CSC/ECE 517 Summer 2008/wiki2 6 cc
Cohesion and coupling. Cohesion and coupling are concepts that are reasonably easy to understand, but nonetheless, it is worthwhile to gather readable illustrations of where they apply. Browse the hundreds of Web pages that attempt to explain these concepts, picking your favorite examples. Categorize these examples, so that the reader will see the big picture, rather than just a set of redundant illustrations. Many of these pages mention related concepts; list some of them and explain how they relate to cohesion and coupling.
Introduction
Cohesion and Coupling are concepts often discussed when referring to object-oriented programming. In fact, one of the primary goals of object-oriented programming is “to build highly cohesive classes and to maintain loose coupling between those classes” [1]. This begs the question as to what is meant by cohesive classes and how do we know when classes are highly cohesive? Further, what does loose coupling mean and how do we know it has been maintained? This article combines information and examples gathered from the Web in an effort to shed some light on these very relevant questions.
In the first section of this article, we provide the basic definition of cohesion along with examples that describe the different types of cohesion. Next, we explain the concept of coupling and use more examples to illustrate the different types of coupling. In the third section of this article, we highlight some of the concepts related to cohesion and coupling, including ways to measure both and an illustration of Demeter’s Law. Lastly, we provide a conclusion to the topics discussed herein.
Cohesion
Cohesion refers to the degree that a module performs one and only one function. In this context, a module refers to any logical collection of “program entities” [2]. This logical collection of program entities could be a function, a class, or even a package. As stated earlier, the goal of object-oriented programming is to achieve highly cohesive classes. High cohesion is desired because it increases the likelihood that a module will be comprehendible, reliable, robust, and reusable [3]. Alternatively, a module that exemplifies low cohesion is difficult to comprehend, costly to maintain, and less likely to be reused [4]. The following examples illustrate the different types of cohesion and are arranged from lowest (least desirable) to highest (most desirable).
Coincidental Cohesion
Coincidental cohesion describes a module whose operations are unrelated to one another and the module itself can be used to achieve several different types of tasks. The following example illustrates coincidental cohesion [5].
public static class BackendService {
public static float computeNetPay(int orderId) {
// implementation
}
public static float calculateInventoryReorderAmount(int inventoryId) {
// implementation
}
public static boolean generateInvoice(int invoiceId) {
// implementation
}
// ...
}
In this example, the BackendService provides a one stop shop for many different types of tasks. This type of cohesion is the least desirable and should be avoided.
Logical Cohesion
Logical cohesion describes a module that groups operations because categorically they are related but the operations themselves are quite different. Typically, these modules accept a control flag which indicates which operation to execute. The following example illustrates logical cohesion.
public class DataStore {
public void SaveData(int destination, byte[] data) {
switch (destination) {
default:
case 0:
SaveToDb(data)
break;
case 1:
SaveToFile(data)
break;
case 2:
SaveToWebService(data)
break;
}
}
protected void SaveToDb(byte[] data) {
// implementation
}
protected void SaveToFile(byte[] data) {
// implementation
}
protected void SaveToWebService(byte[] data) {
// implementation
}
}
In this example, although all of the operations are logically related by teh fact that they all save data, the truth is they are in fact quite different. Saving to a database likely requires a completely different implementation than saving to a file or web service.
Temporal Cohesion
Temporal cohesion describes a module that has several operations grouped by the fact that the operations are executed within temporal proximity. The following example illustrates temporal cohesion.
public class Startup {
public void Initialize() {
InitializeLogging();
InitializeUI();
InitializeDb();
}
public void InitializeLogging() {
// implementation
}
public void InitializeUI() {
// implementation
}
public void InitializeDb() {
// implementation
}
}
This example highlights a common implementation pattern where tasks are grouped simply by the fact that they need to be executed at around the same time. Aside from needing to be executed at the same time, these operations implement tasks that are indeed quite different.
Procedural Cohesion
Procedural cohesion describes a module whose operations are typically grouped because they are executed within a sequence. Unlike sequential cohesion (discussed below) however, the operations within a procedurally cohesive module can be somewhat unrelated and output from one operation is not necessarily used as input to a subsequently executed operation. The following example illustrates procedural cohesion.
public class GradeBookSystem {
public void Login(int teacherId) {
// implementation
}
public ArrayList GetStudents(int teacherId) {
// implementation
}
public void UpdateGrades(ArrayList grades) {
// implementation
}
public void UpdateAttendance(ArrayList dates) {
// implementation
}
public void Logout(int teacherId) {
// implementation
}
}
In this example, the GradeBookSystem exposes multiple operations consisting of logging in and performing different student tracking tasks. These operations are grouped because they facilitate the higher level level task of upgrading the teacher's grade book.
