Top 50 JUnit Interview Question and Answers

1. What is JUnit?

   • JUnit is a popular Java testing framework used for unit testing. It provides annotations to identify test methods, asserts for verifying expected results, and integration with tools like Maven or Gradle.

2. Why is JUnit important?

   • JUnit automates testing, ensures code reliability, promotes TDD (Test-Driven Development), and integrates easily with CI/CD pipelines.

3. Explain the lifecycle of a JUnit test.

   • Lifecycle includes:
     1. @BeforeAll (setup for all tests, runs once).
     2. @BeforeEach (setup before each test).
     3. Test methods (@Test).
     4. @AfterEach (cleanup after each test).
     5. @AfterAll (cleanup for all tests, runs once).

4. What are some key annotations in JUnit?

   • Common annotations:
     • @Test: Marks a test method.
     • @BeforeAll / @BeforeEach: Setup methods.
     • @AfterAll / @AfterEach: Teardown methods.
     • @Disabled: Disables a test.

5. How does JUnit assert equality?

   • Using methods like:
     • assertEquals(expected, actual).
     • assertNotEquals(expected, actual).

6. What is the difference between @BeforeEach and @BeforeAll?

   • @BeforeEach runs before every test, while @BeforeAll runs once before all tests.

7. What is assertThrows() in JUnit 5?

   • Validates that a specific exception is thrown during test execution.

   assertThrows(IllegalArgumentException.class, () -> { throw new IllegalArgumentException("Invalid argument"); });

8. How do you handle timeouts in JUnit tests?

   • Use the @Test(timeout = ms) annotation in JUnit 4 or @Test with assertTimeout in JUnit 5.

9. What is the difference between assertAll and individual asserts?

   • assertAll allows multiple assertions to execute even if one fails, collecting all errors.

10. How do you disable a test in JUnit?

    • Annotate the method with @Disabled.

11. How can you run tests in a specific order?

    • Use @TestMethodOrder(OrderAnnotation.class) and @Order(n) annotations in JUnit 5.

12. Explain parameterized tests in JUnit.

    • Allows running the same test with different data inputs using @ParameterizedTest.

13. What is the role of @ExtendWith?

    • Used to integrate extensions like Mockito, Spring, or custom behaviors into JUnit tests.

14. How does JUnit support dependency injection?

    • Using @ParameterizedTest and @ValueSource or integration with external frameworks like Spring.

15. What is JUnit’s role in TDD?

    • TDD relies heavily on frameworks like JUnit to write failing tests before implementing functionality.

16. How do you integrate JUnit with Maven?

-   Add the JUnit dependency in the `pom.xml` file:
    
    xml
    
    
    `<dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter</artifactId>
        <version>5.x.x</version>
        <scope>test</scope>
    </dependency>` 

1. What is the difference between JUnit 4 and JUnit 5?

-   JUnit 5 uses a modular architecture with better extension support, Java 8+ features, and new annotations like `@BeforeAll`/`@AfterAll`. 18.  **How do you test private methods with JUnit?**

-   Use reflection or redesign code to test through public interfaces. 19.  **How do you generate JUnit test reports?**

-   Use plugins like Surefire or Gradle to generate HTML/XML reports. 20.  **What is the difference between `@Mock` and `@Spy` in JUnit with Mockito?**

-   `@Mock`: Creates a mock object.
-   `@Spy`: Partially mocks an object, retaining real behavior.

1. What is the difference between assertArrayEquals and assertIterableEquals?

• assertArrayEquals compares arrays element by element to ensure they are equal.
• assertIterableEquals compares two iterable objects (e.g., lists) for equality.

Example:

assertArrayEquals(new int[]{1, 2}, new int[]{1, 2}); // Arrays assertIterableEquals(List.of(1, 2), List.of(1, 2)); // Lists

1. What are assumptions in JUnit?

• Assumptions allow tests to execute conditionally. If an assumption fails, the test is skipped.

Example:

@Test void testOnSpecificOS() { assumeTrue(System.getProperty("os.name").contains("Windows")); // Test code that runs only on Windows }

1. How do you disable tests conditionally in JUnit 5?

• Use annotations like @EnabledOnOs or @DisabledOnOs for OS-specific conditions, or @EnabledIf for custom conditions.

Example:

@EnabledOnOs(OS.WINDOWS) @Test void windowsOnlyTest() { assertTrue(true); }

1. What is the assertThat method in JUnit?

• assertThat provides advanced assertions using Hamcrest matchers, enabling more readable test conditions.

Example:

assertThat("JUnit", containsString("Unit"));

1. How do you write repeated tests in JUnit 5?

• Use the @RepeatedTest annotation to run a test multiple times.

