com.r3corda.core.contracts / UniqueIdentifier / <init>

# <init>

`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.

### Parameters

`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.