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Design by contract

- Object-Oriented Software Construction by Bertrand Meyer, Prentice Hall
- The presence of a precondition or postcondition in a routine is viewed as a contract.

Rights and obligations

- Parties in the contract: class and clients
- require pre, ensure post with method r: If you promise to call r with pre satisfied then I, in return, promise to deliver a final state in which post is satisfied.
- Contract: entails benefits and obligations for both parties

Rights and obligations

- Precondition binds clients
- Postcondition binds class

If precondition is not satisfied

- If client’s part of the contract is not fulfilled, class can do what it pleases: return any value, loop indefinitely, terminate in some wild way.
- Advantage of convention: simplifies significantly the programming style.

Source of complexity

- Does data passed to a method satisfy requirement for correct processing?
- Problem: no checking at all or: multiple checking.
- Multiple checking: conceptual pollution: redundancy; complicates maintenance
- Recommended approach: use preconditions

Class invariants and class correctness

- Preconditions and postconditions describe properties of individual methods
- Need for global properties of instances which must be preserved by all routines
- 0<=nb_elements; nb_elements<=max_size
- empty=(nb_elements=0);

Class invariants and class correctness

- A class invariant is an assertion appearing in the invariant clause of the class.
- Must be satisfied by all instances of the class at all “stable” times (instance in stable state):
- on instance creation
- before and after every remote call to a routine (may be violated during call)

Class invariants and class correctness

- A class invariant only applies to public methods; private methods are not required to maintain the invariant.

Invariant Rule

- An assertion I is a correct class invariant for a class C iff the following two conditions hold:
- The constructor of C, when applied to arguments satisfying the constructor’s precondition in a state where the attributes have their default values, yields a state satisfying I.
- Every public method of the class, when applied to arguments and a state satisfying both I and the method’s precondition, yields a state satisfying I.

Invariant Rule

- Precondition of a method may involve the initial state and the arguments
- Postcondition of a method may only involve the final state, the initial state (through old) and in the case of a function, the returned value.
- The class invariant may only involve the state

Invariant Rule

- The class invariant is implicitly added (anded) to both the precondition and postcondition of every exported routine
- Could do, in principle, without class invariants. But they give valuable information.
- Class invariant acts as control on evolution of class
- A class invariant applies to all contracts between a method of the class and a client

Resource Allocation

reqs

0..*

type

provides

0..*

when: TimeInterval

schedule

allocated

0..*

0..1

inv Job::allocated<>0 ==> allocated.provides->includesAll(type.reqs)

--Any allocated resource must have the required facilities

inv Resource::jo1, jo2: Job::

(schedule->includesAll({jo1,jo2}) ==>

jo1.when.noOverlap(jo2.when)

-- no double-booking of resources

Collaborations in UMLUML Distilled: page 113

- A collaboration is a name given to the interaction among two or more classes.
- You can also add a class diagram to show the classes that participate in the collaboration.

Collaborations

- Use them to show common behavior.
- Often you may find that the same collaboration is used by different classes in the system.
- You can illustrate this by parameterizing the collaboration.

Roles

- Show roles that objects play in collaboration.
- UML also uses the term pattern for a parameterized collaboration.

Roles

- Roles in a play script
- Actors will play the roles; not every actor can play every role; roles have certain requirements that only some actors can fulfill. Roles have expectations on actors.

Collaboration basics

collaboration checkDef {

roleCd_graph {

out check(){}; // out: provided

getClasses(){}; // local method

in getAll = // in: expected

from Cd_graph bypassing Neighbors to Vertex; }

roleNeighbors { }

roleVertex { }

}

A collaboration is self-contained and parameterized

by roles and expected members.

Java Code: What is the collaboration?

