软件设计原则(第二部分)

1、 Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 1 of 48Software Design Principle An IntroductionPart TwoDr. ZHOU Xiao

2、-http:/ of Computer ScienceSUN YAT-SEN UNIVERSITY, GZ 510275 Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 2 of 48lI

3、ntroductionlSingle Responsibility Principle (SRP)lOpen Closed Principle (OCP) lLiskov Substitution Principle (LSP)lDependency Inversion Principle (DIP)lInterface Segregation Principle (ISP)lLeast Knowledge Principle (LKP) Department of Computer ScienceDepartment of Computer ScienceDepartment of Comp

4、uter ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 3 of 48lIntroductionlSingle Responsibility Principle (SRP)lOpen Closed Principle (OCP) lLiskov Substitution Principle (LSP)lDependency Inversion Pri

5、nciple (DIP)lInterface Segregation Principle (ISP)lLeast Knowledge Principle (LKP) Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.

6、R.China Page 4 of 48lThe first woman in the United States to be awarded a Ph.D. from a computer science departmentIn 1968, Stanford University lShe received 2008 Turing Award from the ACMfor her work in the design of programming languages and software methodology that led to the development of objec

7、t-oriented programming ACM: Association for Computing MachineryThe Turing Award lThe highest distinction in Computer Science lThe Nobel Prize of computing Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-se

8、n University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 5 of 48lDesign by contractlBehavioral subtypingLet q(x) be a property provable about objects x of type T. Then q(y) should be true for objects y of type S where S is a subtype of T. Department of Computer ScienceDep

9、artment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 6 of 48lA subtype is a datatype that is related to another datatype (the supertype) by some notion of s

10、ubstitutabilityProgram constructs written to operate on elements of the supertype can also operate on elements of the subtypelTypical program construct: subroutines, functions, methodsPolymorphism in object-oriented programming languagelSubtyping relation is a relation on typesOn specification level

11、More abstractly, on concepts Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 7 of 48lA subclass is a class that inheri

12、ts some properties from its superclassA subclass is an extension of its superclassA subclass reuses the properties or services of its superclass lSubclass relation is a relation on implementation levelsuperclass, base class, or parent classchild classes, derived classes, or subclasses Department of

13、Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 8 of 48lstandard requirements on signaturesContravariance of method arguments in the

14、 subtypeCovariance of return types in the subtypeNo new exceptions should be thrown by methods of the subtypelexcept where those exceptions are themselves subtypes of exceptions thrown by the methods of the supertypeThe members of derived classes must expect no more than the corresponding members of

15、 the base class; and should promise no less Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 9 of 48lBehavioral conditi

16、ons that subtype must meetPreconditions cannot be strengthened in a subtypePostconditions cannot be weakened in a subtypeInvariants of the supertype must be preserved in a subtypeHistory constraint: the history properties of the supertype must be preseved in a subtypelHistory properties are properti

17、es true of all sequences of states e.g. the bound of an array does not change Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.Chi

18、na Page 10 of 48lThe square-rectangle problemAlso circle-ellipse problem/wiki/Circle-ellipse_problemA square is a rectangle ?lThe square class inherits the rectangle class will lead to many problems! Department of Computer ScienceDepartment of Computer ScienceDepartment of Comp

19、uter ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 11 of 48lbenefitsdesign is more flexiblela base type can be confidently substituted for a derived typecode is potentially easier to testrequired to

20、support the Open/Closed Principlelcall to actionensure thatlthe base type can be substituted for a derived typelderived types are substitutable for their base typeavoid runtime type checking Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen Univers

21、ity, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 12 of 48lIntroductionlSingle Responsibility Principle (SRP)lOpen Closed Principle (OCP) lLiskov Substitution Principle (LSP)lDependency Inversion Principle (DIP)lInterface Segrega

22、tion Principle (ISP)lLeast Knowledge Principle (LKP) Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 13 of 48lRigidity

23、It is hard to change because every change affects too many other parts of the systemVs. flexiblelFragilityWhen you make a change, unexpected parts of the system breakVs. robustlImmobilityIt is hard to reuse in another application because it cannot be disentangled from the current applicationVs. reus

24、able Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 14 of 48lThe interdependence of the modules makes a design rigid,

25、 fragile, immobile Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 15 of 48 Department of Computer ScienceDepartment o

26、f Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 16 of 48lThe module containing the high level policy, i.e. the Copy() module, is dependent upon the low level de

27、tailed modules that it controls. (i.e. WritePrinter() and ReadKeyboard()lIf we could find a way to make the Copy() module independent of the details that it controls, then we could reuse it freely Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen U

28、niversity, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 17 of 48 Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guang

29、zhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 18 of 48lMore traditional software development methods tend to create software structures in which high level modules depend upon low level modules, and in which abstractions depend upon detailsone of the goals of these methods is to

