What does low in coupling and high in cohesion mean

Question

I have problems understanding the statement low in coupling and high in cohesion  I have googled and read a lot about this  but still finding it hard to understand   To what I understand is High cohesion means  that we should have classes that are specialized to perform a particular function  Hope this is correct  Like a credit card validation class  which is specialized to validate credit cards only   And still don t understand what low Coupling means

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Low Coupling and High Cohesion is a recommended phenomenon.

Coupling means to what extent various modules are interdependent and how the other modules are affected on changing some/considerable functionality of a module. Low coupling is emphasized as the dependency has to be maintained low so that very least/negligible changes are made to other modules.

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Do you have a smart phone  Is there one big app or lots of little ones  Does one app reply upon another  Can you use one app while installing  updating  and or uninstalling another  That each app is self-contained is high cohesion  That each app is independent of the others is low coupling  DevOps favours this architecture because it means you can do discrete continuous deployment without disrupting the system entire

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Short and clear answer   High cohesion  Elements within one class module should functionally belong together and do one particular thing  Loose coupling  Among different classes modules should be minimal dependency

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Inheritance or generalization is an example of high coupling  i e  high interdependence   What I meant by this is that in inheritance often the parent class defines base functionalities that is used by its child class and change in methods of parent class directly impact its child classes  Hence we can say that there is a greater degree of interdependence between classes    Realization or using interface is an example of high cohesion  i e  low interdependence   What this means is that an interface put forward a contract for any class that implements it but each class has the right to implement methods declared in interface in its own way and changes in method declared in one class doesn t affect any other class

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Cohesion - how closely related everything is with one another  Coupling - how everything is connected to one another   Let s take an example - We want to design a self-driving car    1  We need the motor to run properly    2  We need the car to drive on its own   All of the classes and functions in  1  starting the motor and making it run work great together  but do not help the car steer  So we place those classes behind an Engine Controller   All of the classes and functions in  2  work great to make the car steer  accelerate and brake  They do not help the car start or send gasoline to the pistons  So we place these classes behind its own Driving Controller    These controllers are used to communicate with all of the classes and functions that are available  The controllers then communicate only with each other  This means I can t call a function in the piston class from the gas pedal class to make the car go faster   The pedal class has to ask the Driving Controller to talk to the Engine Controller which then tells the piston class to go faster  This allows us programmers to be able to find issues and allows us to combine large programs without worrying  This is because the code was all working behind the controller

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Long story short  low coupling as I understood it meant components can be swapped out without affecting the proper functioning of a system   Basicaly modulize your system into functioning components that can be updated individually without breaking the system

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In software design high cohesion means that class should do one thing and one thing very well  High cohesion is closely related to Single responsibility principle   Low coupling suggest that class should have least possible dependencies  Also  dependencies that must exist should be weak dependencies - prefer dependency on interface rather than dependency on concrete class  or prefer composition over inheritance    High Cohesion and low coupling give us better designed code that is easier to maintain

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What I believe is this    Cohesion refers to the degree to which the elements of a module class belong together  it is suggested that the related code should be close to each other  so we should strive for high cohesion and bind all related code together as close as possible  It has to do with the elements within the module class   Coupling refers to the degree to which the different modules classes depend on each other  it is suggested that all modules should be independent as far as possible  that s why low coupling  It has to do with the elements among different modules classes   To visualize the whole picture will be helpful     The screenshot was taken from Coursera

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I think you have red so many definitions but in the case you still have doubts or In case you are new to programming and want to go deep into this then I will suggest you to watch this video  https   youtu be HpJTGW9AwX0 It s just reference to get more info about polymorphism    Hope you get better understanding with this

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Low coupling is in the context of two or many modules  If a change in one module results in many changes in other module then they are said to be highly coupled  This is where interface based programming helps  Any change within the module will not impact the other module as the interface  the mean of interaction   between them has not changed    High cohesion- Put the similar things together  So a class should have method or behaviors to do related job  Just to give an exaggerated bad example  An implementation of List interface should not have operation related to String  String class should have methods  fields which is relevant for String and similarly  the implementation of List should have corresponding things   Hope that helps

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Low Coupling -- Will keep it very simple   If you change your module how does it impact other modules   Example - If your service API is exposed as JAR  any change to method signature will break calling API  High Tight coupling    If your module and other module communicate via async messages  As long as you get messages  your method change signature will be local to your module  Low coupling    Off-course if there is change in message format  calling client will need to make some change

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An example might be helpful  Imagine a system which generates data and puts it into a data store  either a file on disk or a database   High Cohesion can be achieved by separate the data store code from the data production code   and in fact separating the disk storage from the database storage    Low Coupling can be achieved by making sure that the data production doesn t have any unnecessary knowledge of the data store  e g  doesn t ask the data store about filenames or db connections

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Here is an answer from a bit of an abstract  graph theoretic angle   Let s simplify the problem by only looking at  directed  dependency graphs between stateful objects   An extremely simple answer can be illustrated by considering two  limiting cases of dependency graphs   The 1st limiting case   a cluster graphs     A cluster graph is the most perfect realisation of a high cohesion and low coupling  given a set of cluster sizes  dependency graph   The dependence between clusters is maximal  fully connected   and inter cluster dependence is minimal  zero     This is an abstract illustration of the answer in one of the limiting cases   The 2nd limiting case is a fully connected graph  where everything depends on everything    Reality is somewhere in between  the closer to the cluster graph the better  in my humble understanding   From another point of view  when looking at a directed dependency graph  ideally it should be acyclic  if not then cycles form the smallest clusters components   One step up down the hierarchy corresponds to  one instance  of loose coupling  tight cohesion in a software but it is possible to view this loose coupling tight cohesion principle as a repeating phenomena at different depths of an acyclic directed graph  or on one of its spanning tree s     Such decomposition of a system into a hierarchy helps to beat exponential complexity  say each cluster has 10 elements   Then at 6 layers it s already 1 million objects   10 clusters form 1 supercluster  10 superclusters form 1 hypercluster and so on     without the concept of tight cohesion  loose coupling  such a hierarchical architecture would not be possible   So this might be the real importance of the story and not just the high cohesion low coupling within two layers only  The real importance becomes clear when considering higher level abstractions and their interactions

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Cohesion in software engineering  as in real life  is how much the elements consisting a whole in our case let s say a class  can be said that they actually belong together  Thus  it is a measure of how strongly related each piece of functionality expressed by the source code of a software module is   One way of looking at cohesion in terms of OO is if the methods in the class are using any of the private attributes    Now the discussion is bigger than this but High Cohesion  or the cohesion s best type - the functional cohesion  is when parts of a module are grouped because they all contribute to a single well-defined task of the module   Coupling in simple words  is how much one component  again  imagine a class  although not necessarily  knows about the inner workings or inner elements of another one  i e  how much knowledge it has of the other component   Loose coupling is a method of interconnecting the components in a system or network so that those components  depend on each other to the least extent practically possible     I wrote a blog post about this  It discusses all this in much detail  with examples etc  It also explains the benefits of why you should follow these principles

[oop] What does 'low in coupling and high in cohesion' mean

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