Test-Driven Development: Advantages, Examples, and Refactoring Techniques

1. Chapter 21 Test-Driven Development CS6359 Fall 2011 John Cole

2. Write the Tests First • Tests are written before the code • Developer writes unit tests for nearly all production code CS6359 Fall 2011 John Cole

3. Advantages • The tests actually get written • Programmer satisfaction leads to more consistent test writing • Clarification of detailed interface and behavior • Provable, repeatable, automated verification • Confidence to change things CS6359 Fall 2011 John Cole

4. Example • To test the Sale class, create a SaleTest class that: • Creates a Sale (the thing to be tested; a fixture) • Adds some line items to it with the makeLineItem method • Asks for the total and verifies that it is the expected value CS6359 Fall 2011 John Cole

5. Pattern of Testing Methods • Create the fixture • Do some operation you want to test • Evaluate the results • Point: Don’t write all of the tests for Sale first; write one test method, implement the solution in Sale to make it pass, then repeat CS6359 Fall 2011 John Cole

6. Using xUnit • Create a class that extends the TestCase class • Create a separate testing method for each Sale method you want to test. This will generally be every public method. CS6359 Fall 2011 John Cole

7. Refactoring • A structured, disciplined method to rewrite or restructure existing code without changing its external behavior, applying small transformations and re-testing each step. • Unit tests applied after each step ensure that the changes didn’t cause a problem CS6359 Fall 2011 John Cole

8. Goals of Refactoring • Remove duplicate code • Improve clarity • Make long methods shorter • Remove the use of hard-coded literal constants CS6359 Fall 2011 John Cole

9. Code Smells • Duplicated code • Big methods • Class with many instance variables • Class with lots of code • Strikingly similar subclasses • Little or no use of interfaces • High coupling CS6359 Fall 2011 John Cole

10. Basic Refactorings • Extract method – Convert a long method into shorter ones by moving part of it into a helper method • Extract constant – Replace a literal with a constant variable • Introduce explaining variable – put the results of an expression into a temporary variable with a name that explains its purpose • Replace constructor call with Factory Method – CS6359 Fall 2011 John Cole

11. More Refactorings • Change method parameters (changes them in the definition, then finds all references) • Replace using new with a call to a method that returns an object. (Factory pattern) CS6359 Fall 2011 John Cole

12. Code example Public class Die { public static final int MAX=6; public intfaceValue; public Die(){ roll(); } Public void roll() { faceValue = (int)((Math.Random() * MAX) + 1; } Public intgetFaceValue() { return faceValue; } } CS6359 Fall 2011 John Cole

13. Another Example • rollDice on 392 CS6359 Fall 2011 John Cole