8.1. Unique Instance Constraints

Objects may have sets of attributes that are required to be unique.

  • a customer's login ID

  • a publisher's ISBN code

  • an order's order number

Object orientation endows each object with its own unique identity, the object handle. This handle uniquely identifies an object, but it is implicit and carries no semantic significance. An object's identifier carries semantics—it's a rule about the domain—and is made explicit.

These attributes constitute ways to identify an individual instance. We call each required-to-be-unique set of attributes an identifier.

Definition: An identifier is a set of one or more attributes that uniquely ...

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