Difference Between Cohesion and Coupling

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What is the difference between cohesion and coupling   How can coupling and cohesion lead to either good or poor software design   What are some examples that outline the difference between the two  and their impact on overall code quality

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cohesion refers all about how a single class is designed. Cohesion is the Object Oriented principle most closely associated with making sure that a class is designed with a single, well-focused purpose. The more focused a class is, the cohesiveness of that class is more. The advantages of high cohesion is that such classes are much easier to maintain (and less frequently changed) than classes with low cohesion. Another benefit of high cohesion is that classes with a well-focused purpose tend to be more reusable than other classes.

In the above image, we can see that in low cohesion only one class is responsible to execute lots of job which are not in common which reduces the chance of re-usability and maintenance. But in high cohesion there is a separate class for all the jobs to execute a specific job, which result better usability and maintenance.

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Coupling   interaction   relationship between two modules    Cohesion   interaction between two elements within a module   A software is consisting of many modules  Module consists of elements  Consider a module is a program  A function within a program is a element    At run time  output of a program is used as input for another program  This is called module to module interaction or process to process communication  This is  also called as Coupling   Within a single program  output of a function is passed to another function  This is called interaction of elements within a module  This is also called as Cohesion   Example    Coupling   communication in between 2 different families     Cohesion   communication in between father-mother-child within a family

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I think the differences can be put as the following    Cohesion represents the degree to which a part of a code base forms a logically single  atomic unit  Coupling represents the degree to which a single unit is independent from others  It   s impossible to archive full decoupling without damaging cohesion  and vice versa    In this blog post I write about it in more detail

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Simply put  cohesion means that a class should represent a single concept   The public interface of a class is cohesive if all the class features are related to the concept that the class represents  For example  instead of having CashRegister class  having CashRegister and Coin features cohesion makes it into 2 classes - CashRegister and Coin class   In coupling  one class depends on another as it uses the objects of the class    The problem with high coupling is that it can create side effects  One change in one class could cause an unexpected error in the other class and could break the whole code   Generally  high cohesion and low coupling is considered high quality OOP

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Cohesion is an indication of how related and focused the responsibilities of an software element are    Coupling refers to how strongly a software element is connected to other elements   The software element could be class  package  component  subsystem or a system  And while designing the systems it is recommended to have software elements that have High cohesion and support Low coupling   Low cohesion results in monolithic classes that are difficult to maintain  understand and reduces re-usablity  Similarly High Coupling results in classes that are tightly coupled and changes tend not be non-local  difficult to change and reduces the reuse   We can take a hypothetical scenario where we are designing an typical monitor-able ConnectionPool with the following requirements  Note that  it might look too much for a simple class like ConnectionPool but the basic intent is just to demonstrate low coupling and high cohesion with some simple example and I think should help    support getting a connection release a connection get stats about connection vs usage count get stats about connection vs time Store the connection retrieval and release information to a database for reporting later    With low cohesion we could design a ConnectionPool class by forcefully stuffing all this functionality responsibilities into a single class as below  We can see that this single class is responsible for connection management  interacting with database as well maintaining connection stats     With high cohesion we can assign these responsibility across the classes and make it more maintainable and reusable     To demonstrate Low coupling we will continue with the high cohesion ConnectionPool diagram above  If we look at the above diagram although it supports high cohesion  the ConnectionPool is tightly coupled with ConnectionStatistics class and PersistentStore it interacts with them directly  Instead to reduce the coupling we could introduce a ConnectionListener interface and let these two classes implement the interface and let them register with ConnectionPool class  And the ConnectionPool will iterate through these listeners and notify them of connection get and release events and allows less coupling     Note Word or Caution  For this simple scenario it may look like an overkill but if we imagine a real-time scenario where our application needs to interact with multiple third party services to complete a transaction  Directly coupling our code with the third party services would mean that any changes in the third party service could result in changes to our code at multiple places  instead we could have Facade that interacts with these multiple services internally and any changes to the services become local to the Facade and enforce low coupling with the third party services

