Junit Interview Questions

Checkout Vskills Interview questions with answers in Junit to prepare for your next job role. The questions are submitted by professionals to help you to prepare for the Interview.

Q.1 How can you perform data-driven testing in JUnit 5 using external data sources like databases?

JUnit 5 provides the @Sql annotation to set up the database state before executing a test method and to clean it up after the test completes.

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Q.2 Explain the use of the @RegisterExtension annotation in JUnit 5.

@RegisterExtension allows you to register test extensions directly on fields in test classes.

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Q.3 How do you test concurrent code with JUnit?

You can use libraries like JUnit Pioneer or ConcurrentUnit to test concurrent code in JUnit.

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Q.4 What are the advantages of using JUnit Jupiter (JUnit 5) over JUnit 4?

JUnit Jupiter offers several new features like nested tests, dynamic tests, improved parameterized tests, and better extension support compared to JUnit 4.

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Q.5 Explain the purpose of the @TestMethodOrder annotation in JUnit 5.

@TestMethodOrder allows you to specify the order in which test methods are executed within a test class.

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Q.6 How can you test Spring Boot applications with JUnit 5?

You can use the @SpringBootTest and @RunWith(SpringRunner.class) annotations to test Spring Boot applications with JUnit 5.

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Q.7 What is the role of the @ExtendWith annotation in JUnit Jupiter (JUnit 5)?

@ExtendWith is used to register extensions for a test class or method in JUnit Jupiter.

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Q.8 How do you create and use custom annotations in JUnit tests?

You can create custom annotations in Java and then use them along with JUnit annotations to add custom behavior or metadata to your test methods.

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Q.9 What is the purpose of using the @DirtiesContext annotation in Spring Boot testing?

@DirtiesContext is used to signal that the Spring application context is dirty after a test, requiring it to be refreshed or recreated before the next test.

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Q.10 Explain the concept of test slicing in Spring Boot testing.

Test slicing allows you to test only specific layers or components of your Spring Boot application by using annotations like @DataJpaTest, @WebMvcTest, etc.

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Q.11 How do you use JUnit 5's @TestInfo parameter in test methods?

@TestInfo is used to access information about the currently executing test method, such as display name, tags, and test class.

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Q.12 What is the role of the @ParameterizedTest and @CsvSource annotations in JUnit 5?

@ParameterizedTest is used to indicate that a test method is a parameterized test, and @CsvSource provides comma-separated test data for parameterized tests.

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Q.13 Explain the use of the @TestConfiguration annotation in Spring Boot testing.

@TestConfiguration allows you to define additional beans or override existing beans for tests.

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Q.14 What are the differences between JUnit 4's @RunWith and JUnit 5's @ExtendWith?

While both annotations serve similar purposes, @RunWith is used to specify custom runners in JUnit 4, and @ExtendWith registers extensions in JUnit 5.

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Q.15 How do you test Spring Data JPA repositories with JUnit and Spring Boot?

You can use the @DataJpaTest annotation to test Spring Data JPA repositories with JUnit and Spring Boot.

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Q.16 What are parameterized tests with JUnitParams in JUnit 4, and how are they useful?

JUnitParams is a library for JUnit 4 that simplifies parameterized testing by allowing you to use annotations like @ParametersMethod and @Parameters.

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Q.17 How do you write JUnit 5 tests for reactive applications using Project Reactor or RxJava?

You can use the StepVerifier from Project Reactor or TestSubscriber from RxJava to test reactive flows.

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Q.18 Explain the concept of test templates in JUnit 5.

Test templates allow you to define a generic test structure with placeholders for dynamic input values using the @TestTemplate and @ValueSource annotations.

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Q.19 What are the differences between JUnit 4's @Rule and JUnit 5's @ExtendWith?

Both @Rule and @ExtendWith allow you to add additional behavior to your tests, but @Rule is limited to JUnit 4, while @ExtendWith is part of JUnit 5's extension model.

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Q.20 How do you write JUnit 5 tests for asynchronous code using CompletableFuture or Future objects?

You can use the assertTimeout method with CompletableFuture.join() or Future.get() to test asynchronous code.

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Q.21 Explain the usage of the @Commit and @Rollback annotations in Spring Boot testing.

@Commit is used to commit the transaction after a test, while @Rollback rolls back the transaction.

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Q.22 What is the purpose of using the @MockBean annotation in Spring Boot testing?

@MockBean is used to create mock implementations of beans to replace actual beans during testing.

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Q.23 How do you test the security aspects of your Spring Boot application with JUnit?