Communicational Cohesion
Communicational cohesion describes modules that perform multiple operations on the same input or output data. The following example illustrates communication cohesion.
public class CustomerInformation {
public CustomerInformation(int accountNum) {
// implementation
}
public String getName() {
// implementation
}
public float getBalance() {
// implementation
}
// ...
}
Information cohesion
Information cohesion can do several things with the same data - a class with various methods using same data. The following example illustrates information cohesion.
class Circle {
double radius;
public double getArea(){return 3.14*radius*radius;}
public double getDiameter(){return 2 * radius;}
public double getCircumference(){return 3.14*2*radius;}
}
Sequential Cohesion
Sequential cohesion describes modules whose operations are intended to be executed in sequence with the output of each operation providing input to the subsequently executed operation. The following example illustrates sequential cohesion.
class Cube {
public ArrayList getInfo(){
ArrayList tempList = new ArrayList();
tempList.add(getArea());
tempList.add(getVolume());
return tempList;
}
public double getArea(){return 6*side*side;}
public double getVolume(){return side*side*side;}
}
Functional Cohesion
Functional cohesion describes a module that is designed to perform one and only one task. A functionally cohesive module may contain multiple methods, but all of these methods are designed to help the user achieve a single task. The following example illustrates functional cohesion.
public class Stack {
public Stack() {
// implementation
}
public void push(Object obj) {
// implementation
}
public Object pop() {
// implementation
}
public Object peek() {
// implementation
}
public int isEmpty() {
// implementation
}
}
Advantages and Disadvantages
Cohesion is the idea that the module does a single task - be it calculating data, checking file etc. The "single task mindedness" drastically reduces code breaking when other modules are changed. If the module uses data from multiple other modules - if even one module changes or breaks, this module might need to be changed thus more time wasted. With single task modules, individual modules can be changed with very little problem.
Coupling
Coupling refers to the degree of connectedness between two modules. Modules that have many connections between one another are said to be have high coupled or be tightly coupled. Alternatively, modules that have very little connection between one another are said to have low coupling or be loosely coupled. Introducing more coupling between modules, increases the interdependencies between modules. As a result, attempting to reuse one module requires the “import” of all of its associated or coupled modules [6]. In addition, high coupling may introduce issues with code reuse, maintenance, testing, and general understanding of relationships between modules [7]. For these reasons, it is important to maintain loose coupling between classes. The following examples illustrate the different types of coupling and are arranged from tightest (least desirable) to loosest (most desirable).
Content coupling
Content coupling occurs when one or more modules access the internals of another module. The following example illustrates content coupling.
public class Rectangle {
public int Top = 0;
public int Left = 0;
public int Width = 0;
public int Height = 0;
public Rectangle(int top, int left, int width, int height) {
this.Top = top;
this.Left = left;
this.Width = width;
this.Height = Height;
}
public int getArea() {
return this.Width * this.Height;
}
}
public class FloorPlan {
Rectangle rectangle = null;
public FloorPlan(int width, int height) {
rectangle = new Rectangle(0, 0, 50, 100);
}
public void modifyDimensions(int width, int height) {
rectangle.Width = width;
rectangle.Height = height;
}
public int getArea() {
return rectangle.getArea();
}
}
In this example, FloorPlan is able to directly modify the Width and Height fields of the Rectangle object. This coupling creates a dependency from FloorPlan on the internals of the Rectangle object that inhibits maintenance of the Rectangle class. If someone wanted to go back and change the Width and Height fields of Rectangle class to use a different data type they would also have to update the FloorPlan class.
Common coupling
Common coupling occurs when two or more modules modify the same same global variable. The following example illustrates common coupling.
