Unit Testing with JUnit 6
Explore how to implement automated unit testing with JUnit 6 to ensure code reliability. Learn to write fast, isolated tests with assertions for behavior verification, handle exceptions, manage test lifecycle with setup and teardown methods, and improve test clarity using annotations like @DisplayName and @Disabled. Develop skills to confidently refactor and maintain Java applications using modern testing standards.
Up until now, we have verified our code by running a main method and manually checking the output printed to the console. While this works for small scripts, it becomes unsustainable as our applications grow. If we change a piece of logic in one module, we cannot easily know if we broke something elsewhere without manually running every scenario again.
Automated unit testing solves this. It gives us a permanent safety net, allowing us to refactor and add features with the confidence that we haven’t introduced bugs into existing functionality.
Why do we write unit tests?
Unit testing means writing automated tests that validate the behavior of a specific unit of code. In practice, we isolate the smallest testable unit of the application, typically a function, method, or class, and assert that it produces the expected output for given inputs. Unit tests should be fast and isolated. They should not depend on external systems such as databases, network services, or file systems.
Manual testing creates a slower feedback loop and depends heavily on human repetition and attention to detail. We write code, build it, start the application, manually trigger the feature, and inspect the results. With automated unit tests, we can validate a class’s expected behavior across defined scenarios in milliseconds. This tight feedback loop supports the Red-Green-Refactor cycle: we write a failing test (Red), implement the minimal code to pass it (Green), then refactor the implementation while keeping the tests green.
Writing your first JUnit 6 test
As of 2026, the industry standard for testing in Java is JUnit 6. Released in late 2025, JUnit 6 unifies the versioning of the platform and sets the baseline requirement to Java 17+. While it introduces powerful new internal features, the code you write remains largely compatible with earlier versions, making it easy to learn.
A JUnit test is simply a Java class containing methods annotated with @Test. A true unit test must self-validate; it should either pass (Green) or fail (Red) without human intervention. We achieve this using Assertions, specifically the org.junit.jupiter.api.Assertions class.
To see this in action, imagine we have a simple Calculator class that we need to verify.