Software Construction & Design 1
The University of Sydney Page 1
Software Design and
Construction 2
SOFT3202 / COMP9202
Software Verification
Theory and Examples
School of Information Technologies
Dr. Basem Suleiman
The University of Sydney Page 2
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Agenda
– Software Verification Theory
– Design by Contract
– Pre-conditions and post-conditions
– Class invariants
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Software Validation and
Verification
Theory
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Software Testing – Revisit
– Software process to
– Demonstrate that software meets its requirements (validation testing)
– Find incorrect or undesired behavior caused by defects/bugs (defect testing)
• E.g., System crashes, incorrect computations, unnecessary interactions and data
corruptions
– Part of software verification and validation (V&V) process
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Software Verification and Validation
– Software testing is part of software Verification and Validation (V&V)
– The goal of V&V is to establish confidence that the software is “fit for purpose”
– Software Validation
– Are we building the right product?
– Ensures that the software meets customer expectations/needs
– Software Verification
– Are we building the product right?
– Ensures that the software meets its stated functional and non-functional
requirements
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Software V&V
– Software Verification
– Concerned with the software requirements/specifications
– Can be ambiguous
– Software Validation
– Customer/end user’s expectation
– Disambiguate the requirements
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V-Model
– Link each phase of the SDLC with its associated testing phase
– Each verification stage relates to a validation stage
https://www.buzzle.com/editorials/4-5-2005-68117.asp
https://www.buzzle.com/editorials/4-5-2005-68117.asp
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Software Verification
– Artifact/specification verification
– Review the resulted software artifacts to verify that specifications are met
– Check the output of each software development phase against the input
specification (specs.)
– Design specifications against requirement specifications
• Detailed design correctly implement the requirements (F & NF)
– Construction (code) against the design specs.
• Source code/user interfaces correctly implements the design specs.
– Include inspections, reviews, walkthrough
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Static Verification
– Static Verification
– Static system analysis to discover problems
– May be applied to requirements, design/models, configuration and test data
– Reviews
– Walk through
– Code inspection
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Software Validation
– Artifact/specification validation
– Validate the requirements from the end user perspective
– Check that the needs of all stakeholders (users, managers, investors) are met
– Validate the output of main development stages from the stakeholder’s point of
view
• Validate if the requirements represent the will and goals of the
stakeholders
– Include Unit testing, integration/functional testing, and user acceptance
testing
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Design by Contract
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Contract
– A lawful agreement between two parties in which both
parties accept obligations and on which both parties can
found their rights
– The remedy for breach of a contract is usually an award of
money to the injured party
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Object-Oriented Contract
– Specifies the expected services that can be provided if
certain conditions are satisfied
– Services = “Obligations” and conditions = “Rights”
– Breach of a contract results in generating an exception
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Design by Contract (DbC)
– A software design approach for program correctness
– Known as contract programing, programming by contract,
design-by-contract programming
– Definition of formal, precise and verifiable interface
specification for software components
– Pre-conditions, postconditions and invariants (contract)
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Design by Contract (DbC)
By Fabuio [CC0], from Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Design_by_contract.svg
https://commons.wikimedia.org/wiki/File:Design_by_contract.svg
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Contracts in OO Design
Object-oriented contract
– Describes the services that are provided by an object if
certain conditions are fulfilled
– services = “obligations”, conditions = “rights”
– An OO-contract for each service specifically describes:
– The conditions under which the service will be provided
– A specification of the result of the service that is provided
– The remedy for breach of an OO-contract is the generation
of an exception
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Object-Oriented Contract –
– Examples:
– A letter posted before 18:00 will be delivered on the next working day
to any address within Australia
– For the price of $7 a letter with a maximum weight of 80 grams will be
delivered anywhere in Australia within 4 hours of pickup
– Exercise: identify the conditions/rights and obligations/services of the
above delivery services.
