UniqueIdentifier(externalId: String? = null, id: UUID = UUID.randomUUID())
This class provides a truly unique identifier of a trade, state, or other business object, bound to any existing external ID. Equality and comparison are based on the unique ID only; if two states somehow have the same UUID but different external IDs, it would indicate a problem with handling of IDs.
externalId - Any existing weak identifier such as trade reference ID.
This should be set here the first time a UniqueIdentifier is created as part of state issuance,
or ledger on-boarding activity. This ensure that the human readable identity is paired with the strong ID.
id - Should never be set by user code and left as default initialised.
So that the first time a state is issued this should be given a new UUID.
Subsequent copies and evolutions of a state should just copy the externalId and id fields unmodified.