prototype based 继承机制

February 12, 2012

prototype based 继承机制

与Java,C#,Ruby中”class-based”继承机制不同,javascript是一种”prototype-based”的继承机制。 关于prototype-based和class-based的区别,我想通过一个例子来描述下:
你是一个陶艺师,我给你一个图纸,说:”照着这个图纸的样子帮我做5个罐子”,于是你照着图纸上描述的形状,颜色,材质做出5个,这就是class-based 的模式

我拿给你一个罐子,说:”照着这个样子帮我做个罐子”,于是你参照这个罐子的形状,颜色,材质做出5个,你就是采用了prototype-based 的模式。

class based OO

我们再来看一段class-based 面向对象语言的代码示例:

class Animal
  private String name
  public Animal(String name)
  {
  this.name = name;
  }
  public void run(){
    ...
  }
}
  class Wolf extends Animal
{
  public void hunt ()
  {
    ...
  }
}

class Wolf继承自class Animal拥有了run技能,并且扩展了自己的hunt技能。

prototype based OO

下面我们来看看一段prototype-based面向对象语言的代码示例:

    function Animal(name)
    {
      this.name = name;
      this.run = function () {
        console.log(this.name + " is runing")
      }
    }

    function Wolf(name)
    {
      this.name = name;
      this.hunt = function () {
        console.log(this.name + " is hunting")
      }
    }
    //继承的魔法在这里显现
    Wolf.prototype = new Animal();
    var someone = new Animal("Some one");
    var wolf = new Wolf("Wolf");
    someone.run();
    wolf.run();
    wolf.hunt();
    //should got exception here
    someone.hunt();

在这里,把function Wolf的prototype设置为一个Animal的实例,当尝试调用wolf上的run方法时,由于Wolf中没有run方法,于是javascript解释器会查找Wolf的prototype(一个Animal的实例)是否有run方法,如果有,则调用此方法,如果没有,则尝试调用此实例的prototype上的run方法,如此持续下去,直到找到这个run方法或者到达继承顶部 - Object.prototype。这就是基于”prototype-chain”的继承机制。

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