LSP It's the only design principle named after a person , And the author is also a “ Female doctor ” 


LSP,Liskov substitution principle, Chinese translation for “ Richter's principle of substitution ”.

This is the only one of the object-oriented principles named after people , Even though Liskov There is no popularity in China UNIX The great masters of the world (Kenneth Thompson、Dennis Ritchie)、GOF The gang of four is so loud , But check the information , You'll find that in fact Liskov It's also very good :2008 Turing prize winner in , The first female computer doctor in history . Its specific information can be found on Wikipedia :

Get down to business , Let's see LSP What are the principles .

LSP The original explanation, of course, comes from Liskov Lady, it's time to , She is in 1987 Year of OOPSLA It was proposed at the meeting that LSP principle ,1988 year , She published her article in ACM Of SIGPLAN Notices The journal , It's explained in detail LSP principle :

A type hierarchy is composed of subtypes and supertypes. The intuitive idea of a subtype is one whose objects provide all the behavior of objects of another type (the supertype) plus something extra.What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.

English is relatively long , I look tired , Let's simply translate and summarize :

1)  The object of the subclass provides all the behavior of the parent class , And add something extra to the subclass ( Can be a function , It can also be attributes );

2)  When a program is implemented based on a parent class , Suppose the subclass is replaced by the parent class and the program does not need to be changed , It means that it conforms to LSP principle

Although we've translated and sorted it out a little bit , But it's still very awkward and hard to understand .

Fortunately, there is also Martin The master also thinks this is not very easy to understand ,Robert Martin stay 1996 Years for 《C++ Reporter》 I wrote an article entitled 《The The Liskov Substitution Principle》 The article , Explain, for example, the following :

Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.

Translation is : When a function uses a pointer or reference to a parent class , Must be able to use subclass objects without knowing the subclass type .

Martin The Master explained , relative easy More understanding . but Martin The master is not satisfied yet , stay 2002 year ,Martin In his published 《Agile   Software   Development   Principles   Patterns   and   Practices》 In a Book , It is further simplified as :

Subtypes   must   be   substitutable   for   their   base   types.

Translation is : Subclasses must be replaceable with their parents .

after Martin Master's two translations , I Believe LSP The principle itself has been explained quite well easy I understand , But the point is : How to satisfy LSP principle ? Or more generally : When can subclass replace parent class ?

We know , For the caller (Liskov What's mentioned in the explanation P), Interacting with a parent class is nothing more than two parts : Calls a method of the parent class 、 Get the output of the parent class method , The intermediate process ,P I don't know .

in other words , The relationship between the caller and the parent class is now two-sided : Function input , Output function . For example, the following figure :

With this picture , How to do LSP The principle is clear :

1)  The subclass must implement or inherit all the public functions of the parent class , Otherwise, the caller calls a function in a parent class , And there's nothing in the subclass , There will be errors when executing ;

2)  The input parameters of each function of a subclass must be the same as the parent class , Otherwise, the code that calls the parent class cannot call the child class ;

3)  The output of each function of the subclass ( Return value 、 Change global variables 、 Insert database 、 Sending network data, etc ) Must be no less than the parent class , Otherwise, the processing based on the output of the parent class cannot be completed .

With these three principles , It is very convenient to infer whether the class design conforms to LSP Principle . It's important to note that 3 The key to this is “ No less than the parent class ”, That is to say, it can be more than the parent class , namely : The output of the parent class is a subset of the output of the subclass .

Some friends may be a little puzzled when they see these three principles : These three principles come out one by one , What's the difference between that subclass , It's all the same , There will be different subclasses ?

In fact, if we study these three principles carefully , You'll find that It's just a convention on input and output , And there's no constraint in the middle of the process . such as : The output of the same write database ,A Class can be read XML The data is then written to the database ,B Class can read data from other databases and then use the local database ,C Class can get data by analyzing the business log and then write it to the database . this 3 The process of each class is different , But it's all written to the database in the end .

LSP The classic example of a principle is “ Rectangles and squares ” This sample . From a mathematical point of view , A square is a special rectangle , But from an object-oriented perspective , Square is not a subclass of rectangle . The reason is that for rectangles , After setting the width and height , area  =  wide  *  high ; But for a square , Set the height and set the width at the same time , Setting the width sets the height at the same time , The final area is not equal to what we set   wide  *  high , It's equal to the square of the last set width or height .

The specific code sample ratio is as follows :

* Rectangle
public class Rectangle { protected int _width;
protected int _height; /**
* Set the width
* @param width
public void setWidth(int width){
this._width = width;
} /**
* Set high
* @param height
public void setHeight(int height){
this._height = height;
} /**
* Get the area
* @return
public int getArea(){
return this._width * this._height;

* Square
public class Square extends Rectangle { /**
* Set up “ wide ”, Unlike a rectangle : Set the width of the square , The height of the square is set at the same time
public void setWidth(int width){
this._width = width;
this._height = width;
} /**
* Set up “ high ”, Unlike a rectangle : Set the height of the square , The width of the square is set at the same time
public void setHeight(int height){
this._width = height;
this._height = height;
} }

public class UnitTester {
public static void main(String[] args){
Rectangle rectangle = new Rectangle();
rectangle.setHeight(5); // For example, the following assert Infer as true
assert( rectangle.getArea() == 20); rectangle = new Square();
rectangle.setHeight(5); // For example, the following assert Infer as false, Assertion failed , Throw out java.lang.AssertionError
assert( rectangle.getArea() == 20);

At the same time, the above example also gives an inference about whether the subclass conforms to LSP It's a clever way to , namely : Unit test cases for parent classes , Whether the incoming subclass can also pass the test . Suppose the test passes , It means that it conforms to LSP principle , Otherwise, it means that it does not meet the requirements LSP principle

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