You can use @WithMockUser or Spring Security Test's @MockUserDetails annotations to test security-related aspects.

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Q.24 What is the purpose of using the @WebFluxTest annotation in Spring Boot testing?

@WebFluxTest is used to test the Spring WebFlux components of a Spring Boot application.

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Q.25 Explain the concept of the @MockMvc in Spring Boot testing.

@MockMvc is a part of Spring's testing support and provides a way to perform integration tests for Spring MVC applications.

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Q.26 How do you test error responses and status codes in Spring Boot with JUnit?

You can use the andExpect method with MockMvcResultMatchers to verify error responses and status codes in Spring Boot tests.

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Q.27 What are parameterized tests with JUnitParams in JUnit 5, and how are they different from JUnit 4's parameterized tests?

JUnitParams can be used with both JUnit 4 and JUnit 5, providing a more concise and readable approach to parameterized testing compared to JUnit 4's approach.

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Q.28 Explain the purpose of using the @DataMongoTest annotation in Spring Boot testing.

@DataMongoTest is used to test Spring Data MongoDB repositories along with embedded MongoDB instances.

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Q.29 How do you perform integration testing with external dependencies in Spring Boot using JUnit?

You can use the @SpringBootTest annotation with a custom @TestConfiguration to replace or mock external dependencies during integration tests.

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Q.30 What is the role of the @DynamicTest annotation in JUnit 5?

@DynamicTest is used to define dynamic tests programmatically in JUnit 5.

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Q.31 Explain the concept of test slice layers in Spring Boot, and when would you use them.

Test slice layers, such as @DataJpaTest, @WebMvcTest, etc., allow you to test specific slices of your application without loading the entire application context.

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Q.32 How do you use @NestedTestConfiguration in JUnit Jupiter (JUnit 5)?

@NestedTestConfiguration allows you to configure the test instance lifecycle for inner nested test classes.

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Q.33 What is the purpose of using the @SqlGroup annotation in Spring Boot testing?

@SqlGroup allows you to define multiple SQL scripts to be executed before and after a test method or a test class.

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Q.34 Explain the usage of the @DisplayNameGeneration annotation in JUnit 5.

@DisplayNameGeneration allows you to specify a custom display name generator for test classes and methods.

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Q.35 How do you test database-related functionality in Spring Boot applications using an in-memory database?

You can use embedded databases like H2 or an in-memory configuration of your primary database for testing.

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Q.36 What are the differences between JUnit 5's @TestFactory and @TestTemplate?

Both @TestFactory and @TestTemplate are used for dynamic test generation, but they differ in how they provide dynamic test cases.

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Q.37 Explain the purpose of using the @SpringBootTest annotation in Spring Boot testing.

@SpringBootTest loads the complete Spring application context and allows you to test the application in an integrated environment.

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Q.38 How do you use @RestClientTest in Spring Boot testing for testing REST clients?

@RestClientTest is used to test REST clients and provides a way to auto-configure only the necessary components for testing REST communications.

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Q.39 What is the purpose of using the @DisplayName annotation in JUnit 5?

@DisplayName allows you to provide custom names to your test classes and test methods, making test results more descriptive and readable.

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Q.40 Explain the concept of testing transactions with JUnit in Spring Boot.

In Spring Boot, you can use @Transactional and @Rollback annotations to test methods involving database transactions.

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Q.41 How do you use the @Profile annotation in Spring Boot testing to control bean creation?

@Profile allows you to specify the active profiles during testing, which controls the creation of beans based on the active profile.

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Q.42 What are the differences between JUnit 5's @BeforeAll and @BeforeClass from JUnit 4?

Both annotations are used to execute methods before running tests, but @BeforeAll is used for static methods in JUnit 5, whereas @BeforeClass is used for static methods in JUnit 4.

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Q.43 Explain the purpose of using the @Mock annotation in JUnit with Mockito.

@Mock is used to create a mock object for a class or interface when using Mockito in JUnit tests.

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Q.44 How do you perform mocking in JUnit with Mockito?

To perform mocking with Mockito, you use annotations like @Mock to create mock objects and @InjectMocks to inject them into the tested class.

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Q.45 What is the role of the @Import annotation in Spring Boot testing?

@Import is used to import additional configuration classes or components into the Spring Boot application context during testing.

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Q.46 Explain the concept of conditional test execution with JUnit 5.

Conditional test execution allows you to control whether a test method should be executed based on specific conditions using custom annotations.

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Q.47 How do you perform security testing on Spring Boot applications with JUnit?

