One of the challenges when doing Property-based testing (PBT) is generating data... All that is shown using Java, Kotlin and [_jqwik_](https://jqwik.net); many ideas can be transferred to other languages and libraries, though. ## Table of Contents - [Introduction](#introduction) ## Introduction ### How to do PBT in Java I've already written a [whole blog series](https://blog.johanneslink.net/2018/03/24/property-based-testing-in-java-introduction/) on that topic, that's why I'm not covering the basics here. Reading [this article in Oracle's Java Magazine](https://blogs.oracle.com/javamagazine/know-for-sure-with-property-based-testing) will also give you an overview. #### How jqwik Works Ok, you're too ~~lazy~~ busy to read one of the articles above. Here's the bare minimum of [jqwik](https://jqwik.net)'s syntax that you should understand when you want to follow along. [Skip](#domain-data) without regret if you already know the fundamentals. ```java class JqwikExample { @Property void stringsAndEvenInts(@ForAll String anyString, @ForAll("evenNumbers") int anEvenInt) { System.out.println("anyString: " + anyString); System.out.println("anEvenInt: " + anEvenInt); } @Provide Arbitrary evenNumbers() { return Arbitraries.integers().filter(i -> i % 2 == 0); } } ``` Above you see a working and runnable but completely meaningless property. You can start it like any JUnit 5 platform test using Gradle, Maven or your favourite IDE. The resulting console output will look similar to: ``` anyString: 靐乺廛䷛㡡㝮屠 anEvenInt: 176926354 anyString: 孯⇳餖᭞ၖ anEvenInt: -568892912 anyString: 谏崢麊⃠栦ꆬㄵᥞꓨṯ跚პ疽퇺꨹冊뀃쁾䌬ص猽툲 anEvenInt: -2147483648 anyString: 俕⍍畽䂍㷼⬀∄髨⋏បᲱ anEvenInt: 10 ... ``` The library will generate values for all parameters annotated with `@ForAll`. It will either use the parameter's type - `@ForAll String anyString` - to invoke one of its various default generators. Or it will refer to a `@Provider` method specified by the annotations `value()`attribute: `@ForAll("evenNumbers") int anEvenInt` refers to the method named`evenNumbers`, which is supposed to return an instance of type `Arbitrary`. You will learn much more about the type `Arbitrary` and the diverse ways to implement and access arbitraries in the following sections. ## Domain Data Default generators for JDK types. Constraining default generators when a property or function is not total. - With existing annotations - With referring to methods that provide arbitraries: Arbitraries, which form a monadic type, can be considered to be a more abstract form of data generators. Arbitraries are typically created by using a DSL (domain-specific language) for specifying data generation. They can be invoked explicitly as above, they can be pre-registered for a given type, or they can be collected into _domains_ and applied whenever you formulate properties for this domain. Example: German Zip Codes - Generate Strings: StringArbitrary, filter, map - Wrap in ZipCode value class: Use map and exception in constructor. Disadvantage of filtering. Interlude about Embedded Shrinking Combining arbitraries into domain object: Person Example? Assumptions: - To restrict single parameter - To keep constraints across several parameters Example: Poker Dependencies between generated data Complex, nested domain types: - UseType - Builders Example: Spotify Recursive Data - Non-deterministic recursion - Deterministic recursion Advanced Stuff: - Data from Schema eg JSON Schema - Decision Trees: https://github.com/regehr/uniform-tree-sampling - Program Code: https://embed.cs.utah.edu/csmith/, https://github.com/intel/yarpgen, https://www.fuzzingbook.org/html/03_Syntactical_Fuzzing.html ## Summary ## Feedback Give feedback, ask questions and tell me about your own PBT experiences on [Twitter](https://twitter.com/johanneslink). ## Sharing, Code and License This article is published under the following license: [Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/) All code is [available at Github](https://github.com/jlink/model-based-testing)