Gets the set of available zone IDs.
!(p)
This set includes the string form of all available region-based IDs.
Offset-based zone IDs are not included _in the returned set.
The ID can be passed to {@link #of(string)} to create a {@code ZoneId}.
!(p)
The set of zone IDs can increase over time, although _in a typical application
the set of IDs is fixed. Each call to this method is thread-safe.
@return a modifiable copy of the set of zone IDs, not null
Gets the set of available zone IDs. !(p) This set includes the string form of all available region-based IDs. Offset-based zone IDs are not included _in the returned set. The ID can be passed to {@link #of(string)} to create a {@code ZoneId}. !(p) The set of zone IDs can increase over time, although _in a typical application the set of IDs is fixed. Each call to this method is thread-safe.
@return a modifiable copy of the set of zone IDs, not null