You can use annotations like @WithMockUser and @WithAnonymousUser to simulate different user roles and test security-related aspects.

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Q.48 What is the purpose of using the @JsonTest annotation in Spring Boot testing?

@JsonTest is used to test JSON serialization and deserialization of Spring Boot components.

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Q.49 Explain the use of the @WithMockUser annotation in Spring Boot testing.

@WithMockUser is used to simulate a mock authenticated user during testing with specific roles and authorities.

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Q.50 How do you use the @FlywayTest annotation in Spring Boot testing for database migration testing?

@FlywayTest is used to test database migrations during Spring Boot tests.

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Q.51 What is the purpose of using the @MockBean annotation with Spring Boot's TestRestTemplate?

@MockBean is used to mock RESTful services when using Spring Boot's TestRestTemplate for integration testing.

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Q.52 Explain the usage of the @PropertySource annotation in Spring Boot testing.

@PropertySource is used to load external property files during testing, allowing you to override properties in different environments.

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Q.53 How do you test Spring Boot applications that use NoSQL databases like MongoDB with JUnit?

You can use @DataMongoTest to test MongoDB repositories and @EmbeddedMongoTest to test with an embedded MongoDB instance.

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Q.54 Explain the purpose of using the @WebIntegrationTest annotation in Spring Boot testing.

@WebIntegrationTest is used to test the complete web stack, including web controllers and interactions with web clients, in Spring Boot applications.

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Q.55 What are the differences between @Test, @TestFactory, and @ParameterizedTest in JUnit 5?

@Test: Marks a test method. @TestFactory: Marks a method that returns dynamic tests using DynamicTest instances. @ParameterizedTest: Marks a method for parameterized testing using argument sources.

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Q.56 Explain the usage of @ConditionalOnProperty in Spring Boot testing.

@ConditionalOnProperty is used to conditionally enable or disable beans or configurations based on property values.

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Q.57 How do you perform load testing with JUnit and Spring Boot?

You can use tools like JMeter or Gatling along with JUnit to perform load testing on Spring Boot applications.

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Q.58 What is the purpose of using the @SpringMockkBean annotation in Spring Boot testing with MockK?

@SpringMockkBean is used to create mock beans for MockK when using Spring Boot tests.

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Q.59 Explain the concept of the @TestConfiguration class in JUnit 5.

@TestConfiguration is used to provide additional configuration for a specific test class, overriding or adding to the application's primary configuration.

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Q.60 How do you use JUnit 5's @BeforeEach and @AfterEach with dependency injection in test classes?

You can use dependency injection with @BeforeEach and @AfterEach to inject resources before and after each test method execution.

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Q.61 What is the purpose of using the @EnableConfigurationProperties annotation in Spring Boot testing?

@EnableConfigurationProperties is used to enable the binding of external properties to configuration classes in Spring Boot.

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Q.62 Explain the usage of the @WebAppConfiguration annotation in Spring Boot testing.

@WebAppConfiguration is used to indicate that the ApplicationContext loaded for testing is a web application context.

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Q.63 How do you test WebSocket-based functionality in Spring Boot applications with JUnit?

You can use libraries like MockWebSocketSession along with @WebMvcTest or @WebFluxTest to test WebSocket functionality.

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Q.64 What is the purpose of using the @Sql annotation in Spring Boot testing with an embedded database?

@Sql is used to specify SQL scripts to be executed before or after test methods or classes to set up the database state.

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Q.65 Explain the usage of the @ConstructorBinding annotation in Spring Boot testing with configuration properties.

@ConstructorBinding is used to create immutable configuration properties by binding them to constructor parameters.

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Q.66 How do you perform mocking in JUnit 5 with extensions instead of annotations?

You can implement a custom extension that provides mock objects for test classes.

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Q.67 Explain the concept of environment profiles in Spring Boot testing.

Environment profiles allow you to define and activate specific configurations based on different environments (e.g., dev, test, prod).

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Q.68 What is the role of the @MockK annotation in JUnit tests using MockK?

@MockK is used to create a mock object for a class or interface when using MockK in JUnit tests.

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Q.69 What is JUnit?

JUnit is a widely used open-source testing framework for Java applications. It provides a platform to write and execute unit tests to ensure code correctness and functionality.

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Q.70 What are the different annotations used in JUnit?

@Test: Indicates that the method is a test method. @Before: Specifies a method to be executed before each test method. @After: Specifies a method to be executed after each test method. @BeforeClass: Specifies a method to be executed once before any test method in the class. @AfterClass: Specifies a method to be executed once after all test methods in the class.

