CSCE 431 Lecture 11

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Interfaces and Contracts

Should be determined at design time:

• specifications of each class interface
• APIs of subsystems

Object refining:

• Type signatures (function name, input types, output type)

Access Control

• Expose as little as possible

Law of Demeter

• Method $M$ of object $O$ may invoke methods of following kinds of objects:
  • $O$ itself
  • $M$'s parameters
  • Any objects created/instantiated within $M$
  • $O$'s direct component objects
  • Global variables, accesible by $O$, in the scope of $M$.

Object Constraint Language

IBM 1995

Constraints: conditions on all states and transitions between states of a system implementing a given model.

context Tournament inv:
  getNumPlayers() < getMaxNumPlayers()
context Tournament::acceptPlayer(p) pre:
  not self.isPlayerAccepted(p)
context Tournament::acceptPlayer(p) post:
  self.getNumPlayers() = self@pre.getNumPlayers() + 1
context Tournament::removePlayer(p) post:
  getNumPlayers() = @pre.getNumPlayers() - 1

Formal Specification

• Invariant: predicate that is always true for all instances of a class
• Pre-condition ("rights"): must be true before an operation is invoked
• Post-condition ("obligation"): must be true after an operation is invoked.

Constraints can span more than one class.