DO NOT REPLY [Bug 52829] implement Iterable interface in FileSet and others

[bu...@apache.org]

https://issues.apache.org/bugzilla/show_bug.cgi?id=52829

Jesse Glick <jg...@netbeans.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jglick@netbeans.org

--- Comment #1 from Jesse Glick <jg...@netbeans.org> 2012-03-05 20:42:14 UTC ---
Such types should indeed implement Iterable. The question is what the type
parameter should be. In the case of ResourceCollection, probably the parameter
should be Resource.

Now if this were actually "? extends Resource" then FileSet could override this
to be an Iterable<FileResource>, or ResourceCollection could even itself take a
type parameter describing the subtype of Resource it contains, but I am not
sure we want to get this complicated with generics - it can be rather tricky to
understand such code and make it work correctly with extensions.

Probably simpler and better would be to leverage the Resource.as method and do
not allow covariant Resource subtypes. Then FileResource can continue to be an
implementation detail, which FileSet and friends would not be obliged to use,
and your client code would do something like:

for (Resource r : someFileSetOrOtherResourceCollection) {
  File f = r.as(FileProvider.class).getFile();
  // ...
}

which extends naturally to other extension interfaces like URLProvider. I
believe this works in Rhino JavaScript as well (just omit the ".class" on the
type token and of course qualify it if needed). Of course if you are unsure
what the collection might consist of, you must check each call to
as(FileProvider) for a null return value.

There could also be convenience methods somewhere (Resource? Resources? Files?)
such as

/** call {@link Resource#as} on each element, throw NPE or similar if missing
*/
public static <P> Iterable<P> as(ResourceCollection rc, Class<P> type);
/** obtains {@link FileProvider} from each element; must be filesystemOnly */
public static Iterable<File> asFiles(ResourceCollection rc);

to simplify loops.

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