Introduction to Error Handling, SUnit, Blackbox Testing, and Refactoring

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Introduction to Error Handling, SUnit, Blackbox
Testing, and Refactoring

• CS2340
• Fall 2003

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Notes from Last Time

• Code snippets at
• http//coweb.cc.gatech.edu/cs2340/3151
• Code changes do propagate across windows!
• I love progress!
• Alt-. is a user interrupt

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Exception Handling in Squeak

• Class Exception and its subclasses provided for
  use.
• Can emulate try/catch block in other languages
• Exceptions pass up inheritance hierarchy. If not
  handled, get default handler at Object.

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Example
x y x7. y0. x / y like a try
block on ZeroDivide exception class
do exception Transcript show
exception description cr. 0 .
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Example
f fFileStream fileNamed 'fred'. f
nextPutAll 'Test'. f close. on
FileDoesNotExistException do exception
Transcript show exception description. .
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Blackbox Testing

• Going to get the theory from 2335.
• Treat code as a blackbox
• Base test cases on specification
• Two major techniques
• Equivalence Partitions
• Boundary Value Tests
• Exhaustive testing not possible

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Equivalence Partitions

• Treat sets of data as same (if one test succeeds
  all tests succeed)
• For ranges, pick one test in range, one test on
  each end.
• For sets, pick one in set, one not in set.
• For unique values, pick value, not value.

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Examples

• Must accept years between 1-2050. Test cases 0,
  1876 , 2076.
• Passwords must be 6-8 chars long
• Test cases ab, abcdef, abcdegujswidn
• Leap years are divisible by four
• Test cases 16, 99
• Phone exchanges must not begin with 555.
• Test cases 555, 498

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Boundary Value Tests

• Most of the errors happen on the boundaries
  between sets
• Must accept years between 1-2050.
• Test cases 0, 1, 2050, 2051.
• Boundary Value Tests compliments Equivalence
  Partitions

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Writing your tests

• We could just write workspace code and DoIt over
  and over
• Hoping that we dont ever close window
• Checking transcript manually to ensure results
  are what we want.
• We could write a custom class to do our test and
  result checking
• We can use the SUnit framework.

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SUnit

• Testing framework for creating and running unit
  tests (regression tests) automatically in
  Smalltalk
• Originally developed by Kent Beck
• Implementations exist for many languages
• JUnit for Java

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SUnit Advantages

• Self-checking and reporting
• Check results against expected results
• Report which test is broken
• Built-in initialize before a test and clean-up
  after a test
• setUp is called before each method test
• tearDown is called after each method test

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When to use SUnit?

• Extreme Programming
• Create unit test before the code
• Helps you understand what the code must do
• Helps you to know when the code is done
• Used for regression testing
• Did you break anything when making changes?
• Maintenance
• Once you find a bug create a unit test to make
  sure it is fixed and stays fixed

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Using SUnit

• Derive class from TestCase
• Do any set up (initialize variables) in setUp
• Do any clean up in tearDown
• Create methods testXxxx
• Use
• self assert expr_that_evals_true
• self deny expr_that_evals_false
• self should block that evals to true .
• self shouldnt block that evals to false .
• TestRunner open.

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TestRunner
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Example
Assume we have a class Muppet with method
name aName nameaName. name. And
name name.
testName oscar oscar Muppet new.
oscar name 'oscar'. self assert oscar name
'Oscar'. self should oscar name 'Oscar'.
self deny oscar name 'Ocsar'.
TestCase subclass TestMuppet
instanceVariableNames classVariableNames
poolDictionaries category Muppet
Classes .
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Key steps

• For each real class X, have a test class TestX
  derived from TestCase
• Decide how to test functionality
• For functionality Y in class X, have a test
  method testY in class TestX.
• Run TestRunner to evaluate tests automatically
  (TestRunner open)
• May use SqueakMap to download TestBrowser if
  desired (more later)

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Testing Set Functionality
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Testing with Exceptions

• Check for exceptions
• self should boolean raise Exception
• Make sure no exceptions
• self shouldnt boolean raise Exception

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Example Testing with Exceptions
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Refactoring

• Improving the design
• To make it cheaper and easier to maintain
• Without adding functionality
• Change the current code to improve it
• Use unit tests to make sure it still works
• Key part of Extreme Programming
• Evolve the design
• Key to Maintenance
• Fix the ugly stuff

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Book

• Refactoring, Improving the Design of Existing
  Code
• By Martin Fowler
• ISBN 0-201-48567-2
• Uses Java
• But ideas can be used for any language

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Why dont People Refactor?

• Doesnt add functionality
• No cool new feature to show off
• People dont want to pay for it
• Afraid of breaking things that work
• Reduce this fear by unit tests
• Afraid of opening a can of worms
• Changing A leads to changing B, C, D, etc.

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What if you Dont?

• Code complexity grows
• Changes are harder to make
• Probability of error increases
• Code gets written to get around problems
• Harder for new people to understand
• Code gets duplicated

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Refactoring Goal

• Anybody can write code that a computer can read
• Good programmers write code that other humans can
  understand

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What makes code hard to read and maintain?

• Bad names (i, aValue, str)
• Good names arrayIndex, listSize, name
• Magic numbers (255, 32767)
• Long methods (contain lots o code)
• Things that dont make sense
• Methods in the wrong place
• Attributes in the wrong place
• Duplicating things
• And much more

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Extract class

• Example class Person has lots of stuff in it
  (name, credit card stuff, contact info etc)
• Problem Class gets big and hard to understand
• Solution Pull out related attributes and make a
  class (like CreditCard)

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Extract Method

• Problem class has a long method that is hard to
  understand
• Solution break the method up into more
  understandable parts

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Extract Method Process

• Pull out any duplicated code and make that a
  method
• Pull out cohesive pieces and make methods for
  each piece
• Be sure to move local variables as needed
• Pass some variables as parameters
• Modify the original method to call the piece
  methods

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Move Method

• Example Clock formatting time
• Problem a method exists on a class but uses data
  found in another class
• Solution move the method to the class that has
  the data the method works on

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Move Attribute

• Example SlideShow has a fileDirectory
• Problem Class has an attribute that it shouldnt
  have
• Solution Move the attribute to the class that
  should have it
• Use delegation to access from other classes

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Replace Conditional with Polymorphism

• Example Drawing Editor with Shape attribute
• Constants for Circle, Rectangle, Triangle
• Draw code checks type and then does correct
  drawing
• Problem conditional on constant is hard to
  extend
• Solution create subclasses Circle, Rectangle,
  Triangle and each overrides an abstract draw
  method in Shape

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SqueakMap

• Software package distribution mechanism with
  Squeak
• http//coweb.cc.gatech.edu/cs2340/1407
• http//minnow.cc.gatech.edu/squeak/2726
• World Menu -gt Open -gt Package Loader
• Asks if you want to install SqueakMap say yes
• Eventually fires up SM Package Loader

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Package Loader

• Select TestBrowser in SqueakMap
• Right-click, choose install from menu

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TestBrowser

• TestBrowser open

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Next on the Menu

• Read chapter 3
• Joe the Box simple 2D graphics
• UML
• Milestone 1 due next Tuesday!