Summary

Change java.time.ZoneId to a sealed abstract class.

Problem

This is a refactoring of the said class with a better construct, as the class' implementation specification has already been limiting the subclassing to two subclasses, i.e., ZoneOffset and ZoneRegion.

Solution

Add sealed modifier to the class declaration, and permit ZoneOffset and ZoneRegion as the subclasses.

Specification

Change the declaration of the class from:

public abstract class ZoneId implements Serializable

to:

public abstract sealed class ZoneId implements Serializable permits ZoneOffset, ZoneRegion

Change the @implSpec in the class description to:

 * @implSpec
 * This abstract sealed class permits two implementations, both of which are immutable and
 * thread-safe. One implementation models region-based IDs, the other is {@code ZoneOffset}
 * modelling offset-based IDs. This difference is visible in serialization.