Ad hoc type restriction

[Brian Goetz]

I just want to point out that there's a sort of "syntax error" in your 
proposal.

Java provides annotations as a means of "structured comments" on 
declarations and type uses, but the Java language does not, and will 
not, impart any semantics to programs on the basis of annotations.  If 
you are talking about writing a static analysis tool, perhaps a 
pluggable checker in the Checkers framework, then (as Ethan points out) 
you can use existing annotations with APs and do so, and the compiler is 
merely a conduit for ferrying the annotations to where an AP can find 
them.  (In fact, there is already a checker for "fake enums", where you 
say that a given `int` is really one of the enumerated set 1, 2, 3, 4, 
which is a restriction type.)

If you mean that the compiler actually is going to get into the act, 
though, then this is not an annotation-driven feature, this is a 
full-blown language feature and should be thought of accordingly.  I 
know its tempting to view annos as a "shortcut" to language features, 
but if something has semantics, its part of the language, and sadly that 
means no shortcuts.  That's not to say it isn't a worthwhile idea with a 
good cost-to-benefit ratio. (Indeed, as we get further into the type 
classes work, the logic of a `newtype` mechanism becomes even more 
compelling as then it becomes possible to affect behavior with 
restrictions such as `CaseInsensitveString`, which doesn't actually 
restrict the value set of the type, but allows you to define `Ord 
CaseInsentiveString` separate from `Ord String`.)



