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From Paul Merlin <p...@nosphere.org>
Subject Re: Type system in Java is bizarre... Help!
Date Wed, 26 Aug 2015 09:59:02 GMT
Niclas Hedhman a écrit :
> Gang and especially the Type System experts....
>
> I am struggling to get the TypeLookup class converted to Stream API, and it
> is because the Java type system (or at least the one built into IntelliJ)
> to behave strangely.

IDEs that don't directly rely on javac (Eclipse, IDEA) have history of
showing errors that javac don't show. Especially when new javac versions
are released, IDE-specific compilers tends to lag. I don't know what's
IDEA status WRT this. IIRC there was a setting to tell it to use
in-process javac.

> For instance; I have the following method in ModuleInstance
>
> public Stream<ModelModule<ObjectDescriptor>> visibleObjects(
> Visibility visibility )
>
> And trying to access it from TypeLookup
>
> Stream<ModelModule<? extends ModelDescriptor>> moduleObjects =
> moduleInstance.visibleObjects( module );
>
> But that tells me that the moduleObjects are of the wrong type, and should
> be Stream<ModelModule<ObjectDescriptor>>. I can also change the
> visibleObjects() method to
>
> public Stream<ModelModule<? extends Descriptor>> visibleObjects(
> Visibility visibility )
>
> without compile error in TypeLookup NOR in ModuleInstance. What magic
> happens (or not) in this distinction.
I tried to reproduce locally and javac behave just has you described. So
IDEA does a good job here.

Here is what works;

public Stream<ModelModule<ObjectDescriptor>> visibleObjects(Visibility visibility)


Usage;

Stream<? extends ModelModule<? extends ModelDescriptor>> visibleObjects
= lookup.visibleObjects();



So, one need to chain the '? extends T' when implicitely casting on
'deep' generics ; eg:

List<List<Integer>> someLists = new ArrayList<>();
List<List<Number>> numberLists = someLists; // Fails to compile!
List<? extends List<? extends Number>> deepGenericCast = someLists; // OK


HTH

Cheers


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