JobExecution Visitor does not allow for side-effectful impls

[cwbriones]

See JobExecution$Visitor, and the test case where this happens.

Scenario: You want to visit JobExecution<T> instances and take some action depending on the variant, but you don't necessarily return a value. You try to specify your visitor as Visitor<T, Void>.

Outcome: Because there's no way to return Void from your visit methods (intentionally) then you try to return null. Spotbugs yells at you that you're returning null from a non-nullable method.

In the test case linked above I worked around this by just using Visitor<T, Integer>, but that doesn't give the same implication of a consumer-like class like what you get when using Void as your return type.

I think we can either

• Mark Visitor methods as explicitly @Nullable by default and require implementations to add it themselves.
  • NOTE: You must do this on the inferface. Adding @Nullable to the implementations is not type-safe due to nullability variance rules.
• Add a second interface that is specifically for side-effectful visitors (like Consumer)

I'm more in favor of the first since it doesn't add even more classes to this and this isn't really a big issue at the moment.

[cwbriones]

This isn't an urgent issue but I remembered it while looking through my TODOs and I thought it should have a task filed.