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Q.71 Explain the JUnit test lifecycle.

The JUnit test lifecycle involves the following steps: Setup: @Before annotated method is executed to prepare the test environment. Test: @Test annotated method is executed to perform the test. Teardown: @After annotated method is executed to clean up after the test.

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Q.72 How do you assert in JUnit tests?

JUnit provides various assert methods in the org.junit.Assert class to verify expected outcomes: assertEquals(expected, actual): Checks if two values are equal. assertTrue(condition): Checks if a condition is true. assertFalse(condition): Checks if a condition is false. assertNotNull(object): Checks if an object is not null. assertThrows(exceptionClass, executable): Checks if a specific exception is thrown during the execution of a block of code.

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Q.73 What is the purpose of using @RunWith annotation in JUnit?

@RunWith allows you to specify a custom test runner. It is used when you need to extend JUnit's behavior or integrate with other frameworks.

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Q.74 Explain the difference between JUnit 4 and JUnit 5.

JUnit 4 uses annotations like @Test, @Before, etc., while JUnit 5 introduces new annotations like @Test, @BeforeEach, @AfterEach, etc. JUnit 5 allows nested test classes, making the test structure more organized. JUnit 5 supports parameterized tests out of the box, whereas JUnit 4 requires external libraries for this functionality.

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Q.75 How do you perform parameterized testing in JUnit 5?

In JUnit 5, you can use the @ParameterizedTest annotation along with @CsvSource, @ValueSource, or other custom sources to provide test data to a test method.

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Q.76 Explain the usage of JUnit assumptions.

JUnit assumptions, provided by org.junit.Assume, allow you to skip test execution based on certain conditions. If an assumption fails, the test is ignored, but not marked as a failure.

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Q.77 What is the purpose of using @Disabled annotation in JUnit?

The @Disabled annotation is used to temporarily disable a test method or an entire test class. This can be helpful when certain tests are not ready or are failing temporarily.

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Q.78 How do you mock objects in JUnit tests?

You can use mocking libraries like Mockito or EasyMock to create mock objects. These libraries allow you to simulate the behavior of objects and their methods, enabling isolated testing.

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Q.79 What is the purpose of using JUnit Test Suites?

A JUnit Test Suite allows you to group multiple test classes into a single test suite, making it easier to execute them collectively.

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Q.80 How do you test for expected exceptions in JUnit 4?

In JUnit 4, you can use the expected attribute of the @Test annotation to specify the expected exception for a test method.

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Q.81 How do you test for expected exceptions in JUnit 5?

In JUnit 5, you can use the assertThrows() method to verify that a specific exception is thrown during the execution of a block of code.

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Q.82 Explain the concept of test fixtures in JUnit.

Test fixtures refer to the set of preconditions that must be satisfied before a test method can be executed. @Before and @BeforeEach methods are used to set up test fixtures.

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Q.83 What is the role of the @Ignore annotation in JUnit?

The @Ignore annotation (or @Disabled in JUnit 5) is used to skip the execution of a particular test method or test class.

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Q.84 How can you run JUnit tests from the command line?

JUnit tests can be executed from the command line using tools like Maven or Gradle, which support running tests from the build process.

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Q.85 What are parameterized tests, and when would you use them?

Parameterized tests allow you to run the same test with different input values. They are useful when testing the same functionality with multiple sets of test data.

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Q.86 Explain the concept of test-driven development (TDD) and how JUnit supports it.

Test-driven development (TDD) is a development approach where tests are written before the actual code. JUnit helps by providing a framework to write and run tests to ensure code correctness incrementally.

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Q.87 What are the different assert methods available in JUnit 5?

JUnit 5 provides various assert methods, such as assertEquals, assertNotEquals, assertSame, assertNotSame, assertArrayEquals, etc., to verify expected outcomes.

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Q.88 What is the purpose of using @DisplayName in JUnit 5?

@DisplayName allows you to provide custom names to your test methods, making test results more descriptive and readable.

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Q.89 How do you handle timeouts in JUnit tests?

JUnit provides the @Timeout annotation to specify a maximum time limit for the execution of a test method. If the method exceeds this limit, the test will fail.

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Q.90 Explain the use of JUnit @Rule and TestWatcher.

@Rule allows you to define custom test rules that can modify the behavior of individual tests. TestWatcher is a base class for creating custom test watchers to listen to test execution events.

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Q.91 What is the purpose of using the @ParameterizedTest annotation in JUnit 5?