Cd_graph{{

void isDefined(ClassGraph cg){

checkDefined(cg, getClasses(cg));}

HashSet getClasses(ClassGraph cg){

Strategy getAll = new Strategy(

"from Cd_graph bypassing Neighbors to Vertex");

TraversalGraph tg = new TraversalGraph(getAll, cg);

Visitor v = new Visitor(){

HashSet return_val = new HashSet();

void before(Vertex v){

return_val.add(v.get_vertex_name());}

public Object getReturnValue(){return return_val;}};

tg.traverse(this,v);

return (HashSet)v.getReturnValue();

}

Java Code: What is the collaboration?

void checkDefined(ClassGraph cg, HashSet tempClassSet){

final HashSet classHash = tempClassSet;

Strategy checkAll = new Strategy(”

from Cd_graph through Neighbors to Vertex");

TraversalGraph tg = new TraversalGraph(checkAll, cg);

tg.traverse(this, new Visitor(){

void before(Vertex v){

if(!classHash.contains(v.get_vertex_name())){

System.out.println("The class "+

v.get_vertex_name() + " is undefined.");

}}});}

}}

Find undefined classes

vertices

getAll

*

Cd_graph

*

Vertex

checkAll

neighbors

*

*

vertices

Neighbors

getAll = from Cd_graph bypassing Neighbors to Vertex

checkAll = from Cd_graph through Neighbors to Vertex

High-level description

- It is useful to have a high-level description of the collaboration besides the Java source code. Useful documentation.
- Ultimately we are interested in the executable form of the collaboration (Java source code).

Collaboration 1: with strategies

collaboration checkDef {

participantCd_graph {

out check(){(uses getClasses, checkDefined)};

getClasses(){(uses getAll)};

checkDefined(){(uses checkAll)};

in getAll =

from Cd_graph bypassing Neighbors to Vertex;

incheckAll =

from Cd_graph through Neighbors to Vertex; }

participantNeighbors { }

participantVertex { }

}

Collaboration 2: with accessors

collaboration checkDef {

participantCd_graph {

out check(){(uses getClasses, checkDefined)};

getClasses(){(uses getAll)};

checkDefined(){(uses checkAll)};

in getNeighbors();

in getVertices();

getAll(){return getVertices()};

// from Cd_graph bypassing Neighbors to Vertex;

checkAll(){return getNeighbors().getVertices();}

// from Cd_graph through Neighbors to Vertex;

participantNeighbors { in getVertices(); }

participantVertex { }

}

Use of 2. collaboration

Need to provide the expected methods (in methods) and provide name map (identity in this case):

Cd_graph: in getNeighbors();

Cd_graph: in getVertices();

Neighbors: in getVertices();

Cd_graph:getNeighbors(){

return cg.gather(this,”from Cd_graph to Neighbors”);}

Cd_graph:getVertices(){

return cg.gather(this,

”from Cd_graph bypassing Neighbors to Vertex;”);}

Neighbors:getVertices(){

return cg.gather(this,”from Neighbors to Vertex”);}

Use of 1. collaboration

Need to provide the expected methods (in methods) and provide name map (identity in this case):

Cd_graph:

in getAll // use default

incheckAll // use default

The Collaboration in Action

- Computer Science
- Project
- Smaller class graph for class graphs
- Mathematics

Class dictionary (part 1)

Cd_graph =

Adjacency = < source > Vertex

< ns > Neighbors "." .

Neighbors : Neighbors_wc .

Neighbors_wc :Construct_ns | Alternat_ns

common < construct_ns > List(Any_vertex).

Construct_ns = "=".

Alternat_ns = ":" < alternat_ns >

Bar_list(Term) [

Common = "common".

Any_vertex : Opt_labeled_term | Optional_term

| Syntax_vertex .

Class dictionary (part 2)

Vertex = < vertex_name > Ident.

Syntax_vertex : Regular_syntax common.

Regular_syntax = < string > String .

Opt_labeled_term : Labeled | Regular

common

Regular = .

Labeled = "<" < label_name > Ident ">" .

