Detailed examples of the three major characteristics of encapsulation, inheritance and polymorphism in PHP object-oriented

PHP object-orientedThree major characteristics of learning Learning goals: Fully understand abstraction, encapsulation, inheritance, polymorphism

The three major characteristics of object-oriented: encapsulation, inheritance, Polymorphism first briefly understands abstraction:
When we define a class earlier, we actually extract the common attributes and behaviors of a class of things to form a physical model (template). This method of studying problems It is called abstraction

1. Encapsulation
Encapsulation is to encapsulate the extracted data and the operations on the data together. The data is protected internally, and other parts of the program Only some authorized operations (methods) can operate on the data.
php provides three access control modifiers
public means global, accessible inside this class, outside the class, and subclasses
protected means protected, only this class or Subclasses can access
private means private, and can only be accessed within this class
The above three modifiers can modify both methods and properties (variables). If the method does not have access modifiers, it defaults to public, and members Attributes must specify access modifiers. In PHP4, there is also this way of writing var $name, which means public attributes. This way of writing is not recommended.
Example:

<?php 
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(){ 
//这表示三个修饰符都可以在本类内部使用 
echo $this->name."||".$this->age."||".$this->salary; 
} 
} 
$p1=new Person(&#39;张三&#39;,20,3000); 
//这里属于类外部，那么如果用下面的方法访问age和salary都会报错 
// echo $p1->age; echo$p1->salary; 
?>

So now I want to access protected and private externally What to do with elements and methods? The usual approach is to access these variable formats through public functions:

public function setxxxx($val){ 
$this->xxxx=$val; 
} 
public function getxxxx(){ 
return $this->xxxx; 
}

The set and get here are just for the convenience of identification, not necessary
For example:

public function getsalary(){ 
return $this->salary; //扩展：这里可以调用一些方法，如判断用户名等，正确才给访问 
}

In Externally, you can use echo $p1->getsalary();
If you want to access protected and private, you can also use the following methods, but they are not recommended, as long as you understand them
set() and get()
set() assigns values ​​to protected or private attributes
set($name,$val);
get() gets the value of protected or private
get($name);
For example:

<?php 
class testa{ 
protected $name; 
//使用set()来管理所有属性 
public function set($pro_name,$pro_val){ 
//上面$pro_name和$pro_val可自定义 
//下面$this->pro_name为既定，不可更改 
$this->pro_name=$pro_val; 
} 
//使用get()来获取所有属性值 
public function get($pro_name){ 
if(isset($pro_name)){ 
return $this->pro_name; 
} else { 
return null; 
} 
} 
} 
$n1=new testa(); 
//正常情况，类外部是不能访问protected属性的，但是用了上面的方法就可以对它们进行操作 
$n1->name=&#39;小三&#39;; 
echo $n1->name; 
?>

//Just understand the above code, it is not recommended to use
2. Inheritance
Let’s look at an example first:

<?php 
class Pupil{ 
public $name; 
protected $age; 
public function getinfo(){ 
echo $this->name.&#39;||&#39;.$this->age; 
} 
public function testing(){ 
echo &#39;this is pupil&#39;; 
} 
} 
class Graduate{ 
public $name; 
protected $age; 
public function getinfo(){ 
echo $this->name.&#39;||&#39;.$this->age; 
} 
public function testing(){ 
echo &#39;this is Graduate&#39;; 
} 
} 
?>

From the above example It can be seen that when multiple classes have many common attributes and methods, the code reusability is not high and the code is redundant. Think about the processing method in CSS
Solution: inheritance

<?php 
class Students{ 
public $name; 
public $age; 
public function construct($name,$age){ 
$this->name=$name; 
$this->age=$age; 
} 
public function showinfo(){ 
echo $this->name.&#39;||&#39;.$this->age; 
} 
} 
class Pupil extends Students{ 
function testing(){ 
echo &#39;Pupil &#39;.$this->name.&#39; is testing&#39;; 
} 
} 
class Graduate extends Students{ 
function testing(){ 
echo &#39;Graduate &#39;.$this->name.&#39; is testing&#39;; 
} 
} 
$stu1=new Pupil(&#39;张三&#39;,20); 
$stu1->showinfo(); 
echo &#39;<br/>&#39;; 
$stu1->testing(); 
?>

