设计模式学习（1）- 设计原则

ISO SQuaRE Quality-in-Use model

从用户的角度去分析一个软件的使用质量：

• effecitiveness 效果

efficience效率

• saticifaction 满意度
• safety 安全性
• context coverage 内容覆盖

ISO System/Software Product Quality Model

软件产品质量：

• performance efficiency
• portability
• compatibility
• security
• reliability
• usability
• functional suitability
• maintainability
  • modularity
  • reusability
  • analysability
  • modifiability
  • testability

The SOLID Principles of class design 设计类

• SRP the single responsibility principle
• OCP open closed
  • modules should be open for extension, but closed for modification 当系统需求变化我们希望继承旧系统而不是更改旧系统
• LSP liskov substitution
  • code that uses a reference to a base type must be able to use objects of derived types without knowing it: subtypes must be substitutable for their base types
  • violation of lsp 违反的例子

  • public interface PersistedResource{void load();void store();}

    public class ApplicationSettings implements PersistedResource{//...public void load(){// Loads the saved application settings from storageSystem.out.println("Got the application settings");}public void store(){// Stores the application settings to preserve changesSystem.out.println("Saved the application settings");}}public class UserSettings implements PersistedResource{//...public void load(){// Loads the saved user settings from storageSystem.out.println("Got the user settings");}public void store(){// Stores the user settings to preserve changesSystem.out.println("Saved the user settings");}}

    public class Application{List<PersistedResource> resources = new ArrayList<PersistedResource>();public void initializeResources(){

    /////////////////////////////////////////////////////////////////////here subtypesresources.add(new ApplicationSettings());resources.add(new UserSettings()); loadAll();}public void loadAll(){for (PersistedResource resource: resources){resource.load();}}public void saveAll(){for (PersistedResource resource: resources){resource.store();}}//continued

    public static void main(String[] args){Application app = new Application();app.initializeResources();//do some work, maybe changing settingsSystem.out.println("Made changes to settings");//save the changes before terminatingapp.saveAll();}}

    CHANGE REQUEST

  • public class SpecialSettings implements PersistedResource{//...public void load(){// Loads the special settings from storageSystem.out.println("Got the system settings");}public void store(){// Special settings are read only and cannot be saved// What to do????}}

    public class Application{List<PersistedResource> resources = new ArrayList<PersistedResource>();public void initializeResources(){resources.add(new ApplicationSettings());resources.add(new UserSettings());resources.add(new SpecialSettings());loadAll();}public void loadAll(){for (PersistedResource resource: resources){resource.load();}}public void saveAll(){for (PersistedResource resource: resources){resource.store();

    } //??? NO!! VERY NASTY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

• ISP interface segregation
• DIP dependency inversion

Design Pattern guiding principles

• 对接口进行编程，而不是对实现进行编程
  • 接口出来干了一件好事，就是将类内部进行分类。对于map对象，我们可以做好几个接口，这些接口中定义不同功能的方法，函数和属性，map类实现这些接口，这样我们可以使用接口定义，实现对象。因此，所谓接口就是一系列相关方法与属性集合的定义。
    
  • 第一个好处是客户不需要知道他们使用的对象的特定类型 ，只须对象有客户所期望的接口；第二个好处是客户无须知道他们使用的对象是用什么类来实现的，他们只须知道定义的接口或者抽象类。这将极大的减少子系统实现之间的相互依赖关系。
    

• 类的聚合比类的继承要好
  • 第一，由于对象的实现是基于接口编写的，所以在实现上存在较少的依赖关系，可以在运行时刻动态的替换引用的对象实现；第二，优先使用对象组合有助于保持每个类背封装，并被集中在单个任务上，这样类和类的继承层次会保持在较少的规模，并且不太可能增长为不可控制的庞然大物；第三，使用对象组合可以最大限度的不用创建新的构件，而使用已有的构件就可以组装获得所需要的功能。
    

• 思考设计中的变量以及封装好这些变量

Contract 建立合同

• a subtype method can weaken the precondition
• a subtype method can strengthen the postcondition

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