Term : Normal common

Normal = .

Optional_term = "["

getAll = from Cd_graph bypassing Neighbors to Vertex

checkAll = from Cd_graph through Neighbors to Vertex

Class dictionary (part 1)Cd_graph =

Adjacency = < source > Vertex

< ns > Neighbors "." .

Neighbors : Neighbors_wc .

Neighbors_wc :Construct_ns | Alternat_ns

common < construct_ns > List(Any_vertex).

Construct_ns = "=".

Alternat_ns = ":" < alternat_ns >

Bar_list(Term) [

Common = "common".

Any_vertex : Opt_labeled_term | Optional_term

| Syntax_vertex .

getAll = from Cd_graph bypassing Neighbors to Vertex

checkAll = from Cd_graph through Neighbors to Vertex

Class dictionary (part 1)Cd_graph =

Adjacency= < source > Vertex

< ns > Neighbors "." .

Neighbors : Neighbors_wc .

Neighbors_wc :Construct_ns | Alternat_ns

common < construct_ns > List(Any_vertex).

Construct_ns = "=".

Alternat_ns = ":" < alternat_ns >

Bar_list(Term) [

Common = "common".

Any_vertex : Opt_labeled_term | Optional_term

| Syntax_vertex .

getAll = from Cd_graph bypassing Neighbors to Vertex

checkAll = from Cd_graph through Neighbors to Vertex

Class dictionary (part 2)Vertex = < vertex_name > Ident.

Syntax_vertex : Regular_syntax common.

Regular_syntax = < string > String .

Opt_labeled_term : Labeled | Regular

common

Regular = .

Labeled = "<" < label_name > Ident ">" .

Term : Normal common

Normal = .

Optional_term = "["

getAll = from ClassG bypassing Body to ClassNameUML class diagram ClassG

0..*

Entry

EParse

entries

ClassG

BParse

ClassDef

Body

parts

Part

className

0..*

ClassName

Concrete

Abstract

AOO / UML / OCL/Strategies

checkAll = from ClassG through Body to ClassNameUML class diagram ClassG

0..*

Entry

EParse

entries

ClassG

BParse

ClassDef

Body

parts

Part

className

0..*

ClassName

Concrete

Abstract

AOO / UML / OCL/Strategies

change of domain: from Computer Science

to Mathematics

Example:

x = 1.0 .

y = (+ x 4.0).

z = (* x y).

Equation SystemEquationSystem =

Equation =

Variable = Ident.

Expression : Simple | Compound.

Simple : Variable | Numerical.

Compound = “(“ Op

Op : Add | Mul.

Add = “+”.

Mul = “*”.

Numerical = float.

Write a program for:

Are all variables in an equation

system defined? (disregard order

of equations)

Make More Reusable

Cd_graph{{

String vi = “from Vertex to edu.neu.ccs.demeter.Ident”;

void isDefined(ClassGraph cg){

checkDefined(cg, getClasses(cg));}

HashSet getClasses(ClassGraph cg){

Strategy getAll = new Strategy(

"from Cd_graph bypassing Neighbors to Vertex");

TraversalGraph tg = new TraversalGraph(getAll, cg);

Visitor v = new Visitor(){

HashSet return_val = new HashSet();

void before(Vertex v){

return_val.add(cg.fetch(v, vi) );}

public Object getReturnValue(){return return_val;}};

tg.traverse(this,v);

return (HashSet)v.getReturnValue();

}

Make More Reusable

void checkDefined(ClassGraph cg, HashSet tempClassSet){

final HashSet classHash = tempClassSet;

Strategy checkAll = new Strategy(”

from Cd_graph through Neighbors to Vertex");

TraversalGraph tg = new TraversalGraph(checkAll, cg);

tg.traverse(this, new Visitor(){

void before(Vertex v){ Ident vn = cg.fetch(v, vi);

if (!classHash.contains(vn)){

System.out.println("The object "+ vn

+ " is undefined.");

}}});}

}}

Example:

x = 1.0 .

y = (+ x 4.0).

z = (* x y).

