What is Richter's substitution principle? (for example)

Liskov Substitution Principle

Liskov Substitution Principle LSP talks about the relationship between base classes and subclasses. Only when this relationship exists does the Richter substitution relationship exist. If the relationship between two specific classes A and B violates the design of LSP (assuming it is an inheritance relationship from B to A), then you can choose one of the following two reconstruction solutions according to the specific situation.

<?php
//举例说明继承的风险，我们需要完成一个两数相减的功能，由类A来负责。
class a{
public $width;
public $height;
public function func1($a, $b){  
        return $a - $b;  
    }
}
$a = new a();
echo $a->func1(100,50);
//运行结果100-50=50

Later, we need to add a new function: complete the addition of two numbers, and then sum it with 100. Class B is responsible for this. That is, class B needs to complete two functions:

The code for using class B to inherit class A is as follows:

class b extends a{  
    public function func1($a, $b){  
        return $a + $b;
    }  
      
    public function func2($a, $b){  
        return $this->func1($a, $b) + 100;
    } 
}
$b = new b();
echo $b->func2(100, 50);

Assume that class B accidentally rewrites the method of the parent class when naming the method, resulting in All codes that run the subtraction function all call the rewritten method of class B, causing an error in the function fun1 that originally ran normally (the reason for the error is that subtraction has become addition and other users do not know it. Don’t Always think that the code is written by one person! And people don’t have time to read your code line by line, they just apply it according to the rules).

The Liskov substitution principle is generally speaking: Subclasses can extend the functions of the parent class, but cannot change the original functions of the parent class. It contains the following 4 levels of meaning:

1. Subclasses can implement abstract methods of the parent class, but cannot override non-abstract methods of the parent class.

2. You can add your own unique methods to subclasses.

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 a parent class, the postconditions of the method (i.e., the return value of the method) are more stringent than those of the parent class.

It seems incredible, because we will find that we often violate the Liskov substitution principle in our own programming, but the program still runs well. So everyone will have this question, what will be the consequences if I insist on not following the Liskov substitution principle?

The consequence is: the chance of problems with the code you write will be greatly increased.

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