What is the idea of ​​Liskov substitution principle in php

In PHP, the Liskov substitution principle is a constraint on class inheritance. The main idea of ​​this principle is that inappropriate classes with redundant methods or attributes cannot be inherited casually. Subclasses can extend their parent classes. The functions of the class, but cannot change the original functions of the parent class.

The operating environment of this article: Windows 10 system, PHP version 7.1, Dell G3 computer.

What is the idea of ​​the Liskov substitution principle in php

The Liskov substitution principle tells us that if a base class object is replaced with its subclass object in the software, the program will not generate For any errors and exceptions, the converse is not true. If a software entity uses a subclass object, it may not necessarily be able to use a base class object.

The Liskov substitution principle is one of the important ways to implement the opening and closing principle. Since subclass objects can be used wherever base class objects are used, try to use base class types to define objects in the program. At runtime, the subclass type is determined and the parent class object is replaced with the subclass object.

When using the Liskov substitution principle, please note that all methods of the subclass must be declared in the parent class, or the subclass must implement all methods declared in the parent class. Try to design the parent class as an abstract class or interface, let the subclass inherit the parent class or implement the parent interface, and implement the methods declared in the parent class. At runtime, the subclass instance replaces the parent class instance, and we can easily expand the system function, and there is no need to modify the code of the original subclass. Adding new functions can be achieved by adding a new subclass.

Understand through a piece of code

<?php
//例子1
class Bird{
  protect function fly(){
  }
}
//翠鸟
class KingFisher extends Bird{
}
//鸵鸟
class Ostrich extends Bird{
  //鸵鸟不会飞啊
}
//例子2
class A{
  protect function add($a, $b){
    return $a + $b;
  }
} 
//重载
class B extends A{
  protected function add($a, $b){
    return $a + $b + 100;
  }
}
?>

The Liskov substitution principle is a constraint on class inheritance. There are two understandings of the Liskov substitution principle:

1. You cannot just inherit inappropriate classes with redundant methods or attributes. (Example 1)

2. A subclass can extend the functions of the parent class, but cannot change the original functions of the parent class. (Example 2)

The Liskov substitution principle contains the following hidden meanings:

① A subclass can implement the abstract method of the parent class, but it cannot override the non-abstract method of the parent class.

② Subclasses can add their own unique methods.

③ When a method of a subclass overrides a method of a parent class, the preconditions of the method (that is, the formal parameters of the method) are looser than the input parameters of the parent class method.

④ When a method of a subclass implements an abstract method of the parent class, the postconditions of the method (ie, the return value of the method) are more stringent than those of the parent class.

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