Detailed explanation of usage examples of PHP interfaces and abstract classes

1. Abstract class abstract class

1． An abstract class refers to a class with the abstract keyword added before class and an abstract method (abstract keyword added before the class method function keyword).

2． Abstract classes cannot be instantiated directly. The abstract class only defines (or partially implements) the methods required by the subclass. Subclasses can make an abstract class concrete by inheriting it and by implementing all the abstract methods in the abstract class.

3． If a subclass needs to be instantiated, it must implement all abstract methods in the abstract class. If the subclass does not implement all the abstract methods in the abstract class, then the subclass is also an abstract class and must be preceded by the abstract keyword in class and cannot be instantiated.

Copy code The code is as follows:

abstract class A
{
/**Variables can be defined in abstract classes*/
protected $value1 = 0;
private $value2 = 1;
public $value3 = 2;
/**You can also define non-abstract methods*/
public function my_print()
{
echo "hello,world/n";
}
/**
* In most cases, abstract classes contain at least one abstract method. Abstract methods are declared with the abstract keyword and cannot have specific content.
* You can declare an abstract method just like a normal class method, but end it with a semicolon instead of a method body. That is to say, abstract methods cannot be implemented in abstract classes, that is, there is no function body "{some codes}".
*/
abstract protected function abstract_func1();
abstract protected function abstract_func2();
}
abstract class B extends A
{
public function abstract_func1()
{
echo "implement the abstract_func1 in class A/n"; If written like this in zend studio 8, an error will be reported.*/
//abstract protected function abstract_func2();
}
class C extends B
{
public function abstract_func2()
{
echo "implement the abstract_func2 in class A/n";
}
}

4． If you create a subclass B that inherits from A as follows, but does not implement the abstract method abstract_func():

Copy code The code is as follows:

Class B extends A{};

Then the program will have the following error:

Copy code The code is as follows:

Fatal error: Class B contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (A::abstract_func)

5. If B implements the abstract method abstract_func(), then the access control of the abstract_func() method in B cannot be stricter than the access control of abstract_func() in A, that is:

(1) If abstract_func() in A is declared as public, then the declaration of abstract_func() in B can only be public and cannot be protected or private

(2) If abstract_func() in A is declared as protected, then the declaration of abstract_func() in B can be public or protected, but cannot be private

(3) If abstract_func() in A is declared as private, hey, it cannot be defined as private! (Fatal error: Abstract function A::abstract_func() cannot be declared private )

2. Interface interface

1． Abstract classes provide standards for concrete implementation, while interfaces are pure templates. Interfaces only define functions, not implementation content. Interfaces are declared with the keyword interface.

2． Interface is completely abstract. It can only declare methods, and only public methods. It cannot declare private and protected methods, cannot define method bodies, and cannot declare instance variables. However, interfaces can declare constant variables. But placing constant variables in an interface violates its purpose of existing as an interface, and also confuses the different values of interfaces and classes. If you really need it, you can put it in the corresponding abstract class or Class.

Copy code The code is as follows:

interface iA
{
const AVAR=3;
public function iAfunc1();
public function iAfunc2();
}
echo iA::AVAR;

3． Any class that implements an interface must implement all methods defined in the interface

Copy code The code is as follows:

class E implements iA
{
public function iAfunc1(){echo "in iAfunc1";}
public function iAfunc2(){echo "in iAfunc2";}
}

Otherwise the class must be declared abstract .

Copy code The code is as follows:

abstract class E implements iA{}

4． A class can implement an interface using the implements keyword in its declaration. After doing this, the specific process of implementing the interface is the same as inheriting an abstract class that only contains abstract methods. A class can inherit a parent class and implement any number of interfaces at the same time. The extends clause should precede the implements clause. PHP only supports inheritance from one parent class, so the extends keyword can only be followed by a class name.