#include <stdio.h>
#include <string.h>
#define NUM_FIELDS 3
class EmployeeRecordParser {
public:
EmployeeRecordParser(char* strRow, int nFields) : m_nCount(nFields), m_aryFields(0) {
m_aryFields = new char*[m_nCount];
char* strField = strtok(strRow, ",");
for (int ct = 0; ct < m_nCount && strField; ++ct) {
m_aryFields[ct] = new char[strlen(strField) + 1];
memcpy(m_aryFields[ct], strField, strlen(strField));
m_aryFields[ct][strlen(strField)] = 0;
strField = strtok(NULL, ",");
}
}
~EmployeeRecordParser() {
if (m_aryFields)
delete [] m_aryFields;
}
int GetCount() { return m_nCount; }
char* operator[](int nIndex) { return GetField(nIndex); }
char* GetField(int nIndex) { return nIndex < m_nCount ? m_aryFields[nIndex] : ""; }
private:
char** m_aryFields;
int m_nCount;
};
void ParseRecords(char* strFile) {
int nRecords = 0;
char* strRow = strtok(strFile, "\n");
while (strRow) {
EmployeeRecordParser record(strRow, NUM_FIELDS);
printf("\nEmployee Record %d\n------------------------\n", ++nRecords);
for (int i = 0; i < record.GetCount(); ++i) {
printf("Field %d: %s\n", i, record[i]);
}
strRow = strtok(NULL, "\n");
}
}
int main() {
char str[] = "Tom,Frank,919-777-2333\nMikel,Dundlin,919-234-5512\nRobert,Skoglund,919-232-2904";
ParseRecords(str);
return 0;
}
In the C++ example above, both the ParseRecords method and the EmployeeRecordParser class make use of the globally accessible strtok function. Internally, strtok uses a static variable to track the position of the current string being tokenized, which is also used to determine when the whole string has been parsed. In this particular example, the coupling on this common function has a side-effect that causes a bug that prevents all the records from being correctly parsed.
Control coupling
Control coupling occurs when one module controls the execution flow of another module. The following example illustrates control coupling.
enum InfoType { id, name, balance }
public class CustomerInfo() {
public Object getCustomerInfo(InfoType type) {
Object returnVal = null;
switch (infoType) {
case InfoType.id:
returnVal = getCustomerId();
break;
case InfoType.name:
returnVal = getCustomerName();
break;
case InfoType.balance:
returnVal = getCustomerBalance();
break;
}
return returnVal;
}
// ...
}
public class Client {
private customerInfo = new CustomerInfo();
public void execute() {
int id = (int)customerInfo.getCustomerInfo(InfoType.id);
// ...
}
}
Stamp coupling
Stamp coupling occurs when two or more modules access or modify the same data of a shared object. The following example illustrates stamp coupling.
public class Customer {
private int id = 0;
private String name = "";
private float balance = 0.0f;
public int getId() { return id; }
public void setId(int _id) { id = _id; }
public String getName() { return name; }
public void setName(String _name) { name = _name; }
public float getBalance() { return balance; }
public void setBalance(float _balance) { balance = _balance; }
}
public class CustomerInfo() {
public void save(Customer customer) {
int id = customer.getId();
String name = gustomer.getName();
// ...
}
}
public class Client {
private customerInfo = new CustomerInfo();
public void execute() {
Customer customer = new Customer();
customer.setId(5);
customer.setName("Example");
customer.setBalance(100f);
customerInfo.save(customer);
}
}
Data coupling
Data coupling occurs when one module passes primitive type or simple data structure to another module as an argument. The following example illustrates data coupling.
public class CustomerInfo
{
public float getCustomerBalance(int customerId)
{
// implementation details
}
}
public class Client
{
private customerInfo = new CustomerInfo();
public void execute(int customerId)
{
float balance = customerInfo.getCustomerBalance(customerId);
// ...
}
}
Message coupling
This is the loosest type of coupling. Modules are not dependent on each other, instead they use a public interface to exchange parameter-less messages. The following example illustrates messag coupling.
class superclass {
public void processData(){ module1.processData(); }
}
public Module2 {
public void doSomething() { superclass.processData(); }
}
No coupling
Modules do not communicate at all with one another.
Advantages and disadvantages
Coupling allows interaction between different modules so more complicated tasks can be done. However, a strong coupling will decrease the flexibility of the modules and it will be harder to main and understand. If coupling is too tight, changing one module might have a "snowball effect" and will require changes of other modules that are dependent on it. Coupling must be used with caution and modules must use exactly what it needs and nothing more.
Related Concepts
Measuring Cohesion
The goal of well-designed systems is to have highly cohesive modules. Below are three metrics that can be used to determine the level of cohesion within a system.
Lack of Cohesion 1 (LCOM1)
LCOM1 = (P > Q) ? (P – Q) : 0 P = Total number of method pairs that do not use a common field of the class. Q = Total number of method pairs that access at least one common field of the class.
Lower LCOM1 values indicate higher cohesion and better overall design.
Lack of Cohesion 2 (LCOM2)
LCOM2 = 1 – sum(mA)/(m*a) m = Total number of methods in the class. a = Total number of attributes in the class. mA = Total number of methods that access attribute a. sum(mA) = Sum of all mA for all attributes of the class.
