What is high cohesion and low coupling?

cause ： Module independence means that each module only completes the independent sub functions required by the system , And it has the least connection with other modules and the interface is simple , Two qualitative measures ―― Coupling and cohesion . Coupling is also called inter block connection . A measure of the degree to which modules in a software system structure are closely related to each other . The closer the connection between modules , The more coupled it is , The less independent the module is . The coupling between modules depends on the complexity of the interface between modules 、 The way of calling and the information transmitted .

Coupling classification ( low ―― high ): No direct coupling ; Data coupling ; Tag coupling ; Control coupling ; Public coupling ; Content coupling ;

1 No direct coupling :

2 Data coupling : It refers to the calling relationship between two modules , What is passed is a simple data value , Equivalent to the value passing of high-level language ;

3 Tag coupling : It refers to the data structure passed between two modules , For example, array names in high-level languages 、 Record name 、 File names and so on are marked , In fact, it is the address of the data structure ;

4 Control coupling : When one module calls another , What is passed is the control variable （ Like a switch 、 Signs, etc ）, The tuned module selectively performs a function within the block through the value of the control variable ;

5 Public coupling : Refers to the coupling between modules that interact through a common data environment . Complex programs with common coupling increase with the number of coupling modules .

6 Content coupling : This is the highest level of coupling , And the worst coupling . When one module directly uses the internal data of another module , Or through the abnormal entrance into another module .

Cohesion is also called intra block connection . A measure of the functional strength of a module , That is, a measure of the closeness of the various elements within a module . If every element in a module （ Between names 、 Between segments ） The more connected , The more cohesive it is . Cohesive bandits ( low ―― high ): To gather by chance ; Logical cohesion ; Time converges ; Communication cohesion ; Order cohesion ; Functional cohesion ;

1 To gather by chance : It means that there is no relationship between the processing elements in a module .

2 Logical cohesion : It refers to the execution of several logically similar functions in the module , Through the parameters to determine which function the module completes .

3 Time converges : The module formed by combining the actions that need to be executed at the same time is a time cohesion module .

4 Communication cohesion : All processing elements in the module operate on the same data structure （ Sometimes it's called information cohesion ）, Or each process uses the same input data or produces the same output data .

5 Order cohesion : It refers to that each processing element in a module is closely related to the same function and must be executed in sequence , The output of the former function element is the input of the next function element .

6 Functional cohesion : This is the strongest cohesion , It means that all elements in the module complete a function together , Be short of one cannot . Coupling with other modules is the weakest .

Coupling and cohesion are two qualitative criteria of module independence , When dividing the software system into modules , Strive for high cohesion and low coupling , Improve the independence of modules , Lay the foundation for designing high quality software structure .

There's an example that's easy to understand ： A program has 50 A function , This program works very well ; But once you modify one of these functions , other 49 All functions need to be modified , This is the consequence of high coupling . Once you understand it , When you write a profile design, the design class or module will naturally take “ High cohesion , Low coupling ”.

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