Timo,
Comments below.
On 30/07/2014 7:19 PM, Timo Kinnunen wrote:
Hi,
Regarding the ArrayContentProvider, something like this seems to
compile just fine:
interface IStructuredContentProvider<T> {
default @Deprecated Object[] getElements(Object inputElement) {
return elements(inputElement).toArray();
}
So we'd need to all switch to Java 8 and our code would become a sea of
deprecation and raw type warnings.
ArrayList<T> elements(Object inputElement);
Of course you'd propose Collection<? extends T> or List<? extends T>
not ArrayList, right?
}
class ArrayContentProvider<T> implements IStructuredContentProvider<T> {
private static ArrayContentProvider<?> instance;
public static <T> ArrayContentProvider<T> getInstance() {
synchronized (ArrayContentProvider.class) {
if (instance == null) {
instance = new ArrayContentProvider<>();
}
@SuppressWarnings("unchecked")
ArrayContentProvider<T> result = (ArrayContentProvider<T>)
instance;
return result;
}
}
public @Deprecated @Override Object[] getElements(Object
inputElement) {
return elements(inputElement).toArray();
}
So you're suggesting that no one should ever use or implement this
method. While that's certainly a way to avoid the whole problem with
generic array, none of this fits with Dani's "the problems should go way
just by inferring types".
public @Override ArrayList<T> elements(Object inputElement) {
if (inputElement instanceof Object[]) {
@SuppressWarnings("unchecked")
List<T> list = Arrays.asList((T[]) inputElement);
return new ArrayList<>(list);
}
if (inputElement instanceof Collection) {
@SuppressWarnings("unchecked")
Collection<T> collection = (Collection<T>) inputElement;
return new ArrayList<>(collection);
}
return new ArrayList<>();
}
}
So do the above look simple than the current code? Was it easier to
write? Did generics provide value or make it more complex without
providing value? It's clear It's not simple and generics actually made
it more complicated.
No only that, but as I've pointed out, the convenience to the author is
generics on the argument side of the equation, not on the return type side.
public class Snippet {
public static void main(String[] args) {
ArrayContentProvider<String> instance =
ArrayContentProvider.getInstance();
ArrayList<String> elements1 = instance.elements(null);
System.out.println(elements1);
}
}
Yeah, there's a lot of unchecked casts but at least they are visible
and localized. That's just the price one has to pay for weak typing.
The real question is, was it easier to write? Because in the end, who
are the callers to these APIs? A bunch of generic viewers that are
already written and that we don't generally write ourselves. And they
don't care currently. So the whole things fundamentally has to come
down to what's easiest for the authors to write content providers.
Otherwise I agree with trying to make Object[] generic not working at
all. Arrays in Java are just too strongly and wrongly typed to work
well with generics. Best just to deprecate them and replace them with
proper collections that are much nicer to work with.
Yes, it's certainly far more reasonable to argue getting rid of the
arrays. But that's even more fundamentally disruptive, and requires
Java 8. Note that EMF heavily uses Collection<?> rather than Object[]
in its platform neutral equivalents of these APIs.
That's also why I picked ArrayList<> rather than some other List<> as
the return type, to make it obvious to the caller that they are free
to do anything and everything with the returned collection.
I think that makes no sense. The client must always create a specific
list implementation and the contract allows the caller to add instances
to that collection? Collection<? extends T> would seem far more
reasonable and convenient. Then one could return just the collection
you already have or use Arrays.asList.
--
Have a nice day,
Timo.
Sent from Windows Mail
*From:* Ed Merks <mailto:[email protected]>
*Sent:* ?Wednesday?, ?July? ?30?, ?2014 ?17?:?46
*To:* [email protected]
<mailto:[email protected]>
Dani,
As I've made pretty clear in the past, I'm a huge non-fan of this
effort. I find it ironic that the platform is rife with raw types
(List), and rather than investing an effort to eliminate those, effort
will be invested on generifying things that just aren't screaming to
be generified, and to do so in a context that is heavily dominated by
Object[], which interacts exceedingly poorly with generics. Thought
I've made that argument already, I think it bears repeating...
Anywhere you see something like T[] where T is a type parameter, you
already have to get suspicious. Of course we see this in
java.util.Collection:
<T> T[] toArray(T[] a);
But note that T is derived from the argument, and argument my not be
null, so even in an erased runtime environment we can determine a
reasonable T from "a", so suspicion alleviated and that's why we can
have generic collection implementations...
Note that however nice it would have been that toArray() method looked
like this:
E[] toArray()
rather than
Object[] toArray
because you can't implement this generically, unless you provide some
object with the necessary *runtime *information about E to the
constructor of an implementation class...
So consider this:
public interface IStructuredContentProvider extends IContentProvider {
public Object[] getElements(Object inputElement);
}
there's no relationship between the input element's type and the type
of the array, so if someone proposes
public interface IStructuredContentProvider<X, Y> extends
IContentProvider<X> {
public Y[] getElements(X inputElement);
}
I'm going to be very suspicious. What this tells me is there must be
some sensible way of being sure that I'll be getting back a real Y[]
instance and not an Object[] instance. What could that sensible way be?
