good methods
DESCRIPTION
Good Methods. Cohesion and Coupling. For structured design These software metrics were used extensively Proven to be effective For object-oriented design Not clearly understood or effectively used No historic proof of effectiveness. Cohesion. - PowerPoint PPT PresentationTRANSCRIPT
Slide 1
Good Methods
Slide 2
Cohesion and Coupling
l For structured design• These software metrics were used
extensively• Proven to be effective
l For object-oriented design• Not clearly understood or effectively used• No historic proof of effectiveness
Slide 3
Cohesion
l Measure of degree of interaction within a module
l Measure of the strength of association of the elements inside a module
l Functionality inside a module should be so related that anyone can easily see what the module does
l Goal is a highly cohesive module
Slide 4
Levels of Cohesion in Structured Design
l Functional cohesion (Good)l Sequential cohesionl Communicational cohesionl Procedural cohesionl Temporal cohesionl Logical cohesionl Coincidental cohesion (Bad)
Slide 5
Comparison of Cohesion Levelsfor Structured Design
Cohesion Level Cleanliness of Implementation
Reusability Modifiability Understand-ability
Functional Good Good Good Good
Sequential Good Medium Good Good
Communicational Good Poor Medium Medium
Procedural Medium Poor Variable Variable
Temporal Medium Bad Medium Medium
Logical Bad Bad Bad Poor
Coincidental Poor Bad Bad Bad
Slide 6
Cohesion
l For structured design• Deals with the cohesion of the actions in
a module (unit) to perform one and only one task
l For object-oriented methods• Deals with the ability of a module to
produce only one output for one module
Slide 7
Levels of Cohesion for Object-oriented Methods
l Functional cohesion (Good)l Sequential cohesionl Communicational cohesionl Iterative cohesionl Conditional cohesionl Coincidental cohesion (Bad)
Slide 8
Functional Cohesion for Object-oriented Methods
l Only one output exists for the modulel Ideal for object-oriented paradigm
Slide 9
Functional Cohesion for Object-oriented Methods
public void deposit (double amount)
{
balance = balance + amount;
}
Slide 10
Sequential Cohesion for Object-oriented Methods
l One output is dependent on the other output
l Modifications result in changing only one instance variable
Slide 11
Sequential Cohesion for Object-oriented Methods
public double withdraw (double amount, double fee){ amount = amount + fee;
if (amount < 0)System.out.println (“Error: withdraw amount is
invalid.”);else if (amount > balance)
System.out.println (“Error: Insufficient funds.”);else
balance = balance – amount;return balance;
}
Slide 12
Communicational Cohesion for Object-oriented Methods
l Two outputs are dependent on a common input
Slide 13
Communicational Cohesion for Object-oriented Methods
public void addCD (String title, String artist, double cost, int tracks)
{if (count = = collection.length)
increaseSize ( );collection [count] = new CD (title, artist, cost, tracks);totalCosts = totalCosts + cost;count++;
}
Slide 14
Iterative Cohesion for Object-oriented Methods
l Two outputs are iteratively dependent on the same input
Slide 15
Iterative Cohesion for Object-oriented Methods
void formDet (float Equations[2][3], float x[2][2], float y[2][2], float D[2][2])
{for (int Row = 0; Row < 2; ++Row)
for (int Col = 0; Col < 2; ++Col){
x[Row][Col] = Equations[Row][Col];y[Row][Col] = Equations[Row][Col];D[Row][Col] = Equations[Row][Col];
}x[0][0] = Equations[0][2];x[1][0] = Equations[1][2];y[0][1] = Equations[0][2];
y[1][1] = Equations[1][2];}
Slide 16
Conditional Cohesion for Object-oriented Methods
l Two outputs are conditionally dependent on the same input
Slide 17
Conditional Cohesion for Object-oriented Methods
public boolean checkBookIn ( ){
if (this.isAvailable ( )){ //this object cannot be checked out
System.out.println (“Error: “ + callNumber + “ is not checked out”);
return false;}else{
dueDate = null;availability = true;return true;
}}
Slide 18
Coincidental Cohesion for Object-oriented Methods
l Two outputs have no dependence relationship with each other and no dependence relation on a common input
Slide 19
Coincidental Cohesion for Object-oriented Methods
public void readInput ( ){
System.out.println (“Enter name of item being purchased: “);name = MyInput.readLine ( );System.out.println (“Enter price of item: “);price = MyInput.readLineDouble ( );System.out.println (“Enter number of items purchased: “);numberBought = MyInput.readLineInt ( );
}
Slide 20
Coincidental Cohesion for Object-oriented Methods
public String AcceptItemName ( ){
System.out.println (“Enter name of item being purchased: “);name = MyInput.readLine ( );return name;
