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Difficulties in defining null annotations for generics [message #1564190]

Wed, 14 January 2015 12:02

Eclipse User

Hello,

I'm facing a hard wall in trying to get null annotations work with generics and inheritance hierarchy.

I have specified @NonNullByDefault on package level.

Then I have a class in that package:

public abstract class Foo<T extends @Nullable Object> {
...

@Nullable
public abstract T process();
}

Now I'm trying to define a session bean, in the same package, that would accept these as a parameter:

@Local
public interface BarLocal {
<T extends @Nullable Object> T executeFoo(Foo<T> foo);
}

Here the method declaration gives an error

"Null constraint mismatch: The type 'T extends @Nullable Object' is not a valid substitute for the type parameter 'T extends @Nullable Object' "

_and_

"Null constraint mismatch: The type 'T extends @Nullable Object' is not a valid substitute for the type parameter 'T extends @NonNull Object' "

What is going on? To me it looks like the @NonNullByDefault would add an implicit @NonNull even to declarations that explicitly state the nullness status. And how can T extends @Nullable Object not be a valid substitute for T extends @Nullable Object? They are exactly the same?

Thanks in advance!

Re: Difficulties in defining null annotations for generics [message #1564542 is a reply to message #1564190]

Wed, 14 January 2015 16:30

Eclipse User

Given those snippets, I can reproduce the first error, but not the second.
I'll track this in https://bugs.eclipse.org/457499

For now, I'd suggest you simply change "<T extends @Nullable Object>" to "<T>", as that type bound doesn't really add anything interesting:

@NonNullByDefault doesn't normally affect type parameters (unless you explicitly include DefaultLocation.TYPE_PARAMETER, see javadoc of NonNullByDefault#value).
everything is a subtype of '@Nullable Object', including '@NonNull Object'.

best,
Stephan

Re: Difficulties in defining null annotations for generics [message #1572624 is a reply to message #1564542]

Mon, 19 January 2015 03:34

Eclipse User

Thank you!

Your point of disabling the generic type null annotation checks was a good one; it seems obvious now, but I had not thought about it.

With them I could not get the code to compile. It's rather complicated web of inherited classes; as a whole we have super class taking generic argument and then subclasses of both @Nullable and @NotNull of it; and some routines need to accept both kind.

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