敏捷开发scrum的理解 敏捷开发技巧-消除代码异味

敏捷开发技巧-消除代码异味  

　　摘要:本文通过简单通俗的例子  告诉我们如何判断代码的稳定性和代码中的异类  并且如何重构此类代码 异味这个词 可能有点抽象 我们先看一下下面的例子这是一个CAD系统  现在 它已经可以画三种形状了:线条 长方形 跟圆 先认真的看一下下面的代码:class Shape {                                                                                          final static int TYPELINE =  ;                                                                      final static int TYPERECTANGLE =  ;                                                                 final static int TYPECIRCLE =  ;                                                                    int shapeType;                                                                                      //线条的开始点     //长方形左下角的点     //圆心     Point p ;                                                                                           //线条的结束点     //长方形的右上角的点     //如果是圆的话 这个属性不用     Point p ;                                                                                           int radius;                                                                                      }                                                                                               class CADApp {                                                                                         void drawShapes(Graphics graphics  Shape shapes[]) {                                                    for (int i =  ; i < shapes length; i++) {                                                               switch (shapes[i] getType()) {                                                                         case Shape TYPELINE:                                                                                    graphics drawLine(shapes[i] getP ()  shapes[i] getP ());                                            break;                                                                                          case Shape TYPERECTANGLE:                                                                               //画四条边               graphics drawLine( );                                                                             graphics drawLine( );                                                                             graphics drawLine( );                                                                             graphics drawLine( );                                                                             break;                                                                                          case Shape TYPECIRCLE:                                                                                  graphics drawCircle(shapes[i] getP ()  shapes[i] getRadius());                                      break;                                                                                       }                                                                                               }                                                                                               }                                                                                                }                                                                                                   代码都是一直在改变的 而这也是上面的代码会碰到的一个问题     现在我们有一个问题: 如果我们需要支持更多的形状(比如三角形)  那么肯定要改动Shape这个类  CADApp里面的drawShapes这个方法也要改 好 改为如下的样子:    class Shape {             final static int TYPELINE =  ;       final static int TYPERECTANGLE =  ;       final static int TYPECIRCLE =  ;       final static int TYPETRIANGLE =  ;       int shapeType;         Point p ;              Point p ;              //三角形的第三个点        Point p ;              int radius;         }                  class CADApp {          void drawShapes(Graphics graphics  Shape shapes[]) {         for (int i =  ; i < shapes length; i++) {             switch (shapes[i] getType()) {                case Shape TYPELINE:                    graphics drawLine(shapes[i] getP ()  shapes[i] getP ());                    break;                case Shape TYPERECTANGLE:                      //画四条边                     graphics drawLine( );                    graphics drawLine( );                    graphics drawLine( );                    graphics drawLine( );                    break;                case Shape TYPECIRCLE:                    graphics drawCircle(shapes[i] getP ()  shapes[i] getRadius());                    break;                case Shape TYPETRIANGLE:                    graphics drawLine(shapes[i] getP ()  shapes[i] getP ());                    graphics drawLine(shapes[i] getP ()  shapes[i] getP ());                    graphics drawLine(shapes[i] getP ()  shapes[i] getP ());                    break;               }                  }                  }                 }                      如果以后要支持更多的形状 这些类又要改动…… 这可不是什么好事情！   理想情况下 我们希望当一个类 一个方法或其他的代码设计完以后 就不用再做修改了 它们应该稳定到不用修改就可以重用      现在的情况恰好相反！    每当我们增加新的形状 都得修改Shape这个类 跟CADApp里面的drawShapes方法     怎么让代码稳定（也就是无需修改）？这个问题是个好问题！不过老规矩 先不说 我们以行动回答     我们先看看另外一个方法  当给你一段代码 你怎么知道它是稳定的？

　　怎么判断代码的稳定性？    要判断代码的稳定性 我们可能会这样来判定 先假设一些具体的情况或者需求变动了 然后来看一看 要满足这些新的需求 代码是否需要被修改？     可惜 这也是一件很麻烦的事 因为有那么多的可能性！我们怎么知道哪个可能性要考虑 哪些不用考虑？    有个更简单的方法  如果发现说 我们已经第三次修改这些代码了 那我们就认定这些代码是不稳定的 这个方法很 懒惰 而且 被动 ！我们被伤到了 才开始处理状况 不过至少这种方法还是一个很有效的方法     此外 还有一个简单 而且 主动 的方法 如果这段代码是不稳定或者有一些潜在问题的 那么代码往往会包含一些明显的痕迹 正如食物要腐坏之前 经常会发出一些异味一样（当然 食物如果有异味了 再怎么处理我们都不想吃了 但是代码可不行 ） 我们管这些痕迹叫做 代码异味 正如并不是所有的食物有异味都不能吃了 但大多数情况下 确实是不能吃了 并不是所有的代码异味都是坏事 但大多数情况下 它们确实是坏事情！因此 当我们感觉出有代码异味时 我们必须小心谨慎的检查了     现在 我们来看看上面例子中的代码异味吧     示例代码中的代码异味     第一种异味 代码用了类别代码（type code）   class Shape {                                                                                        final int TYPELINE =  ;                                                                           final int TYPERECTANGLE =  ;                                                                      final int TYPECIRCLE =  ;                                                                         int shapeType;                                                                                                                                                                                 }                                                                                                   这样的异味 是一种严肃的警告 我们的代码可能有许多问题           第二种异味 Shape这个类有很多属性有时候是不用的 例如 radius这个属性只有在这个Shape是个圆的时候才用到     class Shape {                                                                                                                                                                                            Point p ;                                                                                           Point p ;                                                                                           int radius; //有时候不用                                                                 }                                                                                                   第三种异味 我们想给p p 取个好一点的变量名都做不到 因为不同的情况下 它们有不同的含义     class Shape {                          Point p ; //要取作 起始点 左下点 还是 圆心 ？     