PHP面向?qū)ο笕筇攸c(diǎn)學(xué)習(xí)(充分理解抽象、封裝、繼承、多態(tài))
Jun 13, 2016 pm 12:00 PM
面象對(duì)向的三大特點(diǎn)：封裝性、繼承性、多態(tài)性 首先簡(jiǎn)單理解一下抽象：
我們?cè)谇懊娑x一個(gè)類的時(shí)候，實(shí)際上就是把一類事物共有的屬性和行為提取出來(lái)，形成一個(gè)物理模型（模版），這種研究問(wèn)題的方法稱為抽象
一、封裝性
封裝就是把抽取出來(lái)的數(shù)據(jù)和對(duì)數(shù)據(jù)的操作封裝在一起，數(shù)據(jù)被保護(hù)在內(nèi)部，程序的其他部分只有被授權(quán)的操作（方法）才能對(duì)數(shù)據(jù)進(jìn)行操作。
php提供了三種訪問(wèn)控制修飾符
public 表示全局，本類內(nèi)部，類外部，子類都可以訪問(wèn)
protected 表示受保護(hù)的，只有本類或子類可以訪問(wèn)
private 表示私有的，只有本類內(nèi)部可以訪問(wèn)
以上三種修飾符既可以修飾方法也可以修飾屬性（變量），方法如果沒(méi)有訪問(wèn)修飾符則默認(rèn)是public，成員屬性必須指定訪問(wèn)修飾符，在PHP4中也有這種寫法 var $name，表示公開屬性，不推薦這種寫法
例：
復(fù)制代碼 代碼如下:
class Person{
public $name;
protected $age;
private $salary;
function __construct($name,$age,$salary){
$this->name=$name;
$this->age=$age;
$this->salary=$salary;
}
public function showinfo(){
//這表示三個(gè)修飾符都可以在本類內(nèi)部使用
echo $this->name."||".$this->age."||".$this->salary;
}
}
$p1=new Person('張三',20,3000);
//這里屬于類外部，那么如果用下面的方法訪問(wèn)age和salary都會(huì)報(bào)錯(cuò)
// echo $p1->age; echo$p1->salary;
?>
那么現(xiàn)在就想在外部訪問(wèn)protected和private的元素和方法該怎么辦？ 通常做法是通過(guò)public函數(shù)去訪問(wèn)這些變量 格式：
public function setxxxx($val){
$this->xxxx=$val;
}
public function getxxxx(){
return $this->xxxx;
}
這里帶set和get只是為了識(shí)別方便，并非必要
如：
public function getsalary(){
return $this->salary; //擴(kuò)展：這里可以調(diào)用一些方法，如判斷用戶名等，正確才給訪問(wèn)
}
在外部就可以通過(guò) echo $p1->getsalary();
如果要訪問(wèn) protected和private也可以使用以下方法，但不推薦使用，只要了解即可
__set() 和 __get()
__set()對(duì)protected或private屬性進(jìn)行賦值操作
__set($name,$val);
__get()獲取 protected 或 private的值
__get($name);
如：
復(fù)制代碼 代碼如下:
class testa{
protected $name;
//使用__set()來(lái)管理所有屬性
public function __set($pro_name,$pro_val){
//上面$pro_name和$pro_val可自定義
//下面$this->pro_name為既定，不可更改
$this->pro_name=$pro_val;
}
//使用__get()來(lái)獲取所有屬性值
public function __get($pro_name){
if(isset($pro_name)){
return $this->pro_name;
} else {
return null;
}
}
}
$n1=new testa();
//正常情況，類外部是不能訪問(wèn)protected屬性的，但是用了上面的方法就可以對(duì)它們進(jìn)行操作
$n1->name='小三';
echo $n1->name;
?>
//以上代碼看懂就行，不推薦使用
二、繼承性
先看一個(gè)例子:
復(fù)制代碼 代碼如下:
class Pupil{
public $name;
protected $age;
public function getinfo(){
echo $this->name.'||'.$this->age;
}
public function testing(){
echo 'this is pupil';
}
}
class Graduate{
public $name;
protected $age;
public function getinfo(){
echo $this->name.'||'.$this->age;
}
public function testing(){
echo 'this is Graduate';
}
}
?>
從上面的例子可以看出，當(dāng)多個(gè)類有很多共同屬性和方法時(shí)，代碼的復(fù)用性不高，代碼冗余，思考css中的處理方法
解決方法 ：繼承
復(fù)制代碼 代碼如下:
class Students{
public $name;
public $age;
public function __construct($name,$age){
$this->name=$name;
$this->age=$age;
}
public function showinfo(){
echo $this->name.'||'.$this->age;
}
}
class Pupil extends Students{
function testing(){
echo 'Pupil '.$this->name.' is testing';
}
}