On 10/13/2025 3:17 PM, Archie Cobbs wrote:
> Ethan McCue <ethan at mccue.dev> wrote:
>
>     However there is nothing conceptually preventing the tools
>     validating @NonNull usage from also emitting an error until you
>     have inserted a known precheck.
>     ...
>     But for other single-value invariants, like your @PhoneNumber
>     example, it seems fairly practical. Especially since, as a general
>     rule, arbitrary cost computations really shouldn't be invisible.
>     How would one know if (@B A) is going to thread invocations of
>     some validation method everywhere?
>
>
> This is why 3rd party tools aren't as good as having the compiler 
> handle it, because the compiler is in a position to provide both 
> stronger and more efficient guarantees - think generic types and 
> runtime erasure. Compiler-supported typing allows the developer 
> to move the burden of proof from the method receiving a parameter to 
> the code invoking that method, and onward back up the call chain, so 
> that validations tend to occur "early", when they are first known to 
> be true, instead of "late" at the (many more) points in the code where 
> someone actually cares that they are true.
>
> So if phone numbers are central to your application, and they are 
> passed around and used all over the place as type at PhoneNumber String, 
> then they will only need to actually be validated at a few application 
> entry points, not at the start of every method that has a phone number 
> as a parameter. In other words, the annotation is ideally not a 
> "to-do" list but rather an "it's already done" list.
>
> The guarantee that the compiler would then provide is ideally on the 
> same level as with generics: while it's being provided by the 
> compiler, not the JVM, so you can always get around it if you try hard 
> enough (native code, reflection, class file switcheroo, etc.), as long 
> as you "follow the rules" you get the guarantee - or if not, an error 
> or at least a warning.
>
> Brian Goetz <brian.goetz at oracle.com> wrote:
>
>     I think the best bet for making this usable would be some
>     mechanism like a "view", likely only on value types, that would
>     erase down to the underlying wrapped type, but interpose yourself
>     on construction, and provided a conversion from T to RefinedT that
>     verified the requirement.  But this is both nontrivial and
>     presumes a lot of stuff we don't even have yet...
>
>
> I think that is close to what I was imagining. It seems like it could 
> be done with fairly minimal impact/disruption...? No need for wrappers 
> or views.
>
> But first just to be clear, what I'm getting at here is a fairly 
> narrow idea, i.e., what relatively simple thing might the compiler do, 
> with a worthwhile cost/benefit ratio, to make it easier for developers 
> to reason about the correctness of their code when "type restriction" 
> is being used, either formally or informally (meaning, if you're using 
> an int to pass around the size of collection, you're doing informal 
> type restriction).
>
> What's the benefit? Type restriction is fairly pervasive, and yet 
> because Java doesn't make it very easy to do, it's often not being 
> done at all, and this ends up adding to the amount of manual work 
> developers must do to prove to themselves their code is correct. The 
> more of this burden the compiler could take on, the bigger the benefit 
> would be.
>
> What's the cost? That depends on the solution of course.
>
> To me the giant poster-child for this kind of pragmatic language 
> addition is generics. It had all kinds of minor flaws from the point 
> of view of language design, but the problem it addressed was so 
> pervasive, and the new tool it provided to developers for verifying 
> the correctness of their code was so powerful, that nobody thinks it 
> wasn't worth the trade-off.
>
> OK let me throw out two straw-man proposals. I'll just assume these 
> are stupid/naive ideas with major flaws. Hopefully they can at least 
> help map out the usable territory - if any exists.
>
> *Proposal #1*
>
> This one is very simple, but provides a weaker guarantee.
>
>  1. The compiler recognizes and tracks "type restriction annotations",
>     which are type annotations having themeta-annotation @TypeRestriction
>  2. For all operations assigning some value v of type S to type T:
>      1. If a type restriction annotation A is present on T but not S,
>         the compiler generates a warning in the new lint category
>         "type-restriction"
>
> That's it. A cast likevar pn = (@PhoneNumber String)input functions 
> simply as a developer assertion that the type restriction has been 
> verified, but the compiler does not actually check this. There is no 
> change to the generated bytecode. If the developer chooses to write a 
> validation method that takes a string, validates it (or throws an 
> exception), and then returns the validated string, that method will 
> need to be annotated with at SuppressWarnings("type-restriction") because 
> of the cast in front of the return statement.
>
> Guarantee provided: Proper type restriction as long as 
> "type-restriction" warnings are enabled and not emitted. However, this 
> is a "fail slow" guarantee: it's easy to defeat (just cast!). So if 
> you write a method that takes a at PhoneNumber String parameter that is 
> passed an invalid value, you won't find out until something goes wrong 
> later down the line (or never). In other words, /your/ code will be 
> correct, but you have to be trusting of any code that /invokes/ your 
> code, which in practice is not always a sound strategy.
>
> *Proposal #2*
>
> This is proposal is more complex but provides a stronger guarantee:
>
>  1. The compiler recognizes and tracks "type restriction annotations",
>     which have themeta-annotation @TypeRestriction
>      1. The annotation specifies a user-supplied "constructor" class
>         providing a user-defined construction/validation method
>         validate(v)
>      2. We add class TypeRestrictionException extends
>         RuntimeException and encourage validate() methods to throw
>         (some subclass of) it
>  2. For all operations assigning some value v of type S to type T:
>      1. If a type restriction annotation A is present on T but not S,
>         the compiler generates a "type-restriction" warning AND adds
>         an implicit cast added (see next step)
>  3. For every cast like var pn = (@PhoneNumber String)"+15105551212"
>      the compiler inserts bytecode to invoke the appropriate
>     enforcervalidate(v) method
>  4. The JLS rules for method resolution, type inference, etc., do not
>     change (that would be way over-complicating things)
>      1. Two methodsvoid dial(String pn) andvoid dial(@PhoneNumber
>         String pn) will still collide
>
> Guarantee provided: Proper type restriction unless you are going to 
> extremes (native code, reflection, runtime classfile switcheroo, 
> etc.). This is a "fail fast" guarantee: errors are caught at the 
> moment an invalid value is assigned to a type-restricted variable. If 
> your method parameters have the annotation, you don't have to trust 
> 3rd party code that calls those methods (as long as it was compiled 
> properly). I.e., same level of guarantee as generics.
>
> These are by no means complete or particularly elegant solutions from 
> a language design point of view. They are pragmatic and relatively 
> unobtrusive add-ons, using existing language concepts, to get us most 
> of what we want, which is:
>
>   * User-defined "custom" type restrictions with compile-time
>     checking/enforcement
>       o As with generics, the goal is not language perfection, but
>         rather making it easier for developers to reason about correctness
>   * Compile-time guarantees that type restricted values in source
>     files will be actually type restricted at runtime
>   * Efficient implementation
>       o Validation only happens "when necessary"
>       o No JVM changes needed (erasure)
>   * No changes to language syntax; existing source files are 100%
>     backward compatible
>
> The developer side of me says that the cost/benefit ratio of something 
> like this would be worthwhile, in spite of its pragmatic nature, 
> simply because the problem being addressed seems so pervasive. I felt 
> the same way about generics (which was a much bigger change addressing 
> a much bigger pervasive problem).
>
> But I'm sure there are things I'm missing... ?
>
> -Archie
>
> -- 
> Archie L. Cobbs
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