Unit Testing

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Scott Leberknight
6/28/2019
(Unit) Testing

Why Test?

Does your code work?
How do you know?

Tests provide some level of conﬁdence
your code does what you think it does…

…which is not the same as saying
the code is correct
from a business perspective

Tests cannot and will not ever
catch or prevent all possible problems

You might have a passing test suite but
still have logic bugs…
e.g. due to poorly deﬁned or
understood requirements, poor
...

“In the 737 Max, only one of the ﬂight
management computers is active at a time -
either the pilot’s computer or the copil...

“When the two computers disagree, the
solution for the humans in the cockpit is  
to look across the control panel to see ...

“This means that if a particular angle-of-
attack sensor goes haywire - which happens
all the time in a machine that alter...

“The primary cause of this discrepancy was that one piece of ground
software supplied by Lockheed Martin produced results ...

The act of testing makes you consider more
than just the “happy path”

Many Flavors…
“Unit” Integration
Performance
Functional (QA)
Security
User interface
Other…?Exploratory

Good testing…
Automated
Repeatable
Continuous
(“ARC” testing)

Good unit testing…
Fast (a few milliseconds?)
Isolated (no “real” dependencies)
Consider success & failure modes

What is a “unit” test?
Not always clearly deﬁned
A single class? A single function?
Can it touch external resources?
(e.g....

@Test
fun `contribution should be sum of salary & Roth deferrals`() {
assertThat(payroll.totalContribution)
.isEqualTo(pay...

Potential Benefits
Find regressions quickly
Conﬁdence
Better design (*)
(*) probably…
You can go faster in the long run

Potential Pitfalls
False conﬁdence
Tests can have bugs too…
(*) debatable, but not in my personal experience…
You might be...

What to test?
“Business” logic…
Persistence logic…
User interfaces…
“Anything that can possibly break” ???

What to test?
Test your code…
…not your dependencies

What to test?
More generally you should assume
libraries, frameworks, operating systems,
etc. have been tested and work as...

How to test?
TDD (Test-Driven Development aka “test ﬁrst”)
TAST (test at same time, commit only tested code)
Test after (a...

“Legacy Code”
“To me, legacy code is simply
code without tests”
- Michael Feathers,
Working Effectively with Legacy Code

“TDD”
Test-driven development
Usually associated with “test ﬁrst”
Write failing test, write simplest code
to make it pass,...

“TDD”
Can you be test-driven without test ﬁrst?
I think you can…
By letting tests drive the design…
And guide towards “tes...

Tests & Design
What does “testable” code mean?
* Loosely coupled, e.g. injecting dependencies
* Separation of concerns
* S...

Tests & Design
What does “testable” code look like?
* Smaller classes in OO languages
* Smaller methods/functions (7-10 li...

Tests & Design
Why do these things make code testable?
* Facilitates “mocking” dependencies, e.g. a credit
card processing...

Testing Techniques
Prefer smaller, targeted tests (less coupling)
Mock objects (but don’t overdo it)
Perform assertions on...

Testing Techniques
Property-based testing
Test “units” and mock dependencies
Generative testing
Measuring code “covered” b...

Example test using a mock

@ExtendWith(DropwizardExtensionsSupport.class)
class RelationshipResourceTest {
private static final RelationshipService S...

Testing Async Code
No sleeps!
Mock the environment
“If we go by the book…hours could seem like days…”
Wait for conditions
...

Unit vs Integration Tests
* A unit test of a single HTTP endpoint accepts
HTTP requests, but uses mock dependencies, e.g.
...

Code Coverage
How much is “enough”?
Should automated builds fail if
below a threshold? 70%? 80%?
Can/should you test all e...

Code Coverage
By deﬁnition, decoupled, “testable” code
is easier to achieve higher coverage
The goal should not be a numbe...

Key Takeaways…

Gain conﬁdence that code is “correct”
Automated, repeatable, continuous (ARC)
Tests cannot catch all problems
Better desig...

(THE END)

Suggested Reading
Clean Code, Robert “Uncle Bob” Martin
Working Effectively with Legacy Code, Michael Feathers
Pragmatic P...

Suggested Websites
JUnit, de-factor Java unit testing framework, https://junit.org/junit5/
Mockito, mocking library, https...

Boeing 737 MAX
https://commons.wikimedia.org/wiki/File:WS_YYC_737_MAX_1.jpg
https://creativecommons.org/licenses/by-sa/4.0...

Mars Climate Orbiter
https://en.wikipedia.org/wiki/Mars_Climate_Orbiter#/media/
File:Mars_Climate_Orbiter_2.jpg
Public dom...

My Info
sleberknight at
fortitudetec.com
www.fortitudetec.com
@sleberknight
scott.leberknight at
gmail

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