Of course directly implementing that interface in a sensible way is a
good solution, but what about generic solutions?
Consider org.eclipse.jface.viewers.ArrayContentProvider (and note that
EMF generally has *only *content provider implementations analogous to
this). This existing implementation just don't work. You can just
forget about org.eclipse.jface.viewers.ArrayContentProvider.instance,
you can just deprecate
org.eclipse.jface.viewers.ArrayContentProvider.getInstance(), and
you'd better add a public constructor and deprecate it while your at
it, because this implementation
public Object[] getElements(Object inputElement) {
if (inputElement instanceof Object[]) {
return (Object[]) inputElement;
}
if (inputElement instanceof Collection) {
return ((Collection) inputElement).toArray();
}
return new Object[0];
}
simply doesn't work for collections. You'll need new constructors
and new getInstance methods each of which specify the array type,
either as java.lang.Class or as an array prototype, as in the first
form to toArray above. You'd have to provide that even for the
constructor that takes a collection, just the collection instance will
not suffice.
Great, so much for adding generics to JFace being erasure compatible,
counter to what was the case when Java's collection library was
generified. Nothing in the collections library was deprecated and no
new methods were added. In other words, for JFace's changes, you
can't just turn off warnings about raw types, you must deal with the
deprecations and API changes. So if you're trying to maintain
something that works with old versions of Eclipse (i.e., EMF is
compatible with Eclipse 3.5), you're completely hosed. You can't add
generics, because you can't compile against an older target platform
that does have it, so you simply have to live with a sea of raw type
warnings (or turn them all off and lose the value of even having such
warnings). Also, you can't start using the new methods instead of the
deprecated ones, so you have to live with that sea of warning as well,
or just turn them off too. Nor you can you exploit any of it to
provide value to clients (given the premise there is value), because
you can't reasonably address this ArrayContentProvider problem without
inflicting the same pain on the clients (who actually have better
things to do, go figure).
Even the premise that this effort has value is questionable. Granted,
someone writing their first content provider might find it useful, if
(and only if) it's one that's concrete and if (and only if) it doesn't
need to deal with several input types that have no common super type
(and even in that case they'll still typically end up with instanceof
tests to return subtype-appropriate results). That's on the argument
side of the getElements. On the return type side, it's of no benefit
to the author; they just pass this content provider into a generic
viewer that doesn't care whether it's Object[] or X[]. So in fact it's
just a burden with which one must conform.
So is the value of this whole exercise eliminating instance of checks
for the arguments of the provider implementations? Does this tangible
(but small) benefit justify the impact on the long established
community? Do we expect that community to eliminate their
deprecations and generify all their code? (Sorry guys and girls, all
your existing toArray() calls are invalid, or, don't worry about it,
just sprinkle <Object, Object> everywhere.) I wonder, will JDT and
PDE do that? If not, why expect the rest of the community to do it?
And if you don't expect that, what exactly are you expecting will come
from this?
I suggest folks carefully weight the benefits against the disruptive
nature of this type of change.
Regards,
Ed
On 30/07/2014 11:42 AM, Daniel Megert wrote:
Just for the records, here are some constraints that I required in
order to agree to continue that work:
- Some stuff just doesn't make sense to be generified because it
often contains various kinds of objects, e.g. (tree) viewers. See
also
_http://dev.eclipse.org/mhonarc/lists/platform-ui-dev/msg05459.html_.
- If generified types cannot be plugged together unless everything
is again just Object or Class, it's not worth to generify those
types.
- The generified code must be in a shape so that clients can start
to fix their code by invoking Refactor > Infer Generic Type
Arguments... This needs to be validate on existing Platform UI code.
Dani
From: Lars Vogel <[email protected]>
To: [email protected], Jeanderson Candido
<[email protected]>, Hendrik Still <[email protected]>
Date: 30.07.2014 11:23
Subject: [cross-project-issues-dev] Information about the
"Generifing JFace viewers" project
Sent by: [email protected]
------------------------------------------------------------------------
Hi,
as some of you probably remember, the platform.ui team started a
GSoC project last year to generify the JFace viewer framework. We
(platform.ui team together with John Arthone and Dani Megert)
decided that it is worth to finish this project and started a new
GSoC project.
Jeanderson Barros Candido (cc) is working on this project with
Hendrik Still (cc) (GSoC student from last year) and me as mentor.
I personally think the work looks already very good and plan to
integrated it soon into the master. We are trying to learn from
the experience from last year, therefore:
- We plan to integrate it as a whole, not piece wise so people
can fix warning messages created by this change
- We reworking the JFace snippets and tests at the same time to
have a first proof-point
- We plan to use it for platform views to validate that it works
Of course generifying an existing API, will result in certain
limitations and some suggested a complete rewrite of the JFace
viewer framework but this is currently not the scope of this project.
The implementation is currently done at Github:
_https://github.com/jeandersonbc/eclipse.platform.ui_and we do our
planning in
_https://github.com/jeandersonbc/gsoc14-eclipse-planning_.
If someone wants to test the new implementation and provide
feedback, please let us know.
Best regards, Lars_______________________________________________
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