Copy code The code is as follows:

interface iB
{
    public function iBfunc1();
    public function iBfunc2();
}
class D extends A implements iA,iB
{
    public function abstract_func1()
    {
       echo "implement the abstract_func1 in class A/n";
    }
    public function abstract_func2()
    {
       echo "implement the abstract_func2 in class A/n";
    }
    public function iAfunc1(){echo "in iAfunc1";}
    public function iAfunc2(){echo "in iAfunc2";}
    public function iBfunc1(){echo "in iBfunc1";}
    public function iBfunc2(){echo "in iBfunc2";}
}

class D extends B implements iA,iB
{
    public function abstract_func1()
    {
       parent::abstract_func1();
       echo "override the abstract_func1 in class A/n";
    }
    public function abstract_func2()
    {
       echo "implement the abstract_func2 in class A/n";
    }
    public function iAfunc1(){echo "in iAfunc1";}
    public function iAfunc2(){echo "in iAfunc2";}
    public function iBfunc1(){echo "in iBfunc1";}
    public function iBfunc2(){echo "in iBfunc2";}
}

5 ．接口不可以实现另一个接口，但可以继承多个

复制代码代码如下:

interface iC extends iA,iB{}
class F implements iC
{
    public function iAfunc1(){echo "in iAfunc1";}
    public function iAfunc2(){echo "in iAfunc2";}
    public function iBfunc1(){echo "in iBfunc1";}
    public function iBfunc2(){echo "in iBfunc2";}
}

3. Similarities and differences between abstract classes and interfaces

1. Similarities:

(1) Both are abstract classes and cannot be instantiated.

(2) The interface implementation class and the subclasses of abstract class must implement the declared abstract methods.

2. Differences:

(1) Interface needs to be implemented, so use implements , and abstract class needs to be inherited, so use extends .

(2) A class can implement multiple interfaces, but a class can only inherit one abstract class.

(3) Interface emphasizes the implementation of specific functions, while abstract class emphasizes ownership relationships.

(4) Although both interface implementation classes and abstract class subclasses must implement corresponding abstract methods, the implementation forms are different. Every method in the interface is an abstract method, which is only a declaration (declaration, no method body), and the implementation class must implement it. Subclasses of abstract class can be implemented selectively. This choice has two implications: a) Not all methods in abstract class are abstract. Only those methods with abstract are abstract and subclasses must implement them. For those methods without abstract, the method body must be defined in the abstract class; b) When a subclass of abstract class inherits it, it can directly inherit or override non-abstract methods; and for abstract methods, it can choose to implement or Implementation can be left to its subclasses, but this class must also be declared abstract. Since it is an abstract class, of course it cannot be instantiated.

(5) Abstract class is the intermediary between interface and class. abstract class plays a connecting role in interface and class. On the one hand, abstract class is abstract and can declare abstract methods to standardize the functions that subclasses must implement; on the other hand, it can define default method bodies for direct use or override by subclasses. In addition, it can define its own instance variables for use by subclasses through inheritance.

(6) The abstract keyword cannot be added before the abstract method in the interface. The abstract method is implicit by default, and the final keyword cannot be added to prevent the inheritance of abstract methods. The abstract method in an abstract class must be preceded by abstract to indicate that it is explicitly declared as an abstract method.

(7) Abstract methods in interfaces are public by default and can only be public. They cannot be modified with private or protected modifiers. Abstract methods in abstract classes can be modified with public and protected, but cannot be modified with private.

3. Application scenarios of interface

(1) Classes need specific interfaces for coordination, regardless of how they are implemented.

(2) It exists as an identifier that can realize a specific function, or it can be a pure identifier without any interface methods.

(3) It is necessary to treat a group of classes as a single class, and the caller only contacts this group of classes through the interface.

(4) It is necessary to implement multiple specific functions, and these functions may have no connection at all.

4. Application occasions of abstract class

In a word, you can use it when you need both a unified interface and instance variables or default methods. The most common ones are:

(1) Defines a set of interfaces, but does not want to force each implementation class to implement all interfaces. You can use abstract class to define a set of method bodies, or even empty method bodies, and then the subclass can choose the methods that it is interested in to cover.

(2) In some cases, pure interfaces alone cannot satisfy the coordination between classes. Variables representing states in the class are also required to distinguish different relationships. The intermediary role of abstract can serve this well.

(3) Standardizes a set of mutually coordinated methods, some of which are common, independent of state, and can be shared, without the need for subclasses to implement them separately; while other methods require each subclass to implement it according to its own specific status to implement specific functions.