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Troll of Fame - Java training

Getting started

./gradlew build

If you want to import in an IDE

No matter the IDE (Intellij or Eclipse), Lombok plugin must be installed in it.

Intellij (recommended)

  • Import Project
  • Select build.gradle.kts
    • Select Use default Gradle wrapper (recommended)
    • Leave everything else as is


  • Import projects...
  • Gradle
  • Existing Gradle Project
  • Pick the project root directory (e.g. /Users/Sir4ur0n/code/troll-of-fame-java)
  • Finish

Once upon a time...

The King of Trolls Gnonpom coded the Troll of Fame: a wonderful application that would help Trolls learning numbers when they are hunting. Gnonpom was a skilled Test Driven Developer who had just released Troll of Fame where all tests passed.

Unfortunately he was shot by a disgusting Elf.

Here come a new King, Hurrah for the great Troll Aklass!

This time it's for real, let the elf hunting contest begin!

At the end of each battle, the trolls want to compare the number and attributes of the slain elves. And with Troll of Fame it should be easy... Should.


Work with legacy code

You inherit an application that seems to work fine. Run ./gradlew build (•̀ᴗ•́)و

Read TrollTest and ElfTest as a first specification of the software.

Now uncomment the content of ElfProp and run tests again... Oops, seems like our unit tests were not so complete. (╥﹏╥) This is a limitation of unit tests: you don't test many values, and they are never truly random.

Now fix the implementation bug (replace + with * in the Elf value calculation) and check all tests now pass.

We will try to improve the quality of Troll of Frame thanks to Property Based Testing (PBT)!

Property testing

Property Based Testing (a.k.a. PBT) is about generating tests instead of manually writing them. Unlike unit tests where you know what goes in and what comes out (a.k.a. oracle tests), you assess properties that should always be true. The PBT library checks for arbitrary inputs that the property is true.

In Java, we use JUnit-QuickCheck library to write and run Property Based tests.

Step 1 - Configuration and Invariance


No matter the year, the 31st of December is a New Year's Eve

  • For a simpler start, we already configured the build dependencies and created generators for Elf and Troll in the test module
  • Create a Property Based Test file
import com.pholser.junit.quickcheck.Property;
import com.pholser.junit.quickcheck.runner.JUnitQuickcheck;
import org.junit.runner.RunWith;

public class TrollProp {

  public void invariance(Troll troll) {
    // ...
  • Did you notice the property test takes a Troll as input? That's where PBT shines! The library will run this test 100 times by default, and each time will pass a random Troll to it. We no longer care about building input data!
  • The first property test we will write aims to assess Invariance property: it means a property should always be true even if the input varies (e.g. the Troll)
  • As an example, no matter the troll, his score is always >= 0 (i.e. is never negative). Write a test (using AssertJ assertions, just like in unit tests) to check that
  • Does the test pass? You can run it just like unit tests, via Intellij or in CLI with ./gradlew build
  • What would the same check with regular unit tests look like?

📌 Most unit tests can actually be converted to Invariance properties

Another invariance property would be that all High elves' value must be an even number (because they have a multiplier of 2). Remember? This is the first property you uncommented!

Step 2 - Inverse

Inverse properties check that it's possible to transform some input to an output and back to the original input, no matter the input. This is a useful property because it guarantees some functions don't lose information and/or are consistent.


bar and foo are inverse of each other

  • For any troll and any elf, if the troll kills the elf and then realizes the elf survived, what should be the result?
  • Write an inverse property test to check that

Testing it will ensure that iGotOne and oopsHeSurvived are consistent.

Step 3 - Analogy

Analogous properties check that there are at least 2 different ways from any input to reach an output. This is a useful property because it guarantees some functions are consistent (can also be useful for refactors)


Adding any number to itself is the same as multiplying this number by 2

  • For any troll, any elf and any positive quantity of killed elves, what should be the difference between:
    • In JUnit-QuickCheck, you can annotate the quantity parameter with @InRange(minInt = 1, maxInt = 100) to constrain it
    • killing a single elf and repeating this operation quantity times
    • killing in a single strike quantity units of elf?
  • Write an analogous property test to check that

This ensures that iGotOne and iGot are consistent.


For refactors, copy the function to refactor, do your changes, then write an Analogy property test to check for any input that they return the same output, i.e. the refactor has no regression! Now you can delete the test and the legacy function, and rename the refactored function to the legacy name

Step 4 - Idempotence

Idempotent properties check that running a function once or several times leads to exactly the same result, i.e. an idempotent function brings to a stable state from which this function becomes useless. This is a useful properties of functions that clean data (e.g. trim whitespaces, replace dots with hyphens in a phone number, etc.).


Once a list of numbers is sorted, sorting it again doesn't change anything

  • For any troll and any elf, once all elfs have been resurrected, what should happen if these elfs are resurrected again?
  • Write an idempotent property test to check that

This ensures that allElvesOfAKindResurrected brings the troll killing list to a stable state.


More generally, function is idempotent if applying it to its own result doesn't change anything

[Bonus] Step 5 - Metamorphism

Metamorphic properties check that running a function with variants of the same input should lead to equal or consistent outputs. E.g. if the input is multiplied by 2, is the output also multiplied by 2? Divided by 2? The same?

  • For any troll and any elf, what should the troll score be compared to the score of the troll after killing elf?
  • Write a metamorphic property test to check that

This ensures that iGotOne correctly increases the kill list (and thus the score) when an elf is killed.

[Bonus] Step 6 - Injection

Injective properties check that different inputs lead to different outputs, i.e. there aren't 2 different inputs that lead to the same output, i.e. each output has at most 1 input. This is a useful property whenever you need to ensure an output can only be reached by a single input; e.g. a hash function, or a function that takes a person and returns its Social Security Number (imagine if 2 persons had the same SSN!)

  • For any troll and any 2 elves elf1 and elf2, assuming elf1 is different from elf2, troll after killing elf1 must be different from troll after killing elf2
  • Write an injective property test to check that

This ensures that iGotOne always updates the provided troll in a unique way.