As can be seen from the above, inheritance is a subclass (Subclass) inheriting the public and protected properties and methods of the parent class (BaseClass) through the extends parent class. Private properties and methods cannot be inherited.
Syntax Structure:
class parent class name{}
class subclass name extends parent class name{}
Details:
1. A subclass can only inherit one parent class (here refers to direct inheritance); If you want to inherit the properties and methods of multiple classes, you can use multi-layer inheritance
Example:

<?php 
class A{ 
public $name=&#39;AAA&#39;; 
} 
class B extends A{ 
public $age=30; 
} 
class C extends B{} 
$p=new C(); 
echo $p->name;//这里会输出AAA 
?>

2. When creating a subclass object, its parent class’s# will not be automatically called by default. ##Constructor Example:

class A{ 
public function construct(){ 
echo 'A'; 
} 
} 
class B extends A{ 
public function construct(){ 
echo 'B'; 
} 
} 
$b=new B();//这里会优先输出B中的构造方法，如果B中没有构造方法才会输出A中的

3. If you need to access the methods of the parent class in the subclass (the modifiers of the constructor and member methods are protected or private) , then you can use parent class:: method name or parent:: method name to complete [Here parent and self mentioned before are both lowercase, and an error will be reported in uppercase]

class A{ 
public function test(){ 
echo 'a_test'; 
} 
} 
class B extends A{ 
public function construct(){ 
//两种方法都行 
A::test(); 
parent::test(); 
} 
} 
$b=new B();

5. If a child When the method of a class (derived class) is exactly the same as the method of the parent class (public, protected), we call it method overriding or method overriding (override). See the following polymorphism

3. Polymorphism Sex Example:

<?php 
class Animal{ 
public $name; 
public $price; 
function cry(){ 
echo &#39;i don\&#39;t know&#39;; 
} 
} 
class Dog extends Animal{ 
//覆盖、重写 
function cry(){ 
echo &#39;Wang Wang!&#39;; 
Animal::cry();//这里不会报错，能正确执行父类的cry(); 
} 
} 
$dog1=new Dog(); 
$dog1->cry(); 
?>

小结：
1、当一个父类知道所有的子类都有一个方法，但是父类不能确定该方法如何写，可以让子类去覆盖它的方法,方法覆盖(重写)，必须要求子类的方法名和参数个数完全一致
2、如果子类要去调用父类的某个方法(protected/public)，可以使用 父类名::方法名 或者 parent::方法名
3、在实现方法重写的时候，访问修饰符可以不一样，但是子类方法的访问权限必须大于等于父类方法的访问权限（即不能缩小父类方法的访问权限）
如 父类public function cry(){} 子类 protected function cry(){} 则会报错
但是子类的访问权限可以放大，如：
父类private function cry(){} 子类 protected function cry(){} 可以正确执行
扩展：
方法重载（overload）
基本概念：函数名相同，但参数的个数或参数的类型不同，达到调用同一个函数，可以区分不同的函数
在PHP5中虽然也支持重载，但是和其它语言还是有很大区别的，php中不能定义多个同名函数
PHP5中提供了强大的“魔术”函数，使用这些魔术函数，我们可以做到函数重载，
这里我们要到到 call,当一个对象调一个方法时，而该方法不存在，则程序会自动调用call
【官方不推荐使用】
PHP中有以下几个魔术常量：LINE FILE DIR FUNCTION CLASS 等
例： 

<?php 
class A{ 
function test1($p){ 
echo &#39;test1<br/>&#39;; 
} 
function test2($p){ 
echo &#39;test2<br/>&#39;; 
} 
function call($method,$p){ 
//这里$p为数组，上面两个变量名可自定义 
if($method == &#39;test&#39;){ 
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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