What is the difference between unit tