Currently, the java.lang.invoke.CallSite has a package-private constructor, and in its Javadoc it already mentions all the three subclasses: ConstantCallSite, MutableCallSite and VolatileCallSite.
I guess that now that Java has support for sealed classes, the CallSite could be a candidate for it too. So CallSite would be sealed and the permitted subclasses would be non-sealed, to allow existing behavior. https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/invoke/CallSite.html
