PHP object-oriented syntax 5 final class, abstract class, interface structure (not class)

The role of classes
1 Instantiate objects
2 As the base class of other classes, it is inherited!
3Call its static members

Two main functions: instantiating objects and being inherited.

There are two special classes with a single function!
1, ** can only instantiate objects and cannot be inherited. **Final class
2 can only be inherited and cannot instantiate objects. Abstract class

Final class and final method
The final class is the last link in the inheritance chain and cannot have subclasses

final class End{
}
class Test  extends End{
}

运行结果：报错
PHP Fatal error: Class Test may not inherit from final class (End) in /usercode/file.php on line 7

Final method, the restriction method cannot be overridden by subclasses

<?php
class  AmParent{
    final public function end_func(){
        echo &#39;我是final方法&#39;;
    }
}
class AmChild extends AmParent{
     public function end_func(){
        echo &#39;我是子类，我想重写final方法，可是报错了&#39;;
    }
}

运行结果：报错
PHP Fatal error: Cannot override final method AmParentt::end_func() in /usercode/file.php on line 11

Abstract class, abstract class
There is one more thing than regular classes (class constants, static properties, static methods, non-static properties, non-static methods) - abstract methods, which require non-abstract subclasses to be implemented abstract method.
Abstract method: Without {}, it is called a declaration of only a method. Can have parameters.

abstract public function abstract_func($arg1,$arg2)

//不能实例化对象
<?php
abstract class AmParent{
    abstract public function abstract_func($arg1,$arg2);
}
$amParent=new AmParent;

运行结果：
PHP Fatal error: Cannot instantiate abstract class AmParent in /usercode/file.php on line 5

can only be inherited
Two cases of being inherited:
1. The subclass is an abstract class
2. The subclass is not an abstract class, and all abstract methods of the parent class must be included Implementation (plus method body {}).

<?php
abstract class AmParent{
    abstract public function abstract_func($arg1,$arg2);
}
class AmChild extends AmParent{
}

运行结果，报错：
PHP Fatal error: Class AmChild contains 1 abstract method and must therefore be 
declared abstract or implement the remaining methods (AmParent::abstract_func) in /usercode/file.php on line 6

Situation without error reporting:
1 Add abstract to become an abstract class

<?php
abstract class AmParent{
    abstract public function abstract_func($arg1,$arg2);
}
abstract class AmChild extends AmParent{
}

2 Implement abstract method

<?php
abstract class AmParent{
    abstract public function abstract_func($arg1,$arg2);
}
 class AmChild extends AmParent{
     public function abstract_func($arg1,$arg2){
        echo $arg1.$arg2;
         }
}
$amChild=new AmChild;
$amChild->abstract_func( &#39;have implemented &#39;,&#39; abstract_func &#39;);

运行结果，报错：
have implemented abstract_func

Meaning of use
Abstract class, stipulates its Methods that must exist in non-abstract subclasses have the same name but the method body can be different.

<?php
abstract class Goods{
    abstract public function sayName();
}
 class Books extends  Goods{
     public function sayName(){
         echo &#39;我是一本书的名字<br/>&#39;;
     }
}
 class Mobiles extends  Goods{
     public function sayName(){
         echo &#39;我是一部手机的名字<br/>&#39;;
     }
}
$book1=new Books;
$book1->sayName();
$mobile1=new Mobiles;
$mobile1->sayName();

运行结果：
我是一本书的名字
我是一部手机的名字

Interface structure:
Used to limit the public methods (interface methods) that a class (object) should have!

<?php
    interface iGoods{
        const PAI=3.14;
        public function sayName();
        public function sayPrice();
    }

The interface can only contain public abstract methods (incomplete methods) and constants!
Implement implementations
The class that implements the interface structure includes the abstract methods defined in the interface.
Two options, 1 define it as an abstract class, 2 implement the interface method!

//1定义成抽象类
abstract class Books implements iGoods{
    abstract  public function sayName();
    abstract  public function sayPrice();
}

//2实现接口方法
 class Books implements iGoods{
     public function sayName(){
         echo '我实现了接口方法';
     }
     public function sayPrice(){
         echo '我实现了接口方法';
     }
}

Compare the difference between abstract classes and interfaces:
1 There is an inheritance relationship between abstract classes and ordinary classes
The ordinary class inherits the abstract class. First, you can get the existing conventions in the abstract class. Members,
Secondly, you need to implement abstract methods (not necessarily public)
2. There is an implementation relationship between interfaces and ordinary classes!
If a normal class implements an interface, it can only implement its unimplemented public methods!
3. Interfaces are only used to define public methods and constants! And abstract classes can have anything!
4. Multiple interface implementations. A class can implement multiple interfaces at the same time

<?php
interface iA{
        const PAI=3.14;
    public function sayA();
}
interface iB{
    public function sayB();
}
class C implements iA,iB{
    public function sayA(){
    }
    public function sayB(){
    }
}

tip:
The interface is not a class, the interface is an independent structure used to limit the structure of the class!

//用class_exists（@param string），判断类是否存在
var_dump(class_exists('C'));
var_dump(class_exists('iA'));

运行结果：
bool(true)
bool(false)

It is full of public methods and constants without method bodies or abstract keywords.

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