The impl spec of java.time.ZoneId states that ZoneId has two implementations:
    "One implementation models region-based IDs, the other is {@code ZoneOffset} modelling offset-based IDs."

The ZoneId constructor has this check to ensure no other subclass can extend it:
    ZoneId() {
        if (getClass() != ZoneOffset.class && getClass() != ZoneRegion.class) {
            throw new AssertionError("Invalid subclass");
        }
    }

ZoneId can be made a sealed abstract class that permits ZoneOffset and ZoneRegion to extend.  The test in the constructor would no longer be needed.