OCL

Introduction

• How can we express constraints in UML ?
• E.g. the balance attribute in a SavingsAccount class can never be less than zero and more than 100000.
• UML only has a notion of an informal constraint in terms of a note - which can be attached to a model. However its just text.
• To write formal and machine checkable constraints - we use the Object Constraint Language (OCL).

Assertions

• In a program, assertions express constraints on program state that must be true at a specified point during execution.
• In a model/diagram, they document what must be true of an implementation of a modelling element.
• Can have optional preconditions - (must be true when operation is invoked). for e.g. an argument to a certain operation cannot be null.
• Can have optional postconditions - (must be true when operation completes). for e.g. the operation must update the database when it completes.

Design by contract

• A pre and post condition can be viewed as a contract between an operation and its invokers.
• Something like - "If the client promises to invoke the operation with the precondition satisfied, then the operation guarantees that its implementing methods will deliver a final state in which the postcondition is true".
• So the invoker doesn't care about how final state is produced
• And the implementer doesn't need to worry about cases where the precondition is not true.

[TODO] : How about handling errors for invalid preconditions ?

• So a contract can be many things:
  • A specification of what a method should do
  • Documentation - more precise than class interfaces [TODO : e.g. ]
  • Offers runtime checks for correctness.
  • Are a basis for testing.
  • Are a basis for formal proofs of programs correctness.