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Practice: Improve Test Expressiveness

Ready to practice improving test expressiveness with AssertJ? We've prepared a test class with verbose JUnit assertions that you can refactor.

Exercise: DiscountCalculatorTest​

The DiscountCalculatorTest class in the orders module demonstrates common assertion anti-patterns:

What you'll find​

  • ❌ Verbose JUnit assertions: assertEquals, assertTrue, assertNotNull
  • ❌ Manual BigDecimal comparisons: Using compareTo() in assertions

Your task​

Refactor the tests to use AssertJ for better expressiveness.

Hints:

  • AssertJ uses a fluent API that starts with assertThat(actualValue).
  • Look at the Poor expressiveness examples for patterns.
  • BigDecimal comparisons have special assertion methods that are more readable than compareTo().
  • Multiple assertions on the same object can be chained together.
  • The .as() method can add descriptive messages to your assertions.
  • Check the reference table below for common patterns.

Running the test​

You can run the test in your IDE or use the following commands:

# Run the test
mvn test -pl orders -Dtest=DiscountCalculatorTest

# Or run all tests in the orders module
mvn test -pl orders

Automated refactoring​

You can use OpenRewrite to automatically migrate JUnit assertions to AssertJ:

mvn -U org.openrewrite.maven:rewrite-maven-plugin:run \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-testing-frameworks:RELEASE \
  -Drewrite.activeRecipes=org.openrewrite.java.testing.assertj.Assertj

What you'll learn​

By completing this exercise, you'll gain hands-on experience with:

  • Converting verbose JUnit assertions to expressive AssertJ assertions.
  • Using AssertJ's fluent API for better readability.
  • Handling BigDecimal comparisons elegantly.
  • Writing assertions that clearly convey test intent.
  • Creating better failure messages.