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Cohesion refers to what the class  or module  can do  Low cohesion would mean that the class does a great variety of actions - it is broad  unfocused on what it should do  High cohesion means that the class is focused on what it should be doing  i e  only methods relating to the intention of the class   Example of Low Cohesion   -------------------   Staff             -------------------   checkEmail          sendEmail           emailValidate       PrintLetter       -------------------   Example of High Cohesion   ----------------------------   Staff                     ----------------------------   -salary                     -emailAddr                ----------------------------   setSalary newSalary         getSalary                   setEmailAddr newEmail       getEmailAddr              ----------------------------   As for coupling  it refers to how related or dependent two classes modules are toward each other  For low coupled classes  changing something major in one class should not affect the other  High coupling would make it difficult to change and maintain your code  since classes are closely knit together  making a change could require an entire system revamp   Good software design has high cohesion and low coupling

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The term cohesion is indeed a little counter intuitive for what it means in software design   Cohesion common meaning is that something that sticks together well  is united  which are characterized by strong bond like molecular attraction  However in software design  it means striving for a class that ideally does only one thing  so multiple sub-modules are not even involved    Perhaps we can think of it this way  A part has the most cohesion when it is the only part  does only one thing and can t be broken down further   This is what is desired in software design  Cohesion simply is another name for  single responsibility  or  separation of concerns    The term coupling on the hand is quite intuitive which means when a module doesn t depend on too many other modules and those that it connects with can be easily replaced for example obeying liskov substitution principle

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best explanation of Cohesion comes from Uncle Bob s Clean Code   Classes should have a small number of instance variables  Each of the methods of a class should manipulate one or more of those variables  In general the more variables a method manipulates the more cohesive that method is to its class  A class in which each variable is used by each method is maximally cohesive   In general it is neither advisable nor possible to create such maximally cohesive classes  on the other hand  we would like cohesion to be high  When cohesion is high  it means that the methods and variables of the class are co-dependent and hang together as a logical whole   The strategy of keeping functions small and keeping parameter lists short can sometimes lead to a proliferation of instance variables that are used by a subset of methods  When this happens  it almost always means that there is at least one other class trying to get out of the larger class  You should try to separate the variables and methods into two or more classes such that the new classes are more cohesive

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Cohesion  Co-hesion    Co which means together  hesion which means to stick  The System of sticking together of particles of different substances      For real-life example   img Courtesy     Whole is Greater than the Sum of the Parts -Aristotle       Cohesion is an ordinal type of measurement and is usually described as    high cohesion    or    low cohesion     Modules with high cohesion tend to be preferable  because high cohesion is associated with several desirable traits of software including robustness  reliability  reusability  and understandability  In contrast  low cohesion is associated with undesirable traits such as being difficult to maintain  test  reuse  or even understand  wiki    Coupling is usually contrasted with cohesion  Low coupling often correlates with high cohesion  and vice versa  Low coupling is often a sign of a well-structured computer system and a good design  and when combined with high cohesion  supports the general goals of high readability and maintainability  wiki

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Theory Difference  Cohesion   Cohesion is an indication of relative functional strength of module  A cohesive module performs a single task  requiring little interaction with other  components in other parts of program  A module having high cohesion and low coupling is said to be functionally independent  of  other module     Classification of Cohesion  1 Coincidental  2 Logical   3 Temporal  4 Procedural  5 Communication 6 Sequential 7 Functional  Coupling   Coupling is indication of relative interdependence among modules  Degree of coupling between two modules depends on their interface complexity

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Cohesion in software engineering is the degree to which the elements of a certain module 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   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   I wrote a blog post about this  if you want to read up in a little bit more details with examples and drawings  I think it answers most of your questions