The @ParameterizedTest annotation is used to indicate that a test method is a parameterized test that receives input values from various sources.

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Q.92 How can you group and categorize tests in JUnit?

JUnit provides test tagging and filtering mechanisms, such as @Tag annotation and JUnitPlatform configuration files, to group and categorize tests.

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Q.93 What is the use of the @ExtendWith annotation in JUnit 5?

@ExtendWith is used to register extensions (custom classes that extend the behavior of JUnit) for a test class or method.

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Q.94 How do you perform test parameterization using external sources like CSV files in JUnit?

JUnit 5 provides the @CsvFileSource and @CsvSource annotations to load test data from CSV files or directly as comma-separated values within the annotation.

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Q.95 Explain the concept of test ordering in JUnit 5.

JUnit 5 allows you to specify the order of test method execution using the @TestMethodOrder and @Order annotations.

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Q.96 What is the purpose of using the @Tag annotation in JUnit 5?

The @Tag annotation allows you to tag tests with custom labels. It helps in grouping and filtering tests based on these tags.

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Q.97 What are JUnit 5 dynamic tests, and how are they useful?

JUnit 5 dynamic tests allow you to generate tests programmatically at runtime, which is useful when you have tests that cannot be statically defined.

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Q.98 Explain the concept of nested test classes in JUnit 5.

JUnit 5 allows you to create nested test classes, where the inner classes can inherit the test context and configuration of the outer class.

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Q.99 How do you write parameterized tests with JUnit 4?

In JUnit 4, you can use the Parameterized runner and create a test method that takes parameters using the @Parameters annotation.

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Q.100 What is the purpose of using the @ClassRule annotation in JUnit?

The @ClassRule annotation is used to define rules that apply to an entire test class, similar to @Rule, which applies to individual test methods.

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Q.101 Explain the usage of JUnit 5 @TestFactory for dynamic test generation.

@TestFactory is used to create dynamic tests by returning a Collection, Iterable, Stream, or Iterator of DynamicTest instances.

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Q.102 How do you test expected system exit with JUnit?

JUnit 4 provides the @Rule named ExpectedSystemExit to test for expected system exit codes.

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Q.103 What is the role of the @TempDir annotation in JUnit 5?

The @TempDir annotation provides a temporary directory that can be used for testing file-related operations.

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Q.104 Explain the usage of JUnit 5 @TestInstance annotation.

@TestInstance allows you to configure how test instances are created, either per-class (PER_CLASS) or per-method (PER_METHOD) basis.

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Q.105 What is the purpose of the @Nested annotation in JUnit 5?

The @Nested annotation allows you to define inner classes for organizing and grouping related tests.

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Q.106 How do you test private methods in JUnit?

While it's generally considered best practice to test public methods, you can use libraries like PowerMock or reflection to test private methods.

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Q.107 What are JUnit 5 @BeforeEach and @AfterEach used for?

@BeforeEach and @AfterEach methods are used for setting up and tearing down test fixtures for each test method.

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Q.108 Explain the difference between @RunWith and @ExtendWith in JUnit 5.

@RunWith is used in JUnit 4 to specify custom runners, while @ExtendWith is used in JUnit 5 for registering extensions.

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Q.109 How can you test exceptions thrown from asynchronous code in JUnit 5?

JUnit 5 provides the assertThrowsAsync() method for testing exceptions in asynchronous code.

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Q.110 What is the purpose of using @DisplayNameGeneration in JUnit 5?

@DisplayNameGeneration allows you to specify a custom display name generator for test classes and methods.

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Q.111 Explain the concept of parameterized tests with JUnitParams in JUnit 4.

JUnitParams is a library for JUnit 4 that simplifies parameterized testing by using annotations like @Parameters and @TestCaseName.

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Q.112 How do you use JUnit categories to group and run tests selectively?

JUnit categories allow you to assign test classes or methods to specific categories and then execute tests based on those categories.

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Q.113 What are test suites, and how do you create them in JUnit 4 and JUnit 5?

Test suites in JUnit allow you to run multiple test classes together. In JUnit 4, you create test suites using @RunWith(Suite.class) and in JUnit 5, using @SelectClasses.

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Q.114 Explain the purpose of the @Timeout annotation in JUnit.

The @Timeout annotation is used to specify a timeout period for a test method, ensuring that it completes within the defined time limit.

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Q.115 What is the role of the @SpringJUnitConfig annotation in Spring Boot testing?

@SpringJUnitConfig is used to load the Spring application context for integration tests in Spring Boot projects.

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