My favorite Java code I've ever seen is:

    @Nullable Optional<Boolean> foo;

Here are two rules I learned from data modelling and APIs many years ago:

1. If you don't do arithmetic on it, it's not a number. ID int columns and foreign keys are excluded from this. But a phone number or an SSN or a employee ID (that is visible to people) should never be a number; and

2. It's almost never a boolean. It's almost always an enum.

Enums are just better. You can't accidentally pass a strong enum into the wrong parameter. Enums can be extended. There's nothing more depressing than seeing:

    do_stuff(id, true, true, false, true, false, true);

> @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),
    }