[PHP] 抽象类与接口
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？
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如果一个类中，有一个方法是抽象的则这个类就是抽象类
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如果一个类是抽象类，则这个类必须要使用abstract修饰
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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：
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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）
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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
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接口是一种特殊的抽象类， 抽象类又是一种特殊的类
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接口和抽象类是一样的作用
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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。
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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
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作用相同，都不能创建对象， 都需要子类去实现
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接口的声明和抽象类不一样
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接口被实现的方式不一样
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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）
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接口中的成员属性，只能声明常量，不能声明变量
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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
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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
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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
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可以定义一个接口去继承另一个接口
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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
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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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