getPermittedSubclasses() on j.l.rClass returning an array of ClassDesc

Brian Goetz brian.goetz at
Fri Oct 23 15:36:44 UTC 2020

FTR: this was largely a "for consistency" decision, because nestmates 
does it the same way.  (Which is to say, it was a deliberate suboptimal 
choice aimed at minimizing the number of API idioms that users of 
reflection had to deal with.)

On 10/23/2020 11:27 AM, Gavin Bierman wrote:
> Just to follow this up; we have decided to change the signature of permittedSubclasses to the following:
> public Class<?>[] permittedSubclasses() {}
> Thanks!
> Gavin
>> On 8 May 2020, at 23:53, Remi Forax <forax at> wrote:
>> The current draft of the reflection API for the sealed keyword adds a method getPermittedSubclasses() [1] to java.lang.Class.
>> I'm not fully sure that returning an array of ClassDesc is the right choice here, mainly for two reasons,
>> 1/ it's weird to return an array of ClassDesc when all others similar methods return an array of Class,
>>    I know why a ClassDesc might be "better" because it avoid the class loading,
>>    but it also means that now to fully understand java.lang.Class, people has to understand how java.lang.constant works.
>>    The java.lang.constant API was not designed for that, the first line of the overview of this package talks about descriptors, constant pool and indy, not something beginners should worry about.
>> 2/ returning a symbolic view (ClassDesc) instead of a Class is *very* error prone from a user POV, to resolve a ClassDesc to a class, the user as to provide a Lookup
>>    and there is a good chance that users will pick the wrong ones. The number of people that understand classloading and how Lookup works is < 10,
>>    even experts struggle given the number of time the Lookup API as to be patched in recent years. Returning a ClassDesc in this context is like asking a child
>>    to read the serial number of a loaded gun.
>>    Perhaps a way to mitigate that is to provide the code a user should use to get the equivalent classes in the javadoc of getPermittedSubclasses().
>> cheers,
>> Rémi
>> [1]

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