Marking some fields in the returned object of a method NonNull and others Nullable?

[fzyzcjy]

Hi! Thanks for the interesting library! A very simple question: I have a class (actually, a model, if you are interested) like -

class Article {
    int id;
    String text;
    int authorId;
    Date createAt;
}

And I have a class returning it:

interface ArticleRepository {
    Article getFullArticle(); /* return the Article with ALL fields being non-null */ 
    Article getBriefArticle(); /* return the Article with only `id` and `text` fields being non-null */ 
}

Question: How can I annotate that properly?

Thanks very much!

[mernst]

This is a good question.

The solution to your problem is qualifier polymorphism over classes.

Currently, the Checker Framework supports Java generics, and also qualifier polymorphims over methods.

Pull request #2883 implements qualifier polymorphims for classes. We are hoping to complete that feature soon -- in the next release or two -- but this depends on how much time everyone has to work on it.

[fzyzcjy]

Thanks very much! Hoping to see that!

[wmdietl]

The limited form of class qualifier parameters in #2883 will not help in this situation - and it won't help with the Nullness Checker in general. The main qualifier of a type is reused for the qualifier parameter - but if the main qualifier is @Nullable one won't be able to access the field regardless.

In the Article example, you could try making the fields @Nullable and then having a method like this:

    @EnsuresNonNullIf(result=true, expression="createAt")
    boolean isFullArticle() ...

We currently don't have a mechanism to specify as post-condition of getFullArticle(); that the resulting Article will satisfy isFullArticle.

[fzyzcjy]

Thanks very much!

[mernst]

The @Unused annotation might already solve your problem. It doesn't make a field nullable, but makes the field inaccessible, which is usually what you want. You would need to create a pair of new annotations @FullArticle and @BriefArticle, then write a trivial type-checker that processes them.

[fzyzcjy]

Er sorry I do not quite get it... How do I write a "trivial type-checker", do you mean extending the existing Checker Framework?

[mernst]

Yes. The process is explained at https://checkerframework.org/manual/#creating-a-checker .

[fzyzcjy]

Thanks very much!