The difference between abstract classes and interfaces in PHP (PHP interview theory question)

php The difference between abstract class and interface in

1、 Concept

Three concepts of object orientation ： encapsulation , Inherit , polymorphic
Encapsulating properties and methods is a class .
The replication of a class's properties and methods by another class is inheritance ,PHP Any class in it can be inherited , Inherited properties and methods can be redefined by subclasses , This is polymorphism .

Abstract classes and abstract methods ：

• The so-called abstract method is empty with only name “ function ”, And the abstract method must be an empty method .
• An abstract class is a class that contains abstract methods , At the same time, abstract classes can also contain ordinary methods .
• Because the abstract method must be an empty method , So when any subclass inherits an abstract class , Must redefine the concrete connotation of abstract methods , To instantiate .

Interface ：

Special abstract classes that contain only abstract methods .
let me put it another way , Ordinary methods and properties are not allowed in interface classes .

2、 case analysis

(1) php Interface class :interface

In fact, their role is very simple , When many people work together to develop a project , May call some classes written by others , Then you'll ask , How do I know the name of the implementation method of a function , This time Hou php Interface classes work , When we define an interface class , The method in it must be implemented by the following subclasses , such as :

interface Shop 
{
     
public function buy($gid); 
public function sell($gid); 
public function view($gid); 
} 

I declare that shop Interface class , Three methods are defined ： buy (buy), sell (sell), see (view), Then all subclasses that inherit this class must implement this 3 There is no way without one method , If the subclass does not implement these things , Can't run . In fact, interface classes are plain , Is a class template , Provisions of a class , If you belong to this kind of , You must follow my rules , Not without one , But how do you do it , I don't care , That's your business , Such as ：

class BaseShop implements Shop {
     

	public function buy($gid) {
     
		echo(' You bought it ID by  :'.$gid.' The goods '); 
	} 
	public function sell($gid) {
     
		echo(' You sold it ID by  :'.$gid.' The goods '); 
	} 
	public function view($gid) {
     
		echo(' You checked ID by  :'.$gid.' The goods '); 
	} 
} 

Do you think , In a large multi person project , How convenient it is to have interface classes , So you don't have to ask others , What is the method name of your XXX function , Of course, if you like this, I have no way .
Conclusion ： The interface class is the leader of a class , Point out the direction , Subclass must complete its specified method .

(2)php abstract class : abstract

In fact, some abstract classes and interface classes are very similar , Remember where I saw such a sentence , Abstract classes extract the part of the class image , This sentence looks very funny , In fact, it speaks the truth of abstract classes , The function of abstract classes is , When you find that many of your classes use many methods, you are constantly writing repeatedly , Then you can consider using abstract classes , You might say “ I can't rewrite a class. I instantiate each public class , Just call the same method ”, Here you can , In fact, this is what abstract classes do , However, it omits the step of instantiation , It makes you as convenient as calling this class method directly , And you can also overload this method . Such as ：

abstract class BaseShop 
{
     
public function buy($gid) 
{
     
echo(' You bought it ID by  :'.$gid.' The goods '); 
} 
public function sell($gid) 
{
     
echo(' You sold it ID by  :'.$gid.' The goods '); 
} 
public function view($gid) 
{
     
echo(' You checked ID by  :'.$gid.' The goods '); 
} 
} 
class BallShop extends BaseShop 
{
     
var $itme_id = null; 
public function __construct() 
{
     
$this->itme_id = 2314; 
} 
public function open() 
{
     
$this->sell($this->itme_id); 
} 
} 

Here is an example , Like the above, I defined a store class , Pull out all its like parts , buy (buy), sell (sell), see (view), And these methods are implemented in abstract classes , Then the subclasses that inherit it automatically get these methods , Subclasses do their own unique things , Introduce code repetition , Improve reusability .
Conclusion ： An abstract class is a service provider of a class , With many services , You don't have to , You can use it when you need it , If you don't feel satisfied with the service , You can also do it yourself .

3、 Their differences

1. Abstract classes can have non abstract methods, while interfaces can only have abstract methods ！
2. A class can inherit multiple interfaces , A class can only inherit one abstract class ！
3. The interface is used through implements Keyword , Abstract classes inherit extends Keyword ！
4. Member variables cannot be declared in an interface （ Including class static variables ）, But you can declare class constants . Abstract classes can declare various types of member variables , Realize the encapsulation of data .（ another JAVA All member variables in the interface should be declared as public static final type ）
5. Interface does not have a constructor , Abstract classes can have constructors .
6. The default methods in the interface are public Type of , Methods in abstract classes can use private,protected,public To modify .