Why Testing in a Nutshell

Goal of the lecture

• How can you trust that the changes did not destroy something?
• What is my confidence in the system?
• How do I write tests?
• What is unit testing?

Tests in a nutshell

• Tests represent your trust in the system
• Build them incrementally
  • Do not need to focus on everything
  • When a new bug shows up, write a test
• Even better write them before the code
  • Act as your first client, better interface
• Active documentation always in sync
• It has a cost, but pay off is big
  • writing them, maintain them

Testing Style

" The style here is to write a few lines of code, then a test that should run, or even better, to write a test that won't run, then write the code that will make it run. "

• write unit tests that thoroughly test a single class
• write tests as you develop (even before you implement)
• write tests for every new piece of functionality

Developers should spend 25-50% of their time developing tests.

But I can’t cover everything!

• Sure! Nobody can but
• When someone discovers a defect in your code, first write a test that demonstrates the defect.
• Then debug until the test succeeds.

Whenever you are tempted to type something into a print statement or a debugger expression, write it as a test instead. Martin Fowler

Unit Tests

• Unit tests ensure that you get the specified behavior of the public interface of a class
• Normally tests a single class

A Test

A test

• Create a context
• Send a stimulus
• Check the results

A Test

A test

• Create a context: Create an empty set
• Send a stimulus: Add twice the same element
• Check the results: Check that the set contains only one element

Set TestCase

TestCase subclass: # SetTestCase
    ...

SetTestCase >> testAdd 
  | empty |
  empty := Set new.                      "Context"
  empty add: 5.                             "Stimulus"
  empty add: 5. 
  self assert: empty size = 1.        "Check"

SetTestCase run: #testAdd

Good Tests

• Repeatable
• No human intervention
• "self-described"
• Change less often than the system
• Tells a story