Equation SystemEquationSystem =

Equation =

Variable = Ident.

Expression : Simple | Compound.

Simple : Variable | Numerical.

Compound = “(“ Op

Op : Add | Mul.

Add = “+”.

Mul = “*”.

Numerical = float.

Write a program for:

Are all variables in an equation

system defined? (disregard order

of equations)

Adaptive Object-Oriented DesignFind undefined things

definedThings

*

System

*

Thing

usedThings

*

*

*

UsedThingsHolder

definedThings= from System bypassing UsedThingsHolder to Thing

usedThings = from System through UsedThingsHolder to Thing

usedThings = from EquationSystem

through Expression to Variable

Example:

x = 1.0 .

y = (+ x 4.0).

z = (* x y).

Equation SystemEquationSystem =

Equation =

Variable = Ident.

Expression : Simple | Compound.

Simple : Variable | Numerical.

Compound = “(“ Op

Op : Add | Mul.

Add = “+”.

Mul = “*”.

Numerical = float.

definedThings= from EquationSystem

bypassing Expression to Variable

Example:

x = 1.0 .

y = (+ x 4.0).

z = (* x y).

Equation SystemEquationSystem =

Equation =

Variable = Ident.

Expression : Simple | Compound.

Simple : Variable | Numerical.

Compound = “(“ Op

Op : Add | Mul.

Add = “+”.

Mul = “*”.

Numerical = float.

Collaboration CheckUnique:3 Roles

//checks for unique parts.

// Collaboration CheckUnique

// role System

// out void checkUnique(final ClassGraph cg)

// in Strategy getAllUnitsToBeChecked

// role UnitToBeChecked

// in Strategy checkAllParts

// role NotDuplicated

// System = List(UnitToBeChecked).

// UnitToBeChecked = List(NotDuplicated).

// NotDuplicated = Ident.

Collaboration CheckUniquewith role play

//checks for unique parts.

// Collaboration CheckUnique

// role System played by Grammar

// out void checkUnique(final ClassGraph cg)

// in Strategy getAllUnitsToBeChecked

// role UnitToBeChecked played by Statement

// in Strategy checkAllParts

// role NotDuplicated played by NonTerminal

// System = List(UnitToBeChecked).

// UnitToBeChecked = List(NotDuplicated).

// NotDuplicated = Ident.

Collaboration CheckUniquewith role play 2

//checks for unique parts.

// Collaboration CheckUnique

// role System played by Cd_graph

// out void checkUnique(final ClassGraph cg)

// in Strategy getAllUnitsToBeChecked

// role UnitToBeChecked played by Adjacency

// in Strategy checkAllParts

// role NotDuplicated played by Labeled

// in Strategy getIdent

// System = List(UnitToBeChecked).

// UnitToBeChecked = List(NotDuplicated).

// NotDuplicated = Ident.

Program part 1

System{

{{ Strategy getAllUnitsToBeChecked =

… from System to UnitToBeChecked …

void checkUnique(final ClassGraph cg){

cg.traverse(this,

getAllUnitsToBeChecked,

new Visitor(){

void before(UnitToBeChecked a){

a.checkDuplicateParts(cg);} });}}}}

Program part 2

UnitToBeChecked{

{{ Strategy checkAllParts =

… from UnitToBeChecked to NotDuplicated …

void checkDuplicateParts(ClassGraph cg){

cg.traverse(this, checkAllParts,

new Visitor(){

HashSet hParts = new HashSet();

void before(NotDuplicated l){

if(!hParts.add(l.get_ident())){

System.out.println("Element "+ l.get_ident() + " is not unique.");

}}});}}}}

Conclusion Collaborations

- Collaborations are an important architectural element in object-oriented design.
- Separate collaboration from its use.

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