JUnit Interview Questions and Answers

1. What is JUnit?

   • Answer: JUnit is a unit testing framework for Java. It's used to write and run repeatable automated tests for individual units of code (e.g., methods or classes) to ensure they behave as expected.

2. What are the key features of JUnit?

   • Answer: Key features include annotations (@Test, @Before, @After, @BeforeClass, @AfterClass, @Ignore), assertions (assertEquals, assertTrue, assertFalse, etc.), test runners, and support for test suites.

3. Explain the difference between @Before and @BeforeClass annotations.

   • Answer: @Before runs before each test method in a test class, while @BeforeClass runs only once before all test methods in a test class. @Before is suitable for setting up resources needed for individual tests, while @BeforeClass is for setting up resources that are used across all tests (like a database connection).

4. What is the purpose of the @Test annotation?

   • Answer: The @Test annotation marks a method as a test method. JUnit runner identifies and executes methods with this annotation.

5. What are assertions in JUnit? Give examples.

   • Answer: Assertions are methods used to verify the expected outcome of a test. Examples include: assertEquals(expected, actual), assertTrue(condition), assertFalse(condition), assertNull(object), assertNotNull(object).

6. What is a Test Suite in JUnit?

   • Answer: A Test Suite is a collection of test cases grouped together to run as a single unit. It allows running multiple test classes or methods as one test execution.

7. How do you handle exceptions in JUnit tests?

   • Answer: Use the `@Test(expected = ExceptionType.class)` annotation to assert that a specific exception is thrown. Alternatively, use a try-catch block within the test method and assert that the exception was caught.

8. Explain the difference between JUnit 4 and JUnit 5.

   • Answer: JUnit 5 is a major update introducing a new programming model, improved extensibility, and support for various testing styles. It's more modular and offers better integration with other frameworks. JUnit 4 relies heavily on annotations, while JUnit 5 provides more flexible extension mechanisms.

9. What is the role of the JUnit runner?

   • Answer: The JUnit runner is responsible for discovering and executing test methods. It manages the lifecycle of tests, including setting up and tearing down resources.

10. How can you perform parameterized tests in JUnit?

    • Answer: JUnit 5 uses `@ParameterizedTest` and `@CsvSource`, `@MethodSource`, or `@ValueSource` annotations to run the same test with multiple sets of input data. JUnit 4 used `@RunWith(Parameterized.class)` and a custom parameter provider.

11. What is the purpose of the @Ignore annotation?

    • Answer: The @Ignore annotation is used to temporarily exclude a test method from execution. Useful when a test is failing and needs to be fixed later or when a test is not yet implemented.

12. How do you write a test for a method that returns a void?

    • Answer: You can test side effects. This might involve verifying changes to a database, file system, or the state of other objects that the method interacts with.

13. What are some best practices for writing JUnit tests?

    • Answer: Write clear and concise test names, keep tests independent (avoid side effects), follow FIRST principles (Fast, Independent, Repeatable, Self-Validating, Thorough), aim for high test coverage, and use meaningful assertions.

14. How do you test private methods?

    • Answer: You generally shouldn't directly test private methods. Focus on testing the public interface and the overall behavior of the class. If you find you *must* test a private method, consider refactoring your code to make it accessible (often a sign of poor design).

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