15 面向对象的信息隐藏

1、计算与软件工程II Ch15 “面向对象”的信息隐藏丁二玉南京大学软件学院Information Hidingn Each module hides the implementation of an important design decision (secrets) so that only the constituents of that module know thedetailsDesign Secrets need to hiden Primary Secret:ResponsibilityChangen Hidden information that was specified

2、to the software designern From SRSn Secondary Secret: Implementation Changen The implementation decisions made by the designer when implementing the module designed to hide the primary secretn 变化；3Main Contents1.EncapsulationWhat should be hiding?1.2.How to hiding change?Encapsulation (1)n Encapsula

3、tion allows the programmer to group data and the subroutines that operate on them together in one placen ResponsibilityEncapsulation (2)n hide irrelevant details from the usern a class can be divided into two parts, the user interface and the implementation.n The interface is the visible surface of

4、the capsule.q describes the essential characteristics of objects of the class which are visible to the exterior worldn The implementation is hidden in the capsule.q The implementation hiding means that data can only be manipulated, that is updated, within the class, but it does not mean hiding inter

5、face data.NewView of Encapsulationn Old/Beginner/Implementation view of encapsulation:hiding data inside an objectn New view: hiding anything, including:q Data (implementation)q Structure (implementation)q Other object (implementation)q Type (derived classes)q Change/vary (design details)q Encapsula

6、tionCorrectly ADTn ADT = Abstract Data Typeq A concept, not an implementationq A set of (homogeneous) objects together with a set of operations on those objectsq No mention of how the operations are implementedn Encapsulation = data abstraction + typeq data abstraction: group data and operationq Typ

7、e: hiding implementation, make usage correctlyWhy type?A type may be viewed as a set of clothes (or a suit of armor) that protects an underlying untyped representation from arbitrary or unintended use.It provides a protective covering that hides the underlying representation and constrains the way o

8、bjects may interact with other objects.In an untyped system untyped objects are naked in that the underlying representation is exposed for all to see.Encapsulate Datan If needed, use Accessors and Mutators, Not Public Membersn Accessors and Mutators is meaningful behaviorq Constraints, transformatio

9、n, formatpublic void setSpeed(double newSpeed) if (newSpeed 0) sendErrorMessage(.);newSpeed = Math.abs(newSpeed);speed = newSpeed;Encapsulatestructuresn See chapter 16 Iterator PatternReferences and Collection Data-Type !Encapsulate other objectsn Collaboration Designq Composition; delegationEncapsu

10、late type(subclass)n LSPq pointers to superclasses or interfaces;All derived classes must be substituteablefor their base classEncapsulate Change (or vary)n Identify the aspects of your application that may change (or vary) and separate them from what stays the same.n Take the parts that change( var

11、y ) and encapsulate them, so that later you can alter or extend the parts that vary without affecting the parts that dont.n See DIP and OCP LaterEncapsulate Implementation Detailn Datan Structuren Other objectn Typen Change/varyn Principle #1: Minimize The Accessibility of Classes and Membersn Abstr

12、actionq An abstraction focuses on the outside view of an object and separates an objects behavior from its implementationn Encapsulationq Classes should not expose their internal implementation detailsMain Contents1. Encapsulation2. How to hiding change?Example of Responsibility Changevoid Copy(Read

13、Keyboard& r, WritePrinter& wp, WriteDisk& wd, OutputDevice dev)int c;while(c = r.read()!= if(dev = printer)wp.write(c);elseEOF)wd.write(c);void Copy(ReadKeyboard& r, WritePrinter&int c;w)while (c = r.read () != EOF)w.write (c);Write DiskWrite PrinterRead KeyboardCopyHow to n Abstraction is Keyq .usi

14、ng polymorphic dependencies (calls)Example of Responsibility Changevoid Copy(ReadKeyboard& r, WritePrinter& w)int c;while (c = r.read () != EOF) w.write (c);Disk WriterPrinter WriterKeyboard ReaderDiskWriter:Write(c)WriteDisk(c);CopyPrinciple 10: Open/Closed Principle (OCP)Be open for extension4modu

15、les behavior can be extended Be closed for modification4source code for the module must not be changes统计数据表明，修正bug最为频繁，但是影响很小；新增需求数量一般，但造成了绝大多数影响Modules should be written so they can be extended without requiring them to be modifiedSoftware entities should be open for extension, but closed for modif

16、icationB. Meyer, 1988 / quoted by R. Martin, 1996OCPn RTTI is ugly and dangerousq RTTI = Run-Time Type Informationq If a module tries to dynamically cast a base class pointer to several derived classes, any time you extend the inheritance hierarchy, you need to change the moduleq recognize them by t

17、ype switch or if-else-ifstructuresRTTI is Ugly and Dangerous!/RTTIviolating the/open-closed principle and LSPclass Shape class Square extends Shape void drawSquare() / drawclass Circle extends Shape void drawCircle() / drawvoid drawShapes(List shapes) for (Shape shape : shapes) if (shapes instanceof

18、 Square) (Square) shapes).drawSquare(); else if (shape instanceof Circle) (Circle) shape).drawCircle();/Abstraction and Polymorphism that does/ not violate the open-closed principle and LSPinterface Shape void draw();class Square implements Shape void draw() / draw implementationclass Circle impleme

19、nts Shape void draw() / draw implementationvoid drawShapes(List shapes) for (Shape shape : shapes) shape.draw();OCPSummaryn Use abstraction to gain explicit closuren Plan your classes based on what is likely to change.q minimizes future change locationsn OCP needs DIP & LSPNo significant program can

20、 be 100% closedR.Martin, “The Open-Closed Principle,” 1996Principle 11: Dependency Inversion Principle(DIP)I. High-level modules should not depend on low- level modules.Both should depend on abstractions.II. Abstractions should not depend on details. Details should depend on abstractionsR. Martin, 1

21、996DIP : separate interface fromimplementation abstractn Use inheritance to avoid direct bindings to classes:Interface (abstract class)ClientImplementation (concrete class)Design to an interface, not an implementation!DIP Exampleclass Reader public:virtual int read()=0;Copyclass Writer public:Reader

22、Writervirtual voidwrite(int)=0;KeyboardReaderPrinterWritervoid Copy(Reader&r, Writer&w)int c;while(c = r.read() !=EOF)w.write(c);DiskWriterDIPProcedural vs. OO ArchitectureProcedural ArchitectureObject-Oriented ArchitectureDIPsummaryn Abstract classes/interfaces:q tend to change less frequentlyq abs

23、tractions are hinge points where it is easier to extend/modifyq shouldnt have to modify classes/interfaces that represent the abstraction (OCP)n Exceptionsq Some classes are very unlikely to change;n therefore little benefit to inserting abstraction layern Example: String classq In cases like this c

24、an use concrete class directlyn as in Java or C+How to dealwith change OCP states the goal; DIP states the mechanism; LSP is the insurance for DIPExample of Implementation ChangeAll ducks quackand swim.The superclass takes care of the implementation codeDuckquack() swim() display()/other duck-like methodsThe display() method is abstract, since all duck subtypes