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[PHP] 抽象类与接口

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Jun 20, 2016 pm 12:29 PM

抽象类是一种特殊的类，接口是一种特殊的抽象类，而多态就要使用到抽象类或是接口

抽象类

什么是抽象方法？

定义：在一个类中，没有方法体的方法就是抽象方法(就是一个方法没有使用{}而直接使用分号结束)

abstract function test();  //抽象方法 function test(){  //有方法体，但方法体为空的 }

如果一个方法是抽象方法，就必须使用abstract修饰为什么要使用抽象方法？

什么是抽象类？

如果一个类中，有一个方法是抽象的则这个类就是抽象类
如果一个类是抽象类，则这个类必须要使用abstract修饰
抽象类是一种特殊的类，就是因为一个类中有抽象方法，其他不变。也可以在抽象类中声明成员属性，常量，非抽象的方法。
抽象类不能实例化对象（不能通过抽象类去创建一个抽象类的对象）

//普通的类class 类名{   }// 抽象类abstract class 类名 {   } abstract class Demo{  var a;  abstract function fun1();   function fun2()  {   }}

抽象类的作用：

要想使用抽象类，就必须使用一个类去继承抽象类，而且要想使用这个子类，也就是让子类可以创建对象，子类就必须不能再是抽象类，子类可以重写父类的方法（给抽象方法加上方法体）。抽象方法中的方法没有方法体，子类必须实现这个方法（父类中没写具体的实现，但子类必须有这个方法名）
列用抽象类，可以定义一些规范，让子类按这些规范去实现自己的功能

接口

接口是一种特殊的抽象类，抽象类又是一种特殊的类
接口和抽象类是一样的作用
抽象类提供了具体实现的标准，而接口则是纯粹的模版。接口只定义功能，而不包含实现的内容。接口用关键字 interface 来声明。
因为PHP是单继承的，如果使用抽象类，子类实现完抽象类就不能再去继承其它的类了。如果既想实现一些规范，又想继承一个其他类。就要使用接口

声明方式

interface 接口名{  const 常量名 = 常量值;  //接口中的属性必须为常量  function 函数名();}echo 接口名::常量名; interface Demo{  const HOST = "localhost";  function fun();}echoDemo::HOST; //输出为localhost

接口和抽象类的对比

作用相同，都不能创建对象，都需要子类去实现
接口的声明和抽象类不一样
接口被实现的方式不一样
接口中的所有方法必须是抽象方法，只能声明抽象方法（而且不需要使用abstract修饰）
接口中的成员属性，只能声明常量，不能声明变量
接口中的成员访问权限都必须是public, 抽象类中最低的权限protected
使用一个类去实现接口，不是使用extends关键字，而是使用implements这个词
- 如果子类是重写父类接口中抽象方法，则使用 implements ：类–接口，抽象类—接口 implements 。接口—接口 extends

使用 implements 的两个目的

1. 可以实现多个接口，而 extends 词只能继承一个父类

2. extends 和 implements 两个可以同时使用

注意：

可以使用抽象类去实现接口中的部分方法
如果想让子类可以创建对象，则必须实现接口中的全部抽象方法
可以定义一个接口去继承另一个接口
一个类可以去实现多个接口（按多个规范去开发子类）, 使用逗号分隔多个接口名称
一个类可以在继承父类的同时，去实现一个或多个接口(先继承，再实现)

多态

多态是面向对象的三大特性之一，是面向对象设计的重要特性。它展现了动态绑定（dynamic binding）的功能，也称为同名异式”（Polymorphism）。多态的功能可让软件在开发和维护时，达到充分的延伸性（extension）。事实上，多态最直接的定义就是让具有继承关系的不同类对象，可以对相同名称的成员函数调用，产生不同的反应效果。

以下是关于使用多态的一个 Demo ：

<html>  <head>  <metacharset="utf-8">  head>  </html><?php  interface USB  //声明一个USB接口  {    function mount();    function work();    function unMount();  }    class UPan implements USB  //UPan实现了USB的功能  {    function mount()    {      echo "U盘挂载<br>";    }    function work()    {      echo "U盘工作<br>";    }    function unMount()    {      echo "U盘卸载成功<br>";    }  }   class Wire implements USB  //数据线实现了USB的功能  {    function mount()    {        echo "数据线挂载<br>";    }    function work()    {        echo "开始传输数据<br>";    }    function unMount()    {        echo "数据线卸载成功<br>";    }  }  class PC  //声明一个PC类  {    function USB($usb)    {        $usb->mount();        $usb->work();        $usb->unMount();    }  }   class Person  //声明一个Person类  {    function usePC()    {      $pc = new PC;      $upan = new UPan;      $wire = new Wire;       $pc->USB($upan);      $pc->USB($wire);    }  }  $p1 = new Person;  //实例化一个Person类  $p1->usePC();

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