}
Slide 21
Cohesion Decision Tree for Object-oriented Methods
Functional Cohesion yes
Does the module modify fewer than 2 object variables Sequential Cohesion yes no Do all modifications actually result in the change to only one variable Communicational Cohesion yes no Are the output(s) dependent on common input but not derived in a loop or a
select statement Iterative Cohesion yes
no Are the output(s) dependent on common input and are they used in a loop Conditional Cohesion yes no Are the output(s) dependent on common input and are they used in a selection no Coincidental Cohesion
Slide 22
Coupling
l Measure of degree of interaction between two modules
l Measure of interdependence of modulesl Goal is to have so little coupling that
changes can be made within one module without disrupting other modules
Slide 23
Types of Coupling for Structured Methods
l Data (Good)l Stampl Controll Commonl Content (Bad)
Slide 24
Comparison of Coupling Typesfor Structured Design
Coupling Type Susceptibility to Ripple Effect
Modifiability Understand-ability
Module’s Usability in Other Systems
Data Variable Good Good Good
Stamp Variable Medium Medium Medium
Control Medium Poor Poor Poor
Common Poor Medium Bad Bad
Content Bad Bad Bad Bad
Slide 25
Data Coupling for Structured Methods
l Just the data needed is passed between the calling and called modules through calling parameters
Slide 26
Data Coupling for Structured Methods
Compute Bill
Calculate Interest
InterestBill Amount
Slide 27
Stamp Coupling for Structured Methods
l Whole data structure being passed as a parameter to a module but the module needs only part of data in the data structure
l Types of stamp coupling:• natural
• table
• record
• bundle
Slide 28
Control Coupling for Structured Methods
l One module passes a flag as parameter to control the logic in the other module
l Parameter being passed is a control flag which dictates what action to perform within the module
l Example:void ProcessTransactions (bool whichFlag,
DataRec data, bool& successFlag);
Slide 29
Common Coupling for Structured Methods
l Use of global datal Two modules have access to the same
datal Why not?
• Hard to maintain (ripple effect)• Decreases reusability• Hard to debug
• Data change• Module change
Slide 30
Content Coupling for Structured Methods
l One module reaches into the internal code of another module to grab or deposit data or to control its function
Slide 31
Coupling Decision Tree for Structured Methods
No Coupling yes
Is there no data shared between modules?
Data Coupling no Is simple data or a data structure
passed from one module yes to another with all the structure elements used? Stamp Coupling Are only portions of the data no structure being passed actually yes used by the module?
no Control Coupling Does one module pass a flag yes
to control the logic of the other module?
Common Coupling no Does one module read or write yes to the global data?
no Common Coupling Does one module reference yes The contents of another module?
Slide 32
Coupling Example
p
r s
q
t u
1
2
3 4
5 6
# in out
1 Aircraft type Status flag
2 --- List of aircraft parts
3 Function code
---
4 --- List of aircraft parts
5 Part number Part manufacturer
6 Part number Part name
p, t, and u access the same database in update mode
Slide 33
Coupling Example
p q r s t u
p Data Data or Stamp
Common Common
q Control Data or stamp
r Data
s Data
t Common
u
Slide 34
Levels of Coupling for Object-oriented Methods
l No coupling (Good)l Sequential couplingl Computational couplingl Conditional couplingl Common couplingl Content coupling (Bad)
Slide 35
No Coupling for Object-oriented Methods
l No global data sharing or functional calls between two modules
Slide 36
Sequential Coupling for Object-oriented Methods
l Two modules exist where the outputs of one module are the inputs of another
Slide 37
Computational Coupling for Object-oriented Methods
l Two modules exist where one module passes a parameter to another module and the parameter has control or data dependence on the output
Slide 38
Conditional Coupling for Object-oriented Methods
l One module passes a parameter to another module and the parameter has control dependence on an output
Slide 39
Common Coupling for Object-oriented Methods
l One module writes to the global data and another module reads from the global data
Slide 40
Content Coupling for Object-oriented Methods
l One module references the contents of another module
Slide 41
Coupling Decision Tree for Object-oriented Methods
Slide 42
Coupling Decision Tree for Object-oriented Methods
From this point forward, Design Reviews will be done using these metrics.