not what I meant, but sure, this can be done, but it doesn't
> make any sense. There is only one key you can use for such a map
> (since there is only one instance of Class<T> for any T), so why are
> you even using a Map in the first place?
>
> I think the OP wants a map where you given a class can lookup a
> DataConverter for a objects of that class. Something like this:
>
> interface ObjectConverterMap<O> {
> <T> void put(Class<T> key, DataConverter<? super T, O> value);
> <T> DataConverter<? super T, O> get(Class<T> key);
> }
>
> Or maybe he wants a DataConverter for the class object:
>
> interface ClassConverterMap<O> {
> <T> void put(Class<T> key, DataConverter<Class<T>, O> value);
> <T> DataConverter<Class<T>, O> get(Class<T> key);
> }
>
> Neither of these interfaces are compatible with the java.util.Map
> interface.
>
> /Jesper Nordenberg