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Modeling Constraints
with Contracts
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Modeling Constraints with Contracts
– Example of constraints in Arena:
– An already registered player cannot be registered again
– The number of players in a tournament should not be more than
maxNumPlayers
– One can only remove players that have been registered
– We model them with contracts
– Contracts can be written in OCL
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ModelingOO-Contracts – Formal Specifcation
– Natural Language
– Mathematical Notation
– Models and contracts:
– Alanguage for the formulation of constraints with the formal strength
of the mathematical notation and the easiness of natural language:
UML + OCL (Object Constraint Language)
– Uses the abstractions of the UML model
– OCL is based on predicate calculus
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Contracts and Formal Specification
– A Contract share same assumptions about the class
– Three constraints:
– Invariant:
• A predicate that is always true for all instances of a class
– Precondition (“rights”):
• Must be true before an operation is invoked
– Postcondition (“obligation”):
• Must be true after an operation is invoked.
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Formal Specification
– A contract is called a formal specification, if the invariants,
rights and obligations in the contract are unambiguous
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Expressing Constraints in UML Models
HashTable
put(key,entry:Object)
get(key):Object
remove(key:Object)
containsKey(key:Object):boolean
size():int
numElements:int
<
numElements >= 0<
!containsKey(key)
<
containsKey(key)
<
containsKey(key)
<
!containsKey(key)
<
get(key) == entry
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Use Contracts inRequirements Analysis
– Many constraints represent domain-level information
– Why not use them in requirements analysis?
– Increase requirements precision
– Yield more questions for the end user
– Clarify the relationships among several objects
– Constraints are sometimes used during requirements
analysis, however there are trade-offs
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Object Constraint
Language (OCL)
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Object Constraint Language (OCL)
– Formal language for expressing constraints over a set of objects and their
attributes
– Part of the UML standard
– Used to write constraints that cannot otherwise be expressed in a diagram
– Declarative
– No side effects
– No control flow
– Based on Sets and Multi Sets
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OCL Basic Concepts
– OCL expressions
– Return True or False
– Are evaluated in a specified context
– All constraints apply to all instances
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Example – Tournament Class
Tournament
– maxNumPlayers: int
+ acceptPlayer(p:Player)
+ removePlayer(p:Player)
+ getMaxNumPlayers():int
+ getPlayers(): List
+ isPlayerAccepted(p:Player):boolean
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OCL Simple Predicates
“The maximum number of players in any tournament should be a
positive number.”
context Tournament inv: self.getMaxNumPlayers() > 0
Notes:
– OCL uses the same dot notation as Java
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OCL Preconditions – Examples
“The acceptPlayer(p) operation can only be invoked if player p has not yet
been accepted in the tournament.”
context Tournament::acceptPlayer(p) pre:
not self.isPlayerAccepted(p)
Questions:
– What is the context the pre-condtion?
– What is “isPlayerAccepted(p)”?
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OCL Postconditions – Example
“The number of accepted player in a tournament increases by one after
the completion of acceptPlayer()”
context Tournament::acceptPlayer(p) post:
self.getNumPlayers() =
[email protected]() + 1
Notes:
– self@pre: the state of the tournament before the invocation of the operation
– self: denotes the state of the tournament after the completion of the operation
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OCL Contract for acceptPlayer() in Tournament
context Tournament::acceptPlayer(p) pre:
not isPlayerAccepted(p)
context Tournament::acceptPlayer(p) pre:
getNumPlayers() < getMaxNumPlayers()context Tournament::acceptPlayer(p) post:isPlayerAccepted(p)context Tournament::acceptPlayer(p) post:getNumPlayers() = @pre.getNumPlayers() + 1The University of Sydney Page 38OCL Contract for removePlayer()in Tournamentcontext Tournament::removePlayer(p) pre:isPlayerAccepted(p)context Tournament::removePlayer(p) post:not isPlayerAccepted(p)context Tournament::removePlayer(p) post:getNumPlayers() = @pre.getNumPlayers() – 1The University of Sydney Page 39Java Implementation ofTournament class(Contract as a set of JavaDoc comments)public class Tournament {/** The maximum number of players* is positive at all times.* @invariant maxNumPlayers > 0
*/
private int maxNumPlayers;
/** The players List contains
*references to Players who are
*are registered with the
*Tournament. */
private List players;
/** Returns the current number of
* players in the tournament. */
public int getNumPlayers() {…}
/** Returns the maximum number of
* players in the tournament. */
public int getMaxNumPlayers() {…}
/** The acceptPlayer() operation
* assumes that the specified
* player has not been accepted
* in the Tournament yet.
