敏捷开发的必要技巧第3章----消除代码异味

完整：
http://www.blogjava.net/Files/Wingel/第3章%20除去代码异味.rar

http://wingel.iteye.com/topics/download/2f7b5864-fca2-42e5-ba3e-453725fcb885
                第3章 除去代码异味　


    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 = 0;                                                              
       final static int TYPERECTANGLE = 1;                                                         
       final static int TYPECIRCLE = 2;                                                            
       int shapeType;                                                                              
       //线条的开始点
       //长方形左下角的点
       //圆心
       Point p1;                                                                                   
       //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 p2;                                                                                   
       int radius;                                                                                 
    }                                                                                              
    class CADApp {                                                                                 
       void drawShapes(Graphics graphics, Shape shapes[]) {                                        
           for (int i = 0; i < shapes.length; i++) {                                               
               switch (shapes[i].getType()) {                                                      
                  case Shape.TYPELINE:                                                             
                      graphics.drawLine(shapes[i].getP1(), shapes[i].getP2());                     
                      break;                                                                       
                  case Shape.TYPERECTANGLE:                                                        
                      //画四条边
                      graphics.drawLine(...);                                                      
                      graphics.drawLine(...);                                                      
                      graphics.drawLine(...);                                                      
                      graphics.drawLine(...);                                                      
                      break;                                                                       
                  case Shape.TYPECIRCLE:                                                           
                      graphics.drawCircle(shapes[i].getP1(), 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 = 0;
       final static int TYPERECTANGLE = 1;
       final static int TYPECIRCLE = 2;
       final static int TYPETRIANGLE = 3;
       int shapeType; 
       Point p1;      
       Point p2;      
       //三角形的第三个点.
       Point p3;      
       int radius;    
    }                 
    class CADApp {    
       void drawShapes(Graphics graphics, Shape shapes[]) {
           for (int i = 0; i < shapes.length; i++) {
               switch (shapes[i].getType()) {
                  case Shape.TYPELINE:
                      graphics.drawLine(shapes[i].getP1(), shapes[i].getP2());
                      break;
                  case Shape.TYPERECTANGLE:
                      //画四条边.
                      graphics.drawLine(...);
                      graphics.drawLine(...);
                      graphics.drawLine(...);
                      graphics.drawLine(...);
                      break;
                  case Shape.TYPECIRCLE:
                      graphics.drawCircle(shapes[i].getP1(), shapes[i].getRadius());
                      break;
                  case Shape.TYPETRIANGLE:
                      graphics.drawLine(shapes[i].getP1(), shapes[i].getP2());
                      graphics.drawLine(shapes[i].getP2(), shapes[i].getP3());
                      graphics.drawLine(shapes[i].getP3(), shapes[i].getP1());
                      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 = 0;                                                                     
       final int TYPERECTANGLE = 1;                                                                
       final int TYPECIRCLE = 2;                                                                   
       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 p1;                                                                                   
       Point p2;                                                                                   
       int radius; //有时候不用                                                            
    }                                                                                              

    第三种异味：我们想给p1,p2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 p1; //要取作“起始点”，“左下点”，还是“圆心”？
       Point p2;  
    }             

    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 = 0; 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，p1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 = 0; 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 = 0; 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 = 0; 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 = 0; 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;