How to properly annotate methods with generic collections using Eclipse external Annotations

Question

I'm trying to annotate the method java.util.List.toArray using Eclipse's external annotations, but I'm not sure how to annotate the return type. If my List has the following signature:

@NonNull List<@NonNull Element>

List.toArray should return:

@NonNull Element @NonNull[]

If, however, the list can contain nullable elements:

@NonNull List<@Nullable Element>

List.toArray should return an array with nullable elements, too:

@NonNull Element @Nullable[]

I'm using Eclipse Neon, is this even possible? The Eclipse Neon New and Noteworthy page seems to provide an example for List.get() and suggests that I should ommit the nullity for the value, but that doesn't seem to work for arrays? Here is the external annotation definition I'm using:

class java/util/List
toArray
 <T:Ljava/lang/Object;>([TT;)[TT;
 <T:Ljava/lang/Object;>([T1T;)[T1T;

But this doesn't work:

    @NonNull
    List<@NonNull String> collect = // works
    @NonNull
    String @NonNull [] array = collect.toArray(new String[0]);

collect.toArray is marked as error:

Null type safety (type annotations): The expression of type 'String[]' needs unchecked conversion to conform to '@NonNull String []'

How can I fix this? Does this even work with Eclipse Neon, yet?

Answer 1

I found this advice and followed it in Eclipse 2020-03: I placed the cursor between the "T" and the "[]" of the return type, opened the context menu, and selected "Annotate as Not Null". This had the desired effect, as if the method was declared

@NonNull <T> T[] toArray(T[] a)

The generated List.eea file looks like this:

class java/util/List
toArray
 <T:Ljava/lang/Object;>([TT;)[TT;
 <T:Ljava/lang/Object;>([TT;)[1TT;