Patterns 1 - Advanced Software Engineering

Last Time – This time
! Program refactoring ! Design patterns basics
Advanced Software Engineering Topics




Design Patterns - 1




Dr John Kanyaru 1 2




Refactoring

Program Refactoring  Refactoring
 Program transformation that preserves semantics
 Improves software qualities
 Understandability, extensibility, reusability


Introduction  Why is refactoring important?
 Keep up with software evolution
 It may be necessary to improve design to support further
functionality


Dr JM Kanyaru 3 4

, Simple Refactoring So What?
for ( int radius=1, radius < 10; radius++ ) {
area = 3.1416 * radius * radius;  What was gained?
System.out.printf(“area radius %d= %f“,radius,area);
}  Understandability
 By looking at the code you can tell better what the
computation is doing
for ( int radius=1,radius< 10; radius++ )
area = computeArea(radius);  Anything lost?
System.out.printf(“area radius %d= %f“,radius,area);
}
 May be some performance
double computeArea(int radius) {  Short loop and compilers are smart (inlining)
return 3.1416 * radius * radius;
}

5 6




Another Simple Refactoring So What? (Again)
double computeArea(int radius) {
return 3.1416 * radius * radius;
 What was gained?
}  Understandability
 By looking at the code you can tell better what the
computation is doing
 Performance
static final double PI = 3.1416;  Maybe – possible compiler optimization, specially if
double computeArea(int radius) { variable is used in several places
return PI * radius * radius;
}  Anything lost?
 …

7 8

, Insights (1) Insights (2)
 Refactorings are relatively small changes
 A refactoring focus on one change at a time  To apply a refactoring certain conditions should hold
 Several refactorings can be applied to a program incrementally  Example: There should be a magic number in a
computation

program program’ program’’
 Each refactoring is an algorithm that consists of
several steps
refactoring refactoring
 Example:
 Declare a static final field with a meaningful name
 Initialize the field with the magic number
 Substitute the magic number in the original computation with
the new field


9 10




Insights – Recap Why Refactoring?

 Refactorings are relatively small changes  Improve design of software
 A refactoring focus on one change at a time
 Improve understandability
 Several refactorings can be applied to a program
incrementally
 Bug finding
 To apply a refactoring certain conditions should hold

 Each refactoring is an algorithm that consists of  Improve programming productivity
several steps  Code faster – because all of the above

11 12