An immutable external identifier for an item.
This identifier is used as a handle within the system to refer to an externally defined identifier. By contrast, the {@code ObjectId} and {@code UniqueId} represent identifiers within an OpenGamma system.
The external identifier is formed from two parts, the scheme and value. The scheme defines a single way of identifying items, while the value is an identifier within that scheme. A value from one scheme may refer to a completely different real-world item than the same value from a different scheme.
Real-world examples of {@code ExternalId} include instances of:
- Cusip
- Isin
- Reuters RIC
- Bloomberg BUID
- Bloomberg Ticker
- Trading system OTC trade ID
This class is immutable and thread-safe.