Point p ;       }                  第四种异味 drawShapes这个方法里面 有个switch表达式 当我们用到switch（或者一大串的if then else if）时 小心了 switch表达式经常是跟类别代码（type code）同时出现的     现在 让我们将这个示例中的代码异味消除吧     消除代码异味 怎么去掉类别代码（type code）    大多数情况下 要想去掉一个类别代码 我们会为每一种类别建立一个子类 比如     （当然 并不是每次要去掉一个类别代码都要增加一个新类 我们下面的另一个例子里面会讲另一种解决方法）    class Shape {      }                  class Line extends Shape {       Point startPoint;        Point endPoint;     }                  class Rectangle extends Shape {       Point lowerLeftCorner;        Point upperRightCorner;     }                  class Circle extends Shape {       Point center;        int radius;     }                  因为现在没有类别代码了 drawShapes这个方法里面 就要用instanceof来判断对象是哪一种形状了 因此 我们不能用switch了 而要改用if then else      class CADApp {        void drawShapes(Graphics graphics  Shape shapes[]) {           for (int i =  ; i < shapes length; i++) {               if (shapes[i] instanceof Line) {                  Line line = (Line)shapes[i];                  graphics drawLine(line getStartPoint() line getEndPoint());               } else if (shapes[i] instanceof Rectangle) {                  Rectangle rect = (Rectangle)shapes[i];                  graphics drawLine( );                  graphics drawLine( );                  graphics drawLine( );                  graphics drawLine( );               } else if (shapes[i] instanceof Circle) {                  Circle circle = (Circle)shapes[i];                  graphics drawCircle(circle getCenter()  circle getRadius());                                     }                                                                                               }                                                                                               }                                                                                                }                                                                                                   因为没有类别代码了 现在每个类（Shape Line Rectangle Circle）里面的所有属性就不会有时用得到有时用不到了 现在我们也可以给它们取一些好听点的名字了（比如在Line里面 p 这个属性可以改名为startPoint了） 现在四种异味只剩一种了 那就是 在drawShapes里面还是有一大串if then else if 我们下一步 就是要去掉这长长的一串         消除代码异味 如何去掉一大串if then else if（或者switch）            经常地 为了去掉if then else if或者switch 我们需要先保证在每个条件分支下的要写的代码是一样的 在drawShapes这个方法里面 我们先以一个较抽象的方法（伪码）来写吧         class CADApp {                                                                                         void drawShapes(Graphics graphics  Shape shapes[]) {                                                    for (int i =  ; i < shapes length; i++) {                                                               if (shapes[i] instanceof Line) {                                                                       画线条;                                                                                   } else if (shapes[i] instanceof Rectangle) {                                                           画长方形;                                                                              } else if (shapes[i] instanceof Circle) {                                                              画圆;                                                                                 }                                                                                               }                                                                                               }                                                                                                }                                                                                                   条件下的代码还是不怎么一样 不如再抽象一点     class CADApp {                                                                                         void drawShapes(Graphics graphics  Shape shapes[]) {                                                    for (int i =  ; i < shapes length; i++) {                                                               if (shapes[i] instanceof Line) {                                                                       画出形状;                                                                                  } else if (shapes[i] instanceof Rectangle) {                                                           画出形状;                                                                                  } else if (shapes[i] instanceof Circle) {                                                              画出形状;                                                                                  }                                                                                               }                                                                                               }                                                                                                }                                                                                                   好 现在三个分支下的代码都一样了 我们也就不需要条件分支了         class CADApp {                                                                                         void drawShapes(Graphics graphics  Shape shapes[]) {           for (int i =  ; i < shapes length; i++) {               画出形状;           }           }            }               最后 将 画出形状 这个伪码写成代码吧:    class CADApp {       void drawShapes(Graphics graphics  Shape shapes[]) {           for (int i =  ; i < shapes length; i++) {               shapes[i] draw(graphics);           }           }            }                

　　当然 我们需要在每种Shape的类里面提供draw这个方法     abstract class Shape {       abstract void draw(Graphics graphics);    }               class Line extends Shape {       Point startPoint;        Point endPoint;        void draw(Graphics graphics) {           graphics drawLine(getStartPoint()  getEndPoint());       }            }               class Rectangle extends Shape {       Point lowerLeftCorner;        Point upperRightCorner;        void draw(Graphics graphics) {           graphics drawLine( );           graphics drawLine( );           graphics drawLine( );           graphics drawLine( );       }            }               class Circle extends Shape {       Point center;        int radius;       void draw(Graphics graphics) {           graphics drawCircle(getCenter()  getRadius());       }            }               将抽象类变成接口    现在 看一下Shape这个类 它本身没有实际的方法 所以 它更应该是一个接口     interface Shape {                                                                                      void draw(Graphics graphics);                                                                    }                                                                                                   class Line implements Shape {                                                                                                                                                                         }                                                                                                   