class Graduate extends Students{
function testing(){
echo 'Graduate '.$this->name.' is testing';
}
}
$stu1=new Pupil('張三',20);
$stu1->showinfo();
echo '
';
$stu1->testing();
?>
從上面可以看出，繼承就是一個(gè)子類(Subclass)通過(guò) extends 父類 把父類(BaseClass)中的public 和 protected 的屬性和方法繼續(xù)下來(lái)，不能繼承private屬性和方法
語(yǔ)法結(jié)構(gòu)：
class 父類名{}
class 子類名 extends 父類名{}
細(xì)節(jié)：
1、一個(gè)子類只能繼承一個(gè)父類（這里指直接繼承）；如果希望繼承多個(gè)類的屬性和方法，可以使用多層繼承
例：
復(fù)制代碼 代碼如下:
class A{
public $name='AAA';
}
class B extends A{
public $age=30;
}
class C extends B{}
$p=new C();
echo $p->name;//這里會(huì)輸出AAA
?>
2、在創(chuàng)建某個(gè)子類對(duì)象時(shí)，默認(rèn)情況下不會(huì)自動(dòng)調(diào)用其父類的構(gòu)造函數(shù)
例：
class A{
public function __construct(){
echo 'A';
}
}
class B extends A{
public function __construct(){
echo 'B';
}
}
$b=new B();//這里會(huì)優(yōu)先輸出B中的構(gòu)造方法，如果B中沒(méi)有構(gòu)造方法才會(huì)輸出A中的
3、在子類中如果需要訪問(wèn)父類的方法（構(gòu)造方法、成員方法 方法的修飾符為protected或private），那么可以使用 父類::方法名 或者 parent::方法名 來(lái)完成【這里parent和以前提到的self都均為小寫，大寫報(bào)錯(cuò)】
class A{
public function test(){
echo 'a_test';
}
}
class B extends A{
public function __construct(){
//兩種方法都行
A::test();
parent::test();
}
}
$b=new B();
5、如果一個(gè)子類(派生類)的方法與父類的方法完全一樣時(shí)(public，protected)，我們稱為方法覆蓋或方法重寫（override），看下面的多態(tài)性
三、多態(tài)性
例 ：
復(fù)制代碼 代碼如下:
class Animal{
public $name;
public $price;
function cry(){
echo 'i don\'t know';
}
}
class Dog extends Animal{
//覆蓋、重寫
function cry(){
echo 'Wang Wang!';
Animal::cry();//這里不會(huì)報(bào)錯(cuò)，能正確執(zhí)行父類的cry();
}
}
$dog1=new Dog();
$dog1->cry();
?>
小結(jié)：
1、當(dāng)一個(gè)父類知道所有的子類都有一個(gè)方法，但是父類不能確定該方法如何寫，可以讓子類去覆蓋它的方法,方法覆蓋(重寫)，必須要求子類的方法名和參數(shù)個(gè)數(shù)完全一致
2、如果子類要去調(diào)用父類的某個(gè)方法(protected/public)，可以使用 父類名::方法名 或者 parent::方法名
3、在實(shí)現(xiàn)方法重寫的時(shí)候，訪問(wèn)修飾符可以不一樣，但是子類方法的訪問(wèn)權(quán)限必須大于等于父類方法的訪問(wèn)權(quán)限（即不能縮小父類方法的訪問(wèn)權(quán)限）
如 父類public function cry(){} 子類 protected function cry(){} 則會(huì)報(bào)錯(cuò)
但是子類的訪問(wèn)權(quán)限可以放大，如：
父類private function cry(){} 子類 protected function cry(){} 可以正確執(zhí)行
擴(kuò)展：
方法重載（overload）
基本概念：函數(shù)名相同，但參數(shù)的個(gè)數(shù)或參數(shù)的類型不同，達(dá)到調(diào)用同一個(gè)函數(shù)，可以區(qū)分不同的函數(shù)
在PHP5中雖然也支持重載，但是和其它語(yǔ)言還是有很大區(qū)別的，php中不能定義多個(gè)同名函數(shù)
PHP5中提供了強(qiáng)大的“魔術(shù)”函數(shù)，使用這些魔術(shù)函數(shù)，我們可以做到函數(shù)重載，
這里我們要到到 __call,當(dāng)一個(gè)對(duì)象調(diào)一個(gè)方法時(shí)，而該方法不存在，則程序會(huì)自動(dòng)調(diào)用__call
【官方不推薦使用】
PHP中有以下幾個(gè)魔術(shù)常量：__LINE__ __FILE__ __DIR__ __FUNCTION__ __CLASS__ 等
例：
復(fù)制代碼 代碼如下:
class A{
function test1($p){
echo 'test1
';
}
function test2($p){
echo 'test2
';
}
function __call($method,$p){
//這里$p為數(shù)組，上面兩個(gè)變量名可自定義
if($method == 'test'){
if(count($p)==1){
$this->test1($p);
} else if(count($p)==2){
$this->test2($p);
}
}
}
}
$a=new A();
$a->test(5);
$a->test(3,5);
?>
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