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How do I write unit tests in Java?
A unit test checks a small piece of behavior without needing to launch the whole application. Choose the smallest useful boundary: a method or class whose input and output, or other observable effect, can be checked predictably. Prefer a real lightweight collaborator when it is simple to construct; introduce a mock when an external or complex collaborator would make the test slow, unpredictable, or irrelevant to the behavior under test.
JUnit 5 is a collection of components rather than one single library. The JUnit Platform launches test engines, Jupiter provides the programming model and extension support for new tests, and Vintage runs JUnit 3 and JUnit 4 tests on the Platform. The examples below use Jupiter. The official JUnit 5.12.0 guide specifies Java 8 or higher at runtime; check the compatibility requirements for the exact JUnit version and Java runtime your project uses.
A small test: setup, action, assertion
Suppose a price calculator applies a discount and rejects invalid inputs:
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public final class PriceCalculator {
public int discountedPrice(int price, int discountPercent) {
if (price < 0) {
throw new IllegalArgumentException("price must not be negative");
}
if (discountPercent < 0 || discountPercent > 100) {
throw new IllegalArgumentException("discount must be from 0 to 100");
}
return price - (price * discountPercent / 100);
}
}A Jupiter test can arrange the object, call the behavior, and assert the result:
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;
class PriceCalculatorTest {
@Test
void appliesDiscountToPrice() {
PriceCalculator calculator = new PriceCalculator();
int result = calculator.discountedPrice(100, 20);
assertEquals(80, result);
}
}The test name describes the behavior rather than the implementation. The assertion compares the expected value with the actual result, making a failure useful to read. Jupiter includes assertions, assumptions, exception handling, tags, lifecycle controls, and parameterized tests; a third-party assertion library is not required for a first test.
Test specified failure behavior
When an exception is part of the method’s contract, assert that exception directly. This checks both that failure occurs and that it occurs for the intended call:
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import static org.junit.jupiter.api.Assertions.assertThrows;
@Test
void rejectsNegativePrice() {
PriceCalculator calculator = new PriceCalculator();
IllegalArgumentException error = assertThrows(
IllegalArgumentException.class,
() -> calculator.discountedPrice(-1, 10)
);
assertEquals("price must not be negative", error.getMessage());
}Only assert details such as an exception message when that detail is meaningful and stable for callers. Avoid broad tests that pass merely because some unrelated line throws an exception.
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If the same rule should hold across several inputs, a parameterized test keeps the assertion in one place while making the cases visible:
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.CsvSource;
import static org.junit.jupiter.api.Assertions.assertEquals;
@ParameterizedTest
@CsvSource({
"100, 20, 80",
"50, 10, 45",
"80, 0, 80",
"80, 100, 0"
})
void calculatesDiscount(int price, int discountPercent, int expected) {
PriceCalculator calculator = new PriceCalculator();
assertEquals(expected, calculator.discountedPrice(price, discountPercent));
}Include cases that represent meaningful boundaries as well as ordinary values. Parameterized tests require the corresponding Jupiter parameterized-test support to be present in the project’s version-matched dependencies.
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How should test setup and lifecycle work?
By default, Jupiter creates a fresh test-class instance for each test method. This reduces the risk that mutable instance fields changed by one test will leak into another. Tests should still avoid order dependencies: each test needs to establish its own relevant state rather than relying on a previous test having run.
Use lifecycle methods for genuinely shared setup
@BeforeEach runs before each test method; @AfterEach runs after each one. Use them when setup or cleanup is common and clearer than repeating it. For example, a test class can create a fresh calculator in @BeforeEach if many tests use it. Do not move important test-specific conditions into hidden setup: a reader should be able to see what each test is checking.
Mutable state, shared files, clocks, databases, or network services can undermine independence even when each test gets a new class instance. Give each test controlled inputs and clean up resources it creates. Use nested tests when grouping cases by context makes the behavior easier to navigate; do not add nesting just to create more headings.
