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Refactoring

Refactoring. Recap: Class Diagrams. Class diagrams represent design structure Three parts: name, attribute, operations Visibility, attribute type, multiplicity Association, association multiplicity Generalization i.e. interface impl, subclassing Composition i.e. class A contains class B

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Refactoring

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  1. Refactoring

  2. Recap: Class Diagrams • Class diagrams represent design structure • Three parts: name, attribute, operations • Visibility, attribute type, multiplicity • Association, association multiplicity • Generalization i.e. interface impl, subclassing • Composition i.e. class A contains class B • Applied information hiding, DSM, layering

  3. Recap: Interaction Diagrams • Represent functionality • Two major parts: • Life line boxes (represent life time of an object) • Messages between objects (instances) • Also looked at some special case notations • Instances: parameterized instances, singleton instances, array elements • Messages: creation, destruction, conditional messages, mutually exclusive conditional messages, asynchronous messages, polymorphic messages • Abstractions: looping, iteration over collection, nesting of frames, decomposition

  4. Recap: Responsibility Assignment • Information Expert • Creator • Controller • Polymorphism • Pure Fabrication • Indirection • Protected Variations

  5. Eclipse Tutorial • Open source IDE • Available from http://www.eclipse.org • Lot of interesting & advance features • Demo: refactoring features in eclipse • Extract method • Pull up method • Push down method

  6. Refactoring • Reorganize instructions in a programs, perhaps rewriting • Key: Preserve semantics of the program • Objective: Improve structure (aka design), improve readability • Doesn’t add/remove any functionality

  7. Value Argument • Refactoring consumes resources • Doesn’t add any new feature • Impact on near term profit = 0 • Why Improve Structure?

  8. Value Argument • Readability • More readable less chances of inadvertent errors • Adding features in future will be less cumbersome • Invest today, reap later Understand-ability Cost/ feature Time 

  9. Is semantics preserved? • Informal & imprecise verification: Write test suite • Rigorous test suite development before refactoring • Test after you refactor • Question: Why do I call it imprecise verification? • Formal verification: Prove that certain transformation always preserves the semantics

  10. Refactoring that you don't see • Compiler optimizations • Refactoring for performance improvementExample code:radius = ..;while(index < Length){array[index] = 2 * pi * radius;}

  11. Loop Invariant Optimization radius = ..;perimeter = 2 * pi * radiuswhile(index < Length){array[index] = perimeter;} • Compiler optimizations go through precise verification

  12. Common Refactoring Techniques • Extract method • Pull up method • Push down method • Consolidate conditional expressions • Encapsulate Collection • … • Alphabetical list at: http://www.refactoring.com/catalog/index.html

  13. Extract Method: The Problem Statement Logic Movie Frequent Renter Point Presentation

  14. Pull Up Method/Field/Constructor • Methods with identical results on subclasses • Move the method to superclass Fowler: Refactoring, Page 322

  15. Push Down Method • Behavior on a superclass is relevant only for some of its subclasses. • Move the behavior to the subclass Fowler: Refactoring - Page 328

  16. Consolidate Conditional Expression double disabilityAmount() { if (_seniority < 2) return 0; if (_monthsDisabled > 12) return 0; if (_isPartTime) return 0; // compute the disability amount double disabilityAmount() { if (isNotEligableForDisability()) return 0; // compute the disability amount Checking Eligibility for disability Fowler: Refactoring - Page 240

  17. Encapsulate Collection • Method returns a collection • Breaks encapsulation • Allows clients to manipulate collection without objects’ knowledge • Make a read-only view, provide methods to add remove elements Fowler: Refactoring - Page 208

  18. Summary • Improve the structure of code • No value gain at the moment • Easier to add features later • Less chances of errors in maintenance tasks • Key is to preserve semantics • Imprecisely ensure that by developing tests • Also, by code inspection

  19. Exercise • Can some refactoring techniques change the design structure matrix view of the system? • If yes, which refactoring techniques change the DSM view and how?

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