Com S 362: Object-Oriented Analysis and Design Refactoring

Com S 362: Object-Oriented Analysis and Design 2 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

Com S 362: Object-Oriented Analysis and Design 3 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

Com S 362: Object-Oriented Analysis and Design 4 4 Recap: Responsibility Assignment Information Expert Creator Controller Polymorphism Pure Fabrication Indirection Protected Variations

Com S 362: Object-Oriented Analysis and Design 5 5 Eclipse Tutorial Open source IDE Available from Lot of interesting & advance features Demo: refactoring features in eclipse Extract method Pull up method Push down method

Com S 362: Object-Oriented Analysis and Design 6 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

Com S 362: Object-Oriented Analysis and Design 7 7 Value Argument Refactoring consumes resources Doesn’t add any new feature Impact on near term profit = 0 Why Improve Structure?

Com S 362: Object-Oriented Analysis and Design 8 8 Value Argument Readability More readable less chances of inadvertent errors Adding features in future will be less cumbersome Invest today, reap later Time  Understand- ability Cost/ feature

Com S 362: Object-Oriented Analysis and Design 9 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

Com S 362: Object-Oriented Analysis and Design 10 Com S 362: Object-Oriented Analysis and Design 10 Refactoring that you don't see Compiler optimizations Refactoring for performance improvement Example code: radius =..; while(index < Length){ array[index] = 2 * pi * radius; }

Com S 362: Object-Oriented Analysis and Design 11 Com S 362: Object-Oriented Analysis and Design 11 Loop Invariant Optimization radius =..; perimeter = 2 * pi * radius while(index < Length){ array[index] = perimeter; } Compiler optimizations go through precise verification

Com S 362: Object-Oriented Analysis and Design 12 Com S 362: Object-Oriented Analysis and Design 12 Common Refactoring Techniques Extract method Pull up method Push down method Consolidate conditional expressions Encapsulate Collection … Alphabetical list at:

Com S 362: Object-Oriented Analysis and Design 13 Com S 362: Object-Oriented Analysis and Design 13 Extract Method: The Problem Frequent Renter Point Movie Presentation Statement Logic

Com S 362: Object-Oriented Analysis and Design 14 Com S 362: Object-Oriented Analysis and Design 14 Pull Up Method/Field/Constructor Fowler: Refactoring, Page 322 Methods with identical results on subclasses Move the method to superclass

Com S 362: Object-Oriented Analysis and Design 15 Com S 362: Object-Oriented Analysis and Design 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

Com S 362: Object-Oriented Analysis and Design 16 Com S 362: Object-Oriented Analysis and Design 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 Fowler: Refactoring - Page 240 Checking Eligibility for disability

Com S 362: Object-Oriented Analysis and Design 17 Com S 362: Object-Oriented Analysis and Design 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

Com S 362: Object-Oriented Analysis and Design 18 Com S 362: Object-Oriented Analysis and Design 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

Com S 362: Object-Oriented Analysis and Design 19 Com S 362: Object-Oriented Analysis and Design 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?

Com S 362: Object-Oriented Analysis and Design 20 Com S 362: Object-Oriented Analysis and Design 20