重大18_Principles of object-oriented design.ppt

Object-Oriented Programming C++ 18 Agile software development Object-Oriented Programming C++ 01 Course Overview Object-Oriented Programming C++ College of Computer Science, CQU 18 Principles of object-oriented design Liu Ji liujiboy@163.com liujiboy@ * Outline SRP：the single-responsibility principle OCP：the open-closed principle LSP：the liskov substitution principle DIP：the dependency-inversion principle ISP：the interface segregation principle * References SRP In?object-oriented programming, the?single responsibility principle?states that every object should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility. * SRP example The Rectangle class has two responsibilities. The first responsibility is to provide a mathematical model of the geometry of a rectangle (area method). The second responsibility is to render the rectangle on a graphical user interface(draw method). This design violates the SRP. * The violation of SRP causes several nasty problems. Firstly, we must include the GUI in the computational geometry application. If this were a C++ application, the GUI would have to be linked in, consuming link time, compile time, and memory footprint Secondly, if a change to the GraphicalApplication causes the Rectangle to change for some reason, that change may force us to rebuild, retest, and redeploy the ComputationalGeometryApplication. If we forget to do this, that application may break in unpredictable ways. * A better design is to separate the two responsibilities into two completely different classes. This design moves the computational portions of Rectangle into the GeometricRectangle class. Now changes made to the way rectangles are rendered cannot affect the ComputationalGeometryApplication. * OCP OCP?states software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification;?that is, such an ent