Clean Property-Based Contract Tests

Clean Property-based Contract Tests

Dr. Frank Raiser Konzept Informationssysteme GmbH Overview

nProperty-based Testing

Predicate Logic, Contracts, Invariants

How Property-based Testing works nProperty-based Contracts

Definition and Application of contract tests nQuality Assurance

Detect untested contracts

nequals implementation should be

Reflexive, Symmetric, Transitive, Consistent, Null-safe

F.ex. Symmetry: ∀�∀�. (� ≠ �� ∧ � ≠ ��) → �� , � ↔ �� , �

Predicate logic Equals is a uses ∧ for logical predicate that and (&&) needs to be satisfied.

nInterfaces define contracts nMany prominent examples in Java

Comparator#compare, Comparable#compareTo

List#contains, List#remove nNo choice – you have to agree to that contract

java.lang.IllegalArgumentException: Comparison method violates its general contract!

How do you test these contracts?

nCalculation of the premium factor for a car insurance

Adapted from “Developer Testing” by Alexander Tarlinder [ISBN-13: 978-0-13-429106-2]

nPotential indicators:

Younger persons are more likely to have accidents

Female drivers are less likely to claim insurance

Interface and different implementations for premium factors

Represents someone who can request an insurance

1. Create a “test” person Factor == 1.75 2. Run the calculator

Factor == 1.0

AgePremiumCalculator 3. Verify the

correct factor Factor == 1.35

4. Repeat

What new Person(Gender.FEMALE, new Age(19)) about these?

new Person(Gender.FEMALE, new Age(35))

new Person(Gender.FEMALE, new Age(73))

nElderly drivers should get a premium factor of 1.35 ∀�. � �� ∧ �. �� ≥ 60 → �� = 1.35

Factor == 1.35

nHow to get a Person ?

nGenerators do that

Configurable selection of appropriate generator

Configuration of generator Generators can be straightforward possible as well (f.ex. only female persons)

Language Library License

C Theft ISC License https://github.com/silentbicycle/theft Clojure test.check Eclipse Public License https://github.com/clojure/test.check Erlang Triq – Trifork QuickCheck for Erlang Apache 2.0 https://github.com/krestenkrab/triq Haskell QuickCheck BSD 3 https://hackage.haskell.org/package/QuickCheck

Java junit-quickcheck MIT https://github.com/pholser/junit-quickcheck

Javascript qc.js Revised BSD https://bitbucket.org/darrint/qc.js .NET FsCheck Revised BSD https://github.com/fscheck/FsCheck Python factcheck Apache 2.0 https://github.com/npryce/python-factcheck

Scala ScalaCheck Revised BSD http://scalacheck.org/

nMany tests with little code nRandom input data

nBetter readability Can be reproduced (seed) no guarantee for edge-cases Property explicitly formulated

Fewer tests needed nGenerators needed

No object construction code can also get complex

nReusable generators nRuntime penalty

Falsifications tried 100 times

Product of base class factor and other calculators’ factors

Standard PremiumCalculator Factor == 1.925 Factor == 1.1 (male 1.1 * young 1.75) (male)

nVarious factors involved, then a new business rule:

All factors must be within the range of 0.5 and 2.0

‒ Essentially a contract on the PremiumCalculator interface

Codifies contractual agreement for all interface implementations

2017-06-22 Clean Property-based Contract Tests © 2017 Konzept Informationssysteme GmbH 15 Clean Property-based Contracts How much effort and code is it to Explicit verify the adherence contract for an to contract implementation class?

Provides test subject

Sufficient for 100 test cases against the contract!

nBasePremiumCalculator should satisfy the contract

nLSP requires derived classes to adhere to contract

Can be easily tested now

But: What if a developer forgot to add the contract test?

2017-06-22 Clean Property-based Contract Tests © 2017 Konzept Informationssysteme GmbH 17 Validate Define contracts contract adherence jQAssistant is a QA tool which allows the definition and validation of project specific rules on a

structural level. [Source: jqAssistant docs] Define LSP rules

jqassistant any Continuous Integration system Analyze source

Detect contracts Execute tests to Detect verify all contracts classes for for all contracts implementations

Ensure Test classes exist

Ensure contract is

Neo4j’s Cypher Interface query syntax must end in “Contract” and have some @Property

Mark as a contract

* The precise rules may need to be Create tweaked in project- relationship to specific ways (f.ex. If you want a base class the type the for contracts instead contract of interface w/ applies to default methods)

Query types for which contract applies and their test classes

Make sure test class adheres to the contract

nExplicit contract definitionsnEffort to create generators nTiny amount of code nLarge amount of tests nReusable generators being executed nSafety-net against nQuality (dependent on accidentally ignoring generators and contracts properties) may be misleading

Konzept Informationssysteme GmbH Pfarrer-Weiss-Weg 12 89073 Ulm

Dr. Frank Raiser Software Entwicklungsingenieur Tel.: +49 731 1403434 – 51 Fax.: +49 731 1403434 – 34 [email protected] www.konzept-is.de Thank you

Special thanks to pholser for extending junit-quickcheck to make this work Paul Holser

[All images Public Domain.]