30、define the subprogram hierarchy that describes how the high level modules make calls to the low level modules Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-se

31、n University, Guangzhou, P.R.China Page 19 of 48 Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 20 of 48 Department o

32、f Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 21 of 48 Department of Computer ScienceDepartment of Computer ScienceDepartment of

33、 Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 22 of 48lFinding the underlying abstractionWhat is the high level policy?lIt is the abstractions that underlie the application, the truths that

34、 do not vary when the details are changedlIn the Button/Lamp example, the underlying abstraction is to detect an on/off gesture from a user and relay that gesture to a target object What mechanism is used to detect the user gesture? Irrelevant! What is the target object? Irrelevant! These are detail

35、s that do not impact the abstraction Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 23 of 48 Department of Computer S

36、cienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 24 of 48 Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer Sc

37、ienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 25 of 48lThe high level policy is entirely captured within the abstract button classThe Button class knows nothing of the physical mechanism for detecting

38、the users gesturesIt knows nothing at all about the lampThose details are isolated within the concrete derivatives: ButtonImplementation and LamplThe high level policy in Listing 6 is reusable with any kind of button, and with any kind of device that needs to be controlledit is not affected by chang

39、es to the low level mechanisms Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 26 of 48change is easier to managecode

40、is easier to replacedesign is extensiblecode is potentially easier to testrequired to support the Open/Closed Principle Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China

41、Sun Yat-sen University, Guangzhou, P.R.China Page 27 of 48create abstractionsdepend on abstractionsdeclare parameters and variables as an interface or abstract classderive only from abstract classesoverride only abstract members Department of Computer ScienceDepartment of Computer ScienceDepartment

42、of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 28 of 48lIntroductionlSingle Responsibility Principle (SRP)lOpen Closed Principle (OCP) lLiskov Substitution Principle (LSP)lDependency Inver

43、sion Principle (DIP)lInterface Segregation Principle (ISP)lLeast Knowledge Principle (LKP) Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guang

44、zhou, P.R.China Page 29 of 48lOnce an interface has gotten too fat it needs to be split into smaller and more specific interfaces so that any clients of the interface will only know about the methods that pertain to themRole InterfacelMartin Fowlers name for smaller interfaces which only include the

45、 needed methods Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 30 of 48 Department of Computer ScienceDepartment of C

46、omputer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 31 of 48lThe interface of Door has been polluted with an interface that it does not require.The Door class now depe

47、nds upon TimerClientlNot all varieties of Door need timing.It has been forced to incorporate this interface solely for the benefit of one of its subclasseslIf this practice is pursued, then every time a derivative needs a new interface, that interface will be added to the base class.This will furthe

48、r pollute the interface of the base class, making it “fat”. Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 32 of 48lS

49、ome users of Timer will register more than one timeout requestWe include a unique timeOutId code in each timeout registration Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.

50、China Sun Yat-sen University, Guangzhou, P.R.China Page 33 of 48lClearly this change will affect all the users of TimerClientHowever, the design in Figure 1 will also cause Door, and all clients of Door to be affected (i.e. at least recompiled) by this fix!Why should a bug in TimerClient have any af

51、fect on clients of Door derivatives that do not require timing?lWhen a change in one part of the program affects other completely unrelated parts of the programthe cost and repercussions of changes become unpredictable; and the risk of fallout from the change increases dramatically. Department of Co

52、mputer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 34 of 48Separation through delegation (1) Department of Computer ScienceDepartment of

53、Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 35 of 48Separation through delegation (2) Department of Computer ScienceDepartment of Computer ScienceDepartment o

54、f Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 36 of 48lbenefitscohesion is increasedlclients can demand cohesive interfacesdesign is more stablelchanges are isolated and do not cascade thr

55、oughout the codesupports the Liskov Substitution Principlelcall to actionconsider the needs of the client when designing the interfacecreate client-specific interfaces Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.Chi

56、na Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 37 of 48lIntroductionlSingle Responsibility Principle (SRP)lOpen Closed Principle (OCP) lLiskov Substitution Principle (LSP)lDependency Inversion Principle (DIP)lInterface Segregation Principle (ISP)lLe

57、ast Knowledge Principle (LKP) Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 38 of 48lLoD is a specific case of loose

58、 couplingEach unit should have only limited knowledge about other units: only units closely related to the current unit. Each unit should only talk to its friends; dont talk to strangers. Only talk to your immediate friends. Department of Computer ScienceDepartment of Computer ScienceDepartment of C

59、omputer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 39 of 48 Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China S

60、un Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Page 40 of 48 Department of Computer ScienceDepartment of Computer ScienceDepartment of Computer ScienceSun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen University, Guangzhou, P.R.China Sun Yat-sen Unive