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Increased cohesion and decreased coupling do lead to good software design    Cohesion partitions your functionality so that it is concise and closest to the data relevant to it  whilst decoupling ensures that the functional implementation is isolated from the rest of the system    Decoupling allows you to change the implementation without affecting other parts of your software   Cohesion ensures that the implementation more specific to functionality and at the same time easier to maintain   The most effective method of decreasing coupling and increasing cohesion is design by interface    That is major functional objects should only  know  each other through the interface s  that they implement  The implementation of an interface introduces cohesion as a natural consequence   Whilst not realistic in some senarios it should be a design goal to work by   Example  very sketchy    public interface IStackoverFlowQuestion       void SetAnswered IUserProfile user         void VoteUp IUserProfile user         void VoteDown IUserProfile user      public class NormalQuestion implements IStackoverflowQuestion         protected Integer vote    new Integer 0         protected IUserProfile user    null        protected IUserProfile answered    null         public void VoteUp IUserProfile user               vote                   code to     add to user profile                public void VoteDown IUserProfile user              decrement and update profile                public SetAnswered IUserProfile answer               answered    answer               update u            public class CommunityWikiQuestion implements IStackoverflowQuestion        public void VoteUp IUserProfile user       do not update profile        public void VoteDown IUserProfile user       do not update profile        public void SetAnswered IUserProfile user       do not update profile       Some where else in your codebase you could have a module that processes questions regardless of what they are   public class OtherModuleProcessor       public void Process List lt IStackoverflowQuestion gt  questions               process each question

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Cohesion is the indication of the relationship within a module   Coupling is the indication of the relationships between modules     Cohesion   Cohesion is the indication of the relationship within module  Cohesion shows the module   s relative functional strength   Cohesion is a degree  quality  to which a component   module focuses on the single thing   While designing you should strive for high cohesion i e  a cohesive component  module focus on a single task  i e   single-mindedness  with little interaction with other modules of the system  Cohesion is the kind of natural extension of data hiding for example  class having all members visible with a package having default visibility  Cohesion is Intra     Module Concept    Coupling   Coupling is the indication of the relationships between modules  Coupling shows the relative dependence interdependence among the modules  Coupling is a degree to which a component   module is connected to the other modules  While designing you should strive for low coupling i e  dependency between modules should be less Making private fields  private methods and non public classes provides loose coupling  Coupling is Inter -Module Concept    check this link

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Cohesion is an indication of the relative functional strength of a module    A cohesive module performs a single task  requiring little interaction with other components in other parts of a program  Stated simply  a cohesive module should  ideally  do just one thing   Conventional view   the    single-mindedness    of a module  OO view    cohesion implies that a component or class encapsulates only attributes and operations that are closely related to one another and to the class or component itself  Levels of cohesion   Functional   Layer   Communicational   Sequential   Procedural   Temporal   utility   Coupling is an indication of the relative interdependence among modules    Coupling depends on the interface complexity between modules  the point at which entry or reference is made to a module  and what data pass across the interface  Conventional View      The degree to which a component is connected to other components and to the external world OO view  a qualitative measure of the degree to which classes are connected to one another Level of coupling   Content   Common   Control   Stamp   Data   Routine call   Type use   Inclusion or import   External

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simply  Cohesion represents the degree to which a part of a code base forms a logically single  atomic unit  Coupling  on the other hand  represents the degree to which a single unit is independent from others  In other words  it is the number of connections between two or more units  The fewer the number  the lower the coupling   In essence  high cohesion means keeping parts of a code base that are related to each other in a single place  Low coupling  at the same time  is about separating unrelated parts of the code base as much as possible    Types of code from a cohesion and coupling perspective   Ideal is the code that follows the guideline  It is loosely coupled and highly cohesive  We can illustrate such code with this picture     God Object is a result of introducing high cohesion and high coupling  It is an anti-pattern and basically stands for a single piece of code that does all the work at once   poorly selected takes place when the boundaries between different classes or modules are selected poorly  Destructive decoupling is the most interesting one  It sometimes occurs when a programmer tries to decouple a code base so much that the code completely loses its focus   read more here

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High cohesion within modules and low coupling between modules are often regarded as related to high quality in OO programming languages   For example  the code inside each Java class must have high internal cohesion  but be as loosely coupled as possible to the code in other Java classes   Chapter 3 of Meyer s Object-Oriented Software Construction  2nd edition  is a great description of these issues

[oop] Difference Between Cohesion and Coupling

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