1. Test Driven Lasse Koskela Chapter 3: Refactoring in Small Steps
Paul Ammann

2. Overview
Exploring a Potential Solution
Changing Design in a Controlled Manner
Taking the New Design Further
The Most Powerful Designs Are Always the Result of a Continuous Process of Simplification and Refinement.
8/10/2019

3. Recalling the Problem
Existing Design Replaced Variables Via Simple Matching
For all variables v, replace ${v} with its value:
result = result.replaceAll(regex, entry.getValue())
Failing sets template value to “${one}, ${two}, ${three}”
@Test
public void variablesGetProcessedJustOnce() throws Exception {
template.set(“one”, “${one}”);
template.set(“two”, “${three}”);
template.set(“three”, “${two}”);
assertTemplateEvaluatesTo(“${one}, ${three}”, ${two}); }
Tweaking the Current Design Won’t Make This Test Pass.
8/10/2019

4. Exploring A Potential Solution
Exploring a “Segment” Solution for Templates
Prototyping with Spikes
A Spike is A Detour to Learn
In Template Example, Learning is About regex
Learning by Writing Tests
Need to Figure Out An API?
Write Some Test Code That Uses The API
RegexlearningTest
Example Spike for Learning an API
Note Koskela’s Misunderstanding of the Role of Parentheses
Simple Here: We’re Finding Variables in a Template
Compiling Regex Patterns, Matching Expressions
Get Smarter On The Side, Then Apply
8/10/2019

5. Changing Design in Controlled Manner
Creating an Alternative Implementation
Starting With the Low Hanging Fruit
TDD Development of a Template Parser
Removing Duplication From Tests
Refactoring Is Always Important
Applying Learning From the Spike
Final Code Version (note Segment class, originally String)
private void append(String segment, StringBuilder result) {
if (isVariable(segment) { evaluateVariable(segment, result); } // dispatching 
else { result.append(segment);} }
Koskela Refactors Substantially
TemplateParse.java
Now We Can Parse; Next Step Is To Use Parser
8/10/2019

6. Changing Design in a Controlled Manner - Continued
Switching Over Safely
Adopting the New Implementation
Recoding the evaluate() method
Cleaning Up By Extracting Methods
Pull Out The Old Stuff That ‘s No Longer Relevant
Result is new Template Class
Template.java
No New Functionality, But Definitely Refactored
8/10/2019

7. Taking the New Design Further
Keeping Things Compatible
Building on Existing Functionality
Note equals(), hashCode() treatment in Segment classes
Any Worries About This?
Segment, PlainText , Variable
Refactoring Logic into Objects
Tell, Don’t Ask!
Motivation for Segment class
Making the Switchover
Getting Caught By the Safety Net
Adding Exceptional Behavior!
Deleting Dead Code and Further Clean Up
Test Set Makes Requirements Concrete
8/10/2019