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How do I use JUnit 5 with Mockito?
Mockito is optional. It is useful when a class delegates to a collaborator and the test needs to control that collaborator’s response. The JUnit Jupiter extension integrates Mockito with Jupiter. In the following example, the service behavior depends on a repository result, so the mock isolates that boundary:
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.when;
@ExtendWith(MockitoExtension.class)
class GreetingServiceTest {
@Mock
GreetingRepository repository;
@Test
void returnsGreetingFromRepository() {
when(repository.findGreeting("en")).thenReturn("Hello");
GreetingService service = new GreetingService(repository);
String result = service.greetingFor("en");
assertEquals("Hello", result);
}
}This assumes the application has a GreetingRepository interface with findGreeting(String) and a GreetingService constructor and method matching the example. Mockito’s Jupiter extension and the Mockito API version must be available to the test runtime. The Mockito 5.17.0 API documents the extension and strict-stubbing facilities; follow the guidance for the version your project actually uses.
Mock behavior, not every implementation step
The example stubs a response and asserts the service’s result. Verifying an interaction is appropriate when that interaction itself is part of the contract—for example, a message must be sent exactly once. Otherwise, assertions about meaningful outputs or state are generally less brittle than checking every call made internally. Mockito’s default style is described as loose, with strictness features intended to improve debugging; avoid unnecessary stubs and keep the test’s intent clear.
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How do I run Java unit tests?
Use the route the project already supports so local runs and continuous integration discover the same tests. The JUnit Platform has support in common IDEs and build tools, including IntelliJ IDEA, Eclipse, NetBeans, VS Code, Gradle, Maven, and Ant. In an IDE, run the test class or method using the project’s JUnit configuration. In a build, use the repository’s documented test task or command rather than adding a second, unrelated test setup.
Check dependencies and discovery before changing code
JUnit’s 5.12.0 guide points to dependency metadata, build support instructions, and example projects for Gradle, Maven, and Ant. Dependency declarations vary with the build, selected JUnit version, and test-runner configuration, so confirm them against the actual project rather than pasting an unverified snippet. In particular, ensure Jupiter is available and the Platform can launch its engine at test runtime when the chosen setup requires it.
- Identify the project’s Java runtime and existing build tool, then check that the selected JUnit version supports them.
- Inspect the test dependencies and test task or IDE configuration. Confirm that the Jupiter programming model and required test engine are included.
- Run a single test class from the IDE or build tool. Then run the project’s normal test task to confirm the same tests are discovered in the full suite.
- If Mockito is needed, add its version-matched Jupiter integration to the test setup and run a test that exercises the extension.
Tell discovery failures from assertion failures
- No tests found or class not run: check the test source location, naming conventions, IDE runner, build test task, and whether the JUnit Platform and Jupiter engine are configured.
- Compilation or missing-class error: inspect test-scope dependencies, imports, and version alignment. A parameterized test also needs the matching Jupiter parameterized-test support.
- Assertion failure: the test ran, but actual behavior differed from its expected value. Check the input, setup, expected result, and whether the application behavior is correct.
- Mockito extension or annotation failure: verify that the Jupiter extension dependency matches the Mockito version and that the test class is registered with
@ExtendWith(MockitoExtension.class). - Tests pass alone but fail in a suite: look for shared mutable state, reliance on test order, or resources that are not reset or cleaned up.
How can I keep unit tests reliable and maintainable?
- Use deterministic inputs and avoid relying on real network services or uncontrolled external state.
- Give each test a descriptive name tied to a behavior or condition.
- Make at least one meaningful assertion about an observable result or specified failure.
- Keep tests independent, with explicit setup and appropriate cleanup.
- Use real simple collaborators where they make the test clearer; use mocks for boundaries that benefit from isolation.
- Keep JUnit, its engine, parameterized-test support, Mockito, and the project’s Java runtime version-compatible.
- When a test fails, first determine whether it was discovered and executed; then interpret compilation, configuration, and assertion failures separately.
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