Formal Methods in Software Engineering “And they made their lives bitter with hard bondage … all their service … was with rigour.” Ex 1:14

A Spectrum of Methods Less Formal More Formal Cleanroom OCL and Z Traditional Models: Waterfall, Spiral, Incremental Agile Methods: FDD and SCRUM

Characteristics of Formal Methods Well-defined specification language Typically based on set-theoretical concepts Specifies by indicating operational conditions: pre-conditions, post-conditions, and invariants Composed of three components: syntax, semantics, and relations (operational statements) Emphasis on verification Of program correctness Of completeness of description Of refinements to different abstractions Testing De-emphasized

Advantages of Formal Methods Consistency (fewer contradictions) Precision (less ambiguity) Completeness (extent of definitions more clearly defined, so all cases handled) Descriptive Uniformity (less mixture among levels of detail)

Set Theoretical Foundation for Formal Methods See Section 28.3 Set Operators: , ∩, \, and x Logic Operators: and, or, not, implication Sequences:, head, tail, front, last, concatenation See Examples for Block Handler: pgs 775 and (section ) OCL Notation Summary (Table28.1), pg 785 Z Notation Summary (Table 28.2), pg 789

Block Handler Example: Constraints No block will be marked as both free and used All the sets of blocks in the queue are subsets of currently used blocks No elements of the queue contain the same block numbers The collection of used and free blocks make up the total collection of all blocks There are no duplicates among the free blocks There are no duplicates among the used blocks

OCL Example: Block Handler Context BlockHandler inv: (used->intersection(free))-> isEmpty() Context BlockHandler inv: blockQ->forAll(aBlockSet | used->includesAll(aBlockSet)) Context BlockHandler inv: allBlocks = used->union(free) Context BlockHandler::remove() pre: blockQ->size() > 0 post: used = – AND free = first()) AND blockQ = Block Queue Free Used released

Z Example: Block Handler used, free: powerSet BLOCKS blockQ: seq powerSet BLOCKS used ∩ free = Ø used U free = allBlocks forAll i: dom blockQ : blockQ i subset used forAll i, j: dom blockQ : i ≠ j → blockQ i ∩ blockQ j = Ø removes(): Pre: #blockQ > 0 Post: used’=used–head blockQ AND free’=free U head blockQ AND blockQ’=tail blockQ Block Queue Free Used released

Additional Points of Emphasis Why completeness is difficult to achieve (pg 771) Controversy over formal methods (pg 771) Areas in which formal methods may apply (pg 770 – “Why is it important”) Area in which formal methods are difficult to apply (pg 792)