(Advanced) Learn the three major features of PHP object-oriented (encapsulation, inheritance, polymorphism)

The following is the text of the article:

##1. Encapsulation

Encapsulation is to encapsulate the extracted data and the operations on the data together. Protected internally, only authorized operations (methods) in other parts of the program can operate on the data.

php provides three access control modifiers

public means global, accessible within this class, outside the class, and subclasses

protected means protected, only This class or subclass can access

private means private, and can only be accessed within this class

The above three modifiers can modify both methods and attributes (variables). If there is no method The access modifier defaults to public, and member properties 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 what should I do if I want to access protected and private elements and methods externally?

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 are included here 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 
}

You can use echo $p1->getsalary(); externally.

If you want to access protected and private, you can also use the following method, but it is not recommended, as long as you understand it

__set() and __get()

__set() assigns values ​​to protected or private attributes

__set($name,$val);

__get() obtains The value of protected or private

__get($name);

Such as:

<?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; 
?>

2. Inheritance

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;; 
    } 
} 
?>

As can be seen from the above example, when multiple classes have many common attributes and methods,

the reusability of the code 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) that extends the parent class

The public and protected properties and methods in the class (BaseClass) continue,

cannot inherit private properties and methods

Grammar structure:

class 父类名{} 
class 子类名 extends 父类名{}

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 multiple layers 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, the constructor of its parent class will not be automatically called by default

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，

If there is no constructor method in B, the

# in A will be output. 3. If you need to access the method of the parent class in the subclass (the modifier of the constructor method and member method is protected or private) ,

Then you can use parent class::method name or parent::method name to complete

Here parent and the previously mentioned self 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 the method of a subclass (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 below Polymorphism

3. Polymorphism

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(); 
?>

Summary:

1. When a parent class knows all Subclasses have a method, but the parent class cannot determine how to write the method. You can let the subclass override its method. Method overwriting (rewriting) requires that the method name and number of parameters of the subclass be exactly the same

2. If the subclass wants to call a method of the parent class (protected/public), you can use parent class name::method name or parent::method name

3. Override the implementation method When, the access modifiers can be different, but the access rights of the subclass method must be greater than or equal to the access rights of the parent class method (that is, the access rights of the parent class method cannot be reduced)

For example, the parent class public function cry( ){} subclass protected function cry(){} will report an error

But the access rights of the subclass can be enlarged, such as:

父类private function cry(){} 子类 protected function cry(){} 可以正确执行

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