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红椒一个没绷住，嗤地一声笑喷了。香荽是陪着周菡坐车的。The principle of opening and closing means opening to expansion and closing to modification. When the program needs to be expanded, the original code cannot be modified, but the original code should be expanded to achieve a hot plug effect. Therefore, a summary of one sentence is: in order to make the program scalable, easy to maintain and upgrade. To achieve this effect, we need to use interfaces and abstract classes, etc. We will mention this point in the later specific design."Ordinary Highway"The Liskov Substitution Principle (LSP) is one of the basic principles of object-oriented design. The Richter substitution principle says that wherever a base class can appear, subclasses can certainly appear. LSP is the cornerstone of inheritance reuse. Only when derivative classes can replace base classes and the functions of software units are not affected can base classes be truly reused, and derivative classes can also add new behaviors to base classes. The Richter substitution principle is a supplement to the "open-close" principle. The key step to realize the "open-close" principle is abstraction. The inheritance relationship between the base class and the subclass is the concrete realization of abstraction, so the Richter substitution principle is the specification of the concrete steps to realize abstraction. --From BaiduArticle 20 Scope of Application