Formal Specification

• Specifications of a software artifact's properties in a language defined by mathematical logic
• This provides a formal model of the artifact
• Since it is a model, not all information about the artifact is included
  • What would it mean to have a complete formal model of a software artifact?
• Examples:
  • Finite state machines
  • Grammars
  • Alloy
  • Z
• Key advantages of formal models
  • Unambiguous
  • Can prove properties mathematically
  • Can use automated proof techniques
• Disadvantages
  • Learning curve
  • Not always appropriate

Black Box Specification

• Concept developed by Harlan Mills
• Formal specification of the boundary between a software artifact and its environment
• An artifact receives stimuli from its environment
• An artifact issues responses to its environment
• The black box models the mapping of stimuli to responses solely in terms of external events
  • This models less than a state machine does
• One straightforward representation
  • History of interactions
  • Current stimulus
  • Current response
• Current response depends upon both the current stimulus and the history
• Representing interactions
  • Each stimulus and response is represented by a symbol
• Representing a history
  • A history is a boolean expression representing a set of stimulus-response sequences
  • Available operations:
    • Boolean operators (and, or, not)
    • includes: Returns true if its argument is included in the sequence
    • Stimuli: Referred to by name
    • Responses: Referred to by name, prefixed with "response"
    • count: Counts occurrences of its argument
    • Arithmetic operators: Used with count calls
    • Sequencing (:) operator: Matches specified chain
    • any: Matches anything
• Blackbox2 editor/analyzer (An example)