Lower LCOM2 values indicate higher cohesion and better overall design. If the total number of methods or attributes is zero than the value of LCOM2 is undefined.
Lack of Cohesion 3 (LCOM3)
LCOM3 = (m – sum(mA)/a) / (m – 1) m = Total number of methods in the class. a = Total number of attributes in the class. mA = Total number of methods that access attribute a. sum(mA) = Sum of all mA for all attributes of the class.
LCOM3 values greater than one indicates low cohesion and should be addressed. If the total number of methods is less than two or the number of attributes is zero than the value of LCOM3 is undefined.
Measuring Coupling
While it is impossible to avoid some level of coupling within systems, the goal is to reduce coupling as much as possible. Below are three metrics that can be used to determine the level of coupling within a system.
Coupling Between Objects (CBO)
CBO = sum(t) t = Total number of types that are referenced by a particular class, not including any possible super-classes, primitive types or common framework classes.
Lower CBO values indicate lower coupling.
Data Abstraction Coupling (DAC)
DAC = sum(a) a = Total number of types that are used for attribute declarations, not including primitive types, common framework classes, or types that are inherited from any possible super-classes.
Lower DC values indicate lower coupling.
Method Invocation Coupling (MIC)
MIC = nMIC / (N – 1) N = Total number of classes defined within the project. nMIC = Total number of classes that receive a message from the target class.
Lower MIC values indicate lower coupling.
Demeter's Law
Demeter's Law is a design principle that when applied to object-oriented programming means that object A can reference object B but object A cannot use object B to reference object C. Complying with this principle prevents object A from knowing that object B uses object C thereby reducing coupling. If object A needs to access a function of object C then it is up to object B to expose an operation encapsulating the reference to object C. The following example [8] illustrates how this could be done.
public float calculateTotal(Order order) {
return order.getProducts().getTotalCost();
}
In the example object the object which implements calculateTotal() is calling getTotalCost on a Products object which is exposed through order. An alternative to this approach would be for the order object to expose this functionality as suggested by the following example.
public float calculateTotal(Order order) {
return order.getTotalCost()
}
public class Order {
// ...
public float getTotalCost() {
return products.getTotalCost();
}
// ...
}
Conclusion
Coupling and Cohesion go hand in hand. On one hand cohesion wants modules to do exactly a single task thus reduces problems and making a module handle itself while on the other hand coupling is introduced by how dependent the module is on other modules. Good programming desires high cohesion and low coupling - modules that does one task without affecting other modules while at the same time use as less data/objects from other modules as possible to have low coupling. Although low coupling is desirable, that doesn't mean high coupling doesn't have its uses. In some modules the programmer might want it to be tightly coupled for performance reasons - perhaps a series of modules dependent on each other to bring out the maximum output. A non-IT way of seeing this might be our human body - it is tightly coupled and parts/modules are all dependent on one and another - "removing or fixing" one will involve many other; Even though high coupling is bad but the human body functions much better tightly coupled than loosely coupled. So basically even though high cohesion and lose coupling is very desirable, the programmer has to view the design of the system to figure out what the best approach is.
See also
- Wikipedia: Cohesion
- Wikipedia: Coupling
- Software Engineeriing Presentation: Coupling and Cohesion
- More Cohesion Metrics
- More Coupling Metrics
- Example: Measuring Coupling and Cohesion
References
- http://blogs.ittoolbox.com/eai/implementation/archives/design-principles-cohesion-16069
- http://www.waysys.com/ws_content_bl_pgssd_ch06.html
- http://www.site.uottawa.ca:4321/oose/index.html#cohesion
- http://javaboutique.internet.com/tutorials/coupcoh/index-2.html
- http://bmrc.berkeley.edu/courseware/cs169/spring01/lectures/objects/sld001.htm
- http://blogs.ittoolbox.com/eai/implementation/archives/design-principles-coupling-data-and-otherwise-16061
- http://en.wikipedia.org/wiki/Cohesion_%28computer_science%29
- http://www.cs.sjsu.edu/faculty/pearce/modules/lectures/ood/metrics/Cohesion.htm
- http://class.ee.iastate.edu/berleant/home/Courses/SoftwareEngineering/CprE486fall2004/designModularity.htm
- http://www.eli.sdsu.edu/courses/spring99/cs535/notes/cohesion/cohesion.html#Heading8
- http://www.site.uottawa.ca:4321/oose/index.html#sequentialcohesion