A general purpose defaulting annotation, like @Default(NotNull.class)

[dzharkov]

The question was initially asked by @kevin1e100:

More generic defaulting annotation, e.g., @Default(NotNull.class)? This might be handy in particular if we see other annotations unrelated to nullness being added later

[dzharkov]

Comments from the document:

[@amaembo] An my experience says that nothing except NotNullness worth declaring on a class/package. Well, I can imagine probably something like Unmodifiable, but I really doubt. Anything else?

[kevinb9n]

I know of only two other possible uses for this (and let's NOT debate their inclusion in the project here and now; they are just "potential"):

"Caller must use return value of this method"
"Parameter names are exported (so you're allowed to depend on them from e.g. Kotlin)"

But in both of these cases, I don't see why we couldn't just use the same annotation that we use for a single method at the wider scopes.

So I would suggest we plan on a special-purpose annotation for "default not null".

[kevinb9n]

I'm still going to leave the door open here. There are other examples, like "Caller must close any AutoCloseables returned from this package/class", that I think will make sense.

It occurred to me belatedly that defaults are useful for two different reasons:

1. Lessening annotation burden
2. So that new code gets the preferred behavior automatically

I wasn't thinking of #2 enough before.

My main concern about @Default(NotNull.class) and so forth is that it would only work for parameterless annotations, which might push our designs more toward parameterless annotations than they shouldbe?

[kevinb9n]

@Default may be an insufficient name - need something like @DefaultWithin or @DefaultForUsages

[kevinb9n]

Another problem with @Default(NotNull.class) is that there's now TWO classes a user needs to import in every file (both of which are highly likely to be used only once). I think that will make them even more cranky than one.

[kevinb9n]

Another possible approach here is to have @DefaultNotNull but to have that class annotated as @DefaultAnnotationFor(NotNull.class) and have that linkage convey the semantics.

[cpovirk]

Given that we dismissed @DefaultNullable for its added complexity (#41), it seems likely that we'd dismiss this proposal, too?

[kevinb9n]

Yeah, and nothing ever gelled around this idea.