Example:

@RepeatedTest(5) void repeatedTest() { System.out.println("This test runs 5 times"); }

1. What is the purpose of the @ParameterizedTest annotation?

• It allows testing with multiple sets of data inputs without duplicating code.

Example:

@ParameterizedTest @ValueSource(ints = {1, 2, 3}) void testWithMultipleInputs(int number) { assertTrue(number > 0); }

1. How do you use @MethodSource in parameterized tests?

• @MethodSource provides data for parameterized tests from a static method.

Example:

`static Stream provideData() { return Stream.of(Arguments.of(1, 2, 3), Arguments.of(4, 5, 9)); }

@ParameterizedTest @MethodSource(“provideData”) void testWithMethodSource(int a, int b, int result) { assertEquals(result, a + b); }`

1. What is the purpose of the @TestFactory annotation?

• It is used to create dynamic tests at runtime instead of static, predefined test methods.

Example:

@TestFactory Collection<DynamicTest> dynamicTests() { return List.of( DynamicTest.dynamicTest("Test 1", () -> assertTrue(true)), DynamicTest.dynamicTest("Test 2", () -> assertEquals(4, 2 + 2)) ); }

1. How can you group tests in JUnit 5?

• Use @Nested for hierarchical grouping and @Tag for categorization of tests.

Example with @Nested:

@Nested class MathTests { @Test void additionTest() { assertEquals(5, 2 + 3); } }

Example with @Tag:

@Tag("slow") @Test void slowTest() { assertTrue(true); }

1. What are test suites in JUnit?

• Test suites group multiple test classes and execute them together.
• JUnit 5 doesn’t provide built-in test suites but relies on tools like Maven Surefire or Gradle.

1. How do you use @Nested tests in JUnit?

• @Nested organizes related test cases within a class. It improves test readability and structure.

@Nested class InnerClassTest { @Test void testMethod() { assertTrue(true); } }

1. How do you mock objects in JUnit?

• Use frameworks like Mockito with @Mock to create mock objects and verify interactions.

@Mock private MyService service;

1. What is the difference between doReturn and thenReturn in Mockito?

• doReturn is used for stubbing methods with void return types, while thenReturn is used for non-void return types.

1. How do you mock static methods in JUnit?

• Use frameworks like PowerMock or Mockito’s mockStatic introduced in Mockito 3.4.

1. How do you verify method calls in JUnit?

• Use Mockito’s verify method:

  verify(mockService).methodName();

1. What is @Captor in Mockito?

• It is used to capture arguments passed to a mocked method.

1. How do you write tests for interfaces?

• Test the implementations of the interface or mock the interface and verify interactions.

1. What are assertions in JUnit, and why are they important?

• Assertions validate the expected output of the test. Examples include assertTrue, assertFalse, and assertEquals.

1. How do you write parameterized tests for multiple input combinations?

• Use @CsvSource or @CsvFileSource with @ParameterizedTest.

@ParameterizedTest @CsvSource({"1, 2, 3", "4, 5, 9"}) void testAddition(int a, int b, int expected) { assertEquals(expected, a + b); }

1. What is @Timeout in JUnit 5?

• It ensures a test does not exceed the specified execution time. If it does, the test fails.

@Test @Timeout(1) void testWithTimeout() throws InterruptedException { Thread.sleep(500); }

1. What is the role of @Condition in JUnit?

• Allows conditional test execution based on system properties, OS, or environment variables using @EnabledIf or @DisabledIf.

1. How do you integrate JUnit with Gradle?

• Add the JUnit dependency in build.gradle:

  testImplementation 'org.junit.jupiter:junit-jupiter:5.x.x'

1. How can JUnit tests be run in CI/CD pipelines?

• Integrate JUnit with Jenkins, GitHub Actions, or GitLab CI/CD to execute tests as part of the build process.

1. What plugins are used to generate reports for JUnit?

• Use Maven Surefire Plugin or Gradle Test Report for generating HTML/XML test reports.

1. What is the purpose of assertJ in JUnit?

• AssertJ provides a fluent API for assertions, making tests more readable.

1. Can JUnit be used for performance testing?

• JUnit itself is not ideal for performance testing but can integrate with tools like JMH (Java Microbenchmark Harness).

1. How do you test exception scenarios in JUnit?

• Use assertThrows for verifying expected exceptions.

assertThrows(NullPointerException.class, () -> { throw new NullPointerException(); });

1. What is the purpose of assertDoesNotThrow?

• Validates that a block of code does not throw any exception.

1. How do you mock void methods in JUnit?

• Use Mockito’s doNothing or doThrow methods.

doNothing().when(mockService).methodName();

1. What is the role of @Tag in JUnit?

• Tags categorize test cases, enabling selective execution.

@Tag("IntegrationTest") @Test void testIntegration() { assertTrue(true); }

1. How do you test collections in JUnit?

• Use assertIterableEquals or assertArrayEquals.