class Rectangle implements Shape {                                                                                                                                                                    }                                                                                                   class Circle implements Shape {                                                                                                                                                                       }                                                                                                   改进后的代码    改进后的代码就像下面这样         interface Shape {                                                                                      void draw(Graphics graphics);                                                                    }                                                                                                   class Line implements Shape {                                                                          Point startPoint;                                                                                   Point endPoint;                                                                                     void draw(Graphics graphics) {                                                                          graphics drawLine(getStartPoint()  getEndPoint());                                              }                                                                                                }                                                                                                   class Rectangle implements Shape {                                                                     Point lowerLeftCorner;                                                                              Point upperRightCorner;                                                                             void draw(Graphics graphics) {                                                                          graphics drawLine( );                                                                             graphics drawLine( );                                                                             graphics drawLine( );                                                                             graphics drawLine( );                                                                         }                                                                                                }                                                                                                   class Circle implements Shape {                                                                        Point center;                                                                                       int radius;                                                                                         void draw(Graphics graphics) {                                                                          graphics drawCircle(getCenter()  getRadius());                                                  }                                                                                                }                                                                                                   class CADApp {                                                                                         void drawShapes(Graphics graphics  Shape shapes[]) {                                                    for (int i =  ; i < shapes length; i++) {                                                               shapes[i] draw(graphics);                                                                       }                                                                                               }                                                                                                }                                                                                                   如果我们想要支持更多的图形（比如 三角形） 上面没有一个类需要修改 我们只需要创建一个新的类Triangle就行了    另一个例子    让我们来看一下另外一个例子 在当前的系统中 有三种用户 常规用户 管理员和游客     常规用户必须每隔 天修改一次密码（更频繁也行） 管理员必须每 天修改一次密码 游客就不需要修改了     常规用户跟管理员可以打印报表     先看一下当前的代码     class UserAccount {       final static int USERTYPE_NORMAL =  ;       final static int USERTYPE_ADMIN =  ;       final static int USERTYPE_GUEST =  ;       int userType;       String id;         String name;       String password;       Date dateOfLastPasswdChange;       public boolean checkPassword(String password) {                        }               }                  class InventoryApp {       void login(UserAccount userLoggingIn  String password) {           if (userLoggingIn checkPassword(password)) {               GregorianCalendar today = new GregorianCalendar();               GregorianCalendar expiryDate = getAccountExpiryDate(userLoggingIn);               if (today after(expiryDate)) {                  //提示用户修改密码                                  }              }              }                  GregorianCalendar getAccountExpiryDate(UserAccount account) {           int passwordMaxAgeInDays = getPasswordMaxAgeInDays(account);           GregorianCalendar expiryDate = new GregorianCalendar();           expiryDate setTime(account dateOfLastPasswdChange);           expiryDate add(Calendar DAY_OF_MONTH  passwordMaxAgeInDays);           return expiryDate;       }                  int getPasswordMaxAgeInDays(UserAccount account) {           switch (account getType()) {               case UserAccount USERTYPE_NORMAL:                  return  ;               case UserAccount USERTYPE_ADMIN:                  return  ;               case UserAccount USERTYPE_GUEST:                  return Integer MAX_VALUE;           }              }                  void printReport(UserAccount currentUser) {           boolean canPrint;           switch (currentUser getType()) {               case UserAccount USERTYPE_NORMAL:                  canPrint = true;                  break;               case UserAccount USERTYPE_ADMIN:                  canPrint = true;                  break;                                                                                           case UserAccount USERTYPE_GUEST:                                                                       canPrint = false;                                                                            }                                                                                                   if (!canPrint) {                                                                                        throw new SecurityException( You have no right );                                               }                                                                                                   //打印报表       }                                                                                                }