Evolving Software, Conventional Wisdom - Slides Notes | CMSC 433, Study notes of Programming Languages

Download Evolving Software, Conventional Wisdom - Slides Notes | CMSC 433 and more Study notes Programming Languages in PDF only on Docsity! 1 CMSC 433 Programming Language Fall 2004 Refactoring October 14, 2004 Lots of material taken from Fowler, Refactoring: Improving the Design of Existing Code 2 Administrivia Project 3 coming out Midterm October 28 3 Evolving Software Problem The requirements of real software often change in ways that cannot be handled by the current design Moreover, trying to anticipate changes in the initial implementation can be difficult and costly Solution Redesign as requirements change Refactor code to accommodate new design 4 Example (p204) Replace Magic Number with Symbolic Constant double potentialEnergy(double m, double h) { return m * 9.81 * h; } becomes... static final double G = 9.81; double potentialEnergy(double m, double h) { return m * G * h; } 5 Some Motivations for This Refactoring Magic numbers have special values But why they have those values is not obvious So we like to give them a name Magic numbers may be used multiple times Easy to make errors May make a typo when putting in a number May need to change a number later (more digits of G) 6 Conventional Wisdom: The Design is Fixed Software process looks like this: Step 1: Design, design, design Step 2: Build your system Once you re on step 2, don t change the design! You might break something in the code You need to update your design documents You need to communicate your new design with everyone else 7 What if the Design is Broken? You re kind of stuck Design changes are very expensive When you re cleaning up the code, you re not adding features Result: An inappropriate design Makes code harder to change Makes code harder to understand and maintain Very expensive in the long run 8 Refactoring Philosophy It s hard to get the design right the first time So let s not even pretend Step 1: Make a reasonable design that should work, but... Plan for changes As implementers discover better designs As your clients change the requirements (!) But how can we ensure changes are safe? 17 What Code Needs to be Refactored? Bad code exhibits certain characteristics that can be addressed with refactoring These are called smells Different smells suggest different refactorings 18 Feature Envy A method seems more interested in a class other than the one it is actually in E.g., invoking lots of get methods Move Method Move method from one class to another Extract Method Pull out code in one method into a separate method 19 Move Method Should other methods also be moved? What about sub- and superclasses? What about access control (public, protected)? 20 Extract Method Are you ever going to reuse this new method? Local variable scopes? Extra cost of method invocation? void printOwning(double amt) { printBanner(); System.out.println( name + name); System.out.println( amount + amt); } void printDetails(double amt) { System.out.println( name + name); System.out.println( amount + amt); } void printOwning(double amt) { printBanner(); printDetails(amt); } 21 Long Method A method is too long. Long methods are harder to understand than lots of short ones. Can decompose with Extract Method Replace Temp with Query Remove code that assigns a method call to a temporary, and replace references to that temporary with the call Replace Method with Method Object Use the command pattern to build a closure 22 Replace Temp with Query Local variables make it hard to use some refactorings, e.g., Extract Method What about performance? double basePrice = num * price; if (basePrice > 1000) return basePrice * 0.95; else return basePrice * 0.98; double basePrice() { return num * price; } if (basePrice() > 1000) return basePrice() * 0.95; else return basePrice() * 0.98; 23 Switch Statements Usually not necessary in delegation-based OO programming Replace Type Code with State/Strategy Define a class hierarchy, a subclass for each type code Replace Conditional with Polymorphism Call method on state object to perform the check; switching is based on dynamic dispatch 24 Replace Type Code with State/Strategy 25 Replace Conditional with Polymorphism double getSpeed() { switch (kind) { case EUROPEAN: return getBaseSpeed(); case AFRICAN: return getBaseSpeed()-loadFactor()*numberOfCoconuts; case NORWEGIAN_BLUE: return (isNailed) ? 0 : getBaseSpeed(voltage); throw new RuntimeException( Should be unreachable ); } } 26 Duplicated Code The same expression used in different places in the same class Use Extract Method to pull it out into a method The same expression in two subclasses sharing the same superclass Extract Method in each, then PullUp method into parent Duplicated code in two unrelated classes Extract Class - Break a class that does too many things into smaller classes 27 Pull Up Method Might do other refactorings if methods don t quite match What if doesn t appear in all subclasses? 28 Extract Class How do we decide what goes in new class? Do fields still need to be accessed in orig class? 37 Other Bad Smells (cont d) Alternative Classes with Different Interfaces Methods do the same thing but have different interfaces Incomplete Library Class Library code doesn t do everything you d like Data Class Classes that act as structs, with no computation Refused Bequest Subclass doesn t use features of superclass 38 Other Bad Smells (cont d) Comments! If code is heavily commented, either It s very tricky code (e.g., a hard algorithm), or The design is bad, and you re trying to explain it When you feel the need to write a comment, first try to refactor the code so that any comment becomes superfluous. 39 Refactoring with Tools Many refactorings can be performed automatically This reduces the possibility of making a silly mistake Eclipse provides support for refactoring in Java http://www.eclipse.org 40 More information Textbook: Refactoring by M. Fowler Catalog of refactorings: http://www.refactoring.com/catalog/index.html Refactoring to patterns http://industriallogic.com/xp/refactoring/ This document was created with Win2PDF available at http://www.daneprairie.com. The unregistered version of Win2PDF is for evaluation or non-commercial use only.