* @pre !isPlayerAccepted(p)
* @pre getNumPlayers()
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OCL Sets, Bags and Sequences
– Sets, Bags and Sequences are predefined in OCL
– Subtypes of Collection
– OCL offers a large number of predefined operations on
collections. All of the form:
collection->operation(arguments)
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OCL-Collection
– The OCL-Type Collection is the generic superclass of a collection of objects
of Type T
– Subclasses of Collection are
– Set: Set in the mathematical sense. Every element can appear only once
– Bag: A collection, in which elements can appear more than once (also called
multiset)
– Sequence: A multiset, in which the elements are ordered
– Example for Collections:
– Set(Integer): a set of integer numbers
– Bag(Person): a multiset of persons
– Sequence(Customer): a sequence of customers
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OCL-Operations for OCL-Collections (1)
size: Integer
Number of elements in the collection
includes(o:OclAny): Boolean
True, if the element o isin the collection
count(o:OclAny): Integer
Counts how many times an element is contained in the collection
isEmpty: Boolean
True, if the collection is empty
notEmpty: Boolean
True, if the collection is not empty
The OCL-Type OclAny is the most general OCL-Type
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OCL-Operations for OCL-Collections(2)
union(c1:Collection)
Union with collection c1
intersection(c2:Collection)
Intersectionwith Collection c2 (contains only elements, which appear in the collection as
well as in collection c2 auftreten)
including(o:OclAny)
Collection containing all elements of theCollection and element o
select(expr:OclExpression)
Subset of all elements of the collection, for which the OCL-expression expr is true
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Evaluating OCL Expressions
The value of an OCL expression is an object or a collection of objects.
– Multiplicity of the association-end is 1
– The value of the OCL expression is a single object
– Multiplicity is 0..1
– The result is an empty set if there is no object, otherwise a single object
– Multiplicity of the association-end is *
– The result is a collection of objects
• By default, the navigation result is a Set
• When the association is {ordered}, the navigation results in a
Sequence
• Multiple “1-Many” associations result in a Bag
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OCL Quantifiers
forAll
– forAll (variable|expression) is True if expression is True for all
elements in the collection
exist
– exists (variable|expression) is True if there exists at least one
element in the collection for which expression is True
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OCL Quantifiers – forAll Example
Given the following OCL quantifier
context Tournament inv:
matches->forAll(m:Match |
m.start.after(t.start) and m.end.before(t.end))
Exercise: explain what this OCL attempts to verify.
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OCL Quantifiers – Exists Example
Given the following OCL quantifier
context Tournament inv:
matches->exists(m:Match | m.start.equals(start))
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References
– Ian Sommerville. 2016. Software Engineering (10th ed.) Global Edition.
Pearson.
– Wikipedia, Software Verification and Validation,
https://en.wikipedia.org/wiki/Software_verification_and_validation
– Object-Oriented Software Engineering: Using UML, Patterns, and Java, 3rd
Edition, Bernd Bruegge & Allen H. Dutoit, Pearson.
http://vig.prenhall.com/catalog/academic/product/0,4096,0130471100,00.html
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W12 Tutorial: Practical
Exercises
Design Pattern Assignment
Demo
W12 Lecture: Specification
Languages
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Specifying the Model Constraints: Using asSet
Local attribute navigation
context Tournament inv:
end – start <= Calendar.WEEKDirectly related class navigationcontext Tournament::acceptPlayer(p)pre:league.players->includes(p)
players
* tournaments
{ordered}
Tournament
+start:Date
+end:Date
+acceptPlayer(p:Player)
*
League
+start:Date
+end:Date
+getActivePlayers()
*
Player
+name:String
+email:String
* players
tournaments*
Indirectly related class navigation
context League::getActivePlayers
post:
result=tournaments.players->asSet
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