[Coding][代码整洁之道]对象与数据结构

过程式代码（使用数据结构的代码）便于在不该懂既有数据结构的前提下添加新函数，面向对象代码便于在不改动既有函数的前提下添加新类。

过程式代码难以添加新的数据结构，因为必须修改所有的函数；面向对象代码难以添加新的函数，因为必须修改所有类。

过程式代码示例：

public class Square{
    public Point topLeft;
    public double side;
}
public class Rectangle{
    public Point topLeft;
    public double height;
    public double width;
}
public class Circle{
    public Point topLeft;
    public double radius;
}

public class Geometry{
    public double PI = 3.1415926;
    public double area(Object shape)throws NoSuchShapeException
    {   if(shape instanceof Square){
            Square s = (Square)shape;
            return s.side*s.side;
        }
        else if(shape instanceof Rectangle){
            Rectangle r = (Rectangle)shape;
            return r.width*r.height;
        }
        else if(shape instanceof Circle){
            Circle c = (Circle)shape;
            return PI*c.radius*c.radius;
        }
        throw new NoSuchShapeException();
    }
}

如果此时想在Geometry类中添加一个primeter()函数，这实现起来很简单，现有形状不会因此受到影响：

public class Geometry{
    public double PI = 3.1415926;
    public double area(Object shape)throws NoSuchShapeException
    {   if(shape instanceof Square){
            Square s = (Square)shape;
            return s.side*s.side;
        }
        else if(shape instanceof Rectangle){
            Rectangle r = (Rectangle)shape;
            return r.width*r.height;
        }
        else if(shape instanceof Circle){
            Circle c = (Circle)shape;
            return PI*c.radius*c.radius;
        }
        throw new NoSuchShapeException();
    }

    public double primeter(Object shape)throws NoSuchShapeException
    {   if(shape instanceof Square){
            Square s = (Square)shape;
            return 4*s.side;
        }
        else if(shape instanceof Rectangle){
            Rectangle r = (Rectangle)shape;
            return r.width*2+r.height*2;
        }
        else if(shape instanceof Circle){
            Circle c = (Circle)shape;
            return 2*PI*c.radius;
        }
        throw new NoSuchShapeException();
    }
}

如果此时想添加一个新的形状，那Geometry类中的area()和primeter()都要修改，如果Geometry类中有其他方法的话，那要修改的方法可能会更多。

过程式代码难以添加新的数据结构，因为必须修改所有函数。

面向对象代码示例如下：

public class Square implements Shape{
    private Point topLeft;
    private double side;

    public double area(){
        return side*side;
    }
}

public class Rectangle implements Shape{
    private Point topLeft;
    private double height;
    private double width;

    public double area(){
        return height*width;
    }
}

public class Circle implements Shape{
    private Point topLeft;
    private double radius;
    public double PI = 3.1415926;

    public double area(){
        return PI*radius*radius;
    }
}

public class Geometry {
   	public double PI = 3.1415926;
    public double area(Shape shape)throws NoSuchShapeException {
  		shape.area();
	}
}

此时如果你想添加一个新形状，实现起来很简单，现有形状不会因此受到影响：

public class Square implements Shape{
    private Point topLeft;
    private double side;

    public double area(){
        return side*side;
    }
}

public class Rectangle implements Shape{
    private Point topLeft;
    private double height;
    private double width;

    public double area(){
        return height*width;
    }
}

public class Circle implements Shape{
    private Point topLeft;
    private double radius;
    public double PI = 3.1415926;

    public double area(){
        return PI*radius*radius;
    }
}

public class NewShape implements Shape{
    private Point topLeft;
    ......

    public double area(){
        ......
    }
}

但是如果此时你想添加一个新函数primeter()函数，那就要修改Shape接口，继承了Shape接口的现有形状都需要修改。

面向对象代码难以添加新函数，因为必须修改所有类。