> @Nullable Optional<Boolean> foo;

To be (somewhat facetiously) fair, that's just JSON. The key can be not-present, present but null, or it can have a value. I usually use nested Options for that, not nulls, but it's still annoying to represent.

In Rust I could also do

    enum JsonValue<T> {
        Missing,
        Null,
        Present(T),
    }