常识# 统一访问原则 （uniform acces principle）

The Uniform Access Principle was put forth by Bertrand Meyer. It states "All services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation." This principle applies generally to object-oriented programming languages. In simpler form, it states that there should be no difference between working with an attribute, precomputed property, or method/query.

上文是wiki里对统一访问原则的描述，用《scala编程》一书中的说法就是“客户代码不应由属性是通过字段实现还是方法实现而受到影响”。这句话到底什么意思呢？代码说话

elem.width

上面这种表达方式有两种实现形式，当改成任意的另外一种时，都不会改变当前代码的用法。（当然对某些语言是行不通的）

class Elem{
  val width = 1
}

class Elem{
  def width:Int = 1
}

 

 

另附wiki上用python语言写的例子

egg = Egg( 4, "White")
egg.color = "Green"
print egg.weight, egg.color, egg.quack()  # prints: 4 Green quack

字段实现

class Egg(object):
   def __init__(self, weight, color):
      self.weight = weight
      self.color = color
   def quack(self):
      return "quack"

方法实现

class Egg(object):
    def __init__(self, weight, color):
      self.__weight = toGrams(weight)
      self.__color = toRGB(color)
 
    def setColor(self, colorname):
      self.__color = toRGB(colorname)
 
    def getColor(self):
      return toColorName(self.__color)
 
    color = property(getColor, setColor, doc="Color of the Egg")
 
    def setWeight(self, weightOz);
       self.__weight = 29.3*weightOz
 
    def getWeight(self):
       return self.__weight/29.3;
 
    weight = property(setWeight, getWeight, doc="Weight in Ounces")
 
    def quack(self):
       return "quack"