南京大学-软件工程-15 面向对象的信息隐藏.ppt

How to … Abstraction is Key ...using polymorphic dependencies (calls) Example of Responsibility Change DiskWriter::Write(c) { WriteDisk(c); } Copy Keyboard Reader Printer Writer Disk Writer void Copy(ReadKeyboard r, WritePrinter w){ int c; while ((c = r.read ()) != EOF) w.write (c); } Principle 10: Open/Closed Principle (OCP) Software entities should be open for extension, but closed for modification B. Meyer, 1988 / quoted by R. Martin, 1996 Be open for extension modules behavior can be extended Be closed for modification source 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 modified OCP RTTI is ugly and dangerous RTTI = Run-Time Type Information 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 module recognize them by type switch or if-else-if structures RTTI is Ugly and Dangerous! //? RTTI violating?the?//open-closed?principle and LSP ????class?Shape?{}????class?Square?extends?Shape?{????????void?drawSquare()?{????????????//?draw ????????} ????}????class?Circle?extends?Shape?{????????void?drawCircle()?{????????????//?draw ????????} ????}????void?drawShapes(ListShape?shapes)?{????????for?(Shape?shape?:?shapes)?{????????????if?(shapes?instanceof?Square)?{????????????????((Square)?shapes).drawSquare();????????????}?else?if?(shape?instanceof?Circle)?{????????????????((Circle)?shape).drawCircle(); ????????????} ????????} ????}???? //??Abstraction and Polymorphism that?does?// not?violate?the?open-closed?principle and LSP ????interface?Shape?{????????void?draw(); ????}????class?Square?implements?Shape?{????????void?draw()?{????????????//?draw?implementation ????????} ????}????class?Circle?implements?Shape?{????????void?draw()?{????????????//?draw?implementation ????????} ????}????void?drawShapes(L