🧪 What is JUnit?

JUnit is a free and open-source testing framework for Java. It helps you write test methods that automatically check if your code is working correctly.

You’ll use JUnit to:

Run tests quickly Catch bugs early Automate checking your code

🔁 What is Mockito?

Mockito is a Java tool that helps you fake (mock) other parts of the code that your class depends on. This way, you can test just your logic without relying on a real database or API.

For example: If UserService depends on UserRepository, you can use Mockito to mock UserRepository and test only UserService.

📚 Step-by-Step: JUnit Annotations (Explained Simply)

AnnotationWhat It Does (Simple Explanation)@TestMarks a method as a test. JUnit runs this method.@BeforeEachRuns before every test. Used to set up common test data.@AfterEachRuns after every test. Good for cleanup.@BeforeAllRuns once before all tests. Must be static.@AfterAllRuns once after all tests. Also must be static.@DisplayNameAdds a custom name to the test (for better readability).@DisabledTemporarily turns off a test method.

🧪 Step-by-Step: Mockito Annotations (Made Easy)

AnnotationWhat It Does@MockCreates a fake object of a class/interface.@InjectMocksAutomatically injects the mocks into the class you want to test.@SpyWraps a real object but allows you to mock specific methods.@CaptorCaptures the arguments passed to mock methods.@ExtendWith(MockitoExtension.class)Allows Mockito to work with JUnit 5.

🛠️ Common JUnit Assertion Methods (To Check Your Results)

MethodWhat It ChecksassertEquals(a, b)If two values are equal.assertTrue(condition)If the condition is true.assertFalse(condition)If the condition is false.assertThrows(Exception.class, () -> ...)If a method throws the expected exception.

⚙️ Common Mockito Methods (To Control Behavior)

MethodWhat It Doeswhen(...).thenReturn(...)Sets what the mock should return.verify(mock).method()Checks if a method was called.any(), anyString()Argument matchers used in mocks.doReturn(...).when(...)Alternative to when() (for spies).doThrow(...).when(...)Simulates throwing an exception.times(n), never(), atLeastOnce()Checks how many times a method was called.

🔍 Example: JUnit + Mockito in Action (Beginner Friendly)

Let’s write a test case for a real Java class using JUnit and Mockito.

✅ Step 1: Your Class (Service to Test)

javaCopyEditpublic class UserService { private final UserRepository userRepository; public UserService(UserRepository userRepository) { this.userRepository = userRepository; } public User registerUser(String name, String email) { if (userRepository.findByEmail(email).isPresent()) { throw new IllegalArgumentException("User already exists"); } return userRepository.save(new User(0, name, email)); } }

✅ Step 2: Your Test Class

javaCopyEdit@ExtendWith(MockitoExtension.class) public class UserServiceTest { @Mock private UserRepository userRepository; @InjectMocks private UserService userService; @Test void testRegisterUserSuccessfully() { // Arrange when(userRepository.findByEmail("[email protected]")).thenReturn(Optional.empty()); // Act User result = userService.registerUser("Test", "[email protected]"); // Assert assertEquals("Test", result.getName()); verify(userRepository).save(any(User.class)); } @Test void testRegisterUserAlreadyExists() { // Arrange when(userRepository.findByEmail("[email protected]")) .thenReturn(Optional.of(new User(1, "Old", "[email protected]"))); // Act + Assert assertThrows(IllegalArgumentException.class, () -> { userService.registerUser("New", "[email protected]"); }); verify(userRepository, never()).save(any()); } }

🎓 Why This Matters for Interviews

Java interviews don’t just test your ability to write working code — they test if you know how to:

Write clean, modular, and testable code Use JUnit and Mockito to validate your logic Work in teams where testing is standard practice

Mastering these concepts will not only help you ace interviews but also make you a confident and job-ready Java developer.

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