Concrete implementation of this method attempts to compute value for iso3166CodesMap for each IsoCountryCode type key.
This method is called from Locale class to retrieve country code set for getISOCountries(type)
PART1_ALPHA2 is used to represent the ISO3166-1 alpha-2 two letter country codes.
PART1_ALPHA3 is used to represent the ISO3166-1 alpha-3 three letter country codes.
PART3 is used to represent the ISO3166-3 four letter country codes.
Enum for specifying the type defined in ISO 3166. This enum is used to retrieve the two-letter ISO3166-1 alpha-2, three-letter ISO3166-1 alpha-3, four-letter ISO3166-3 country codes.
@see #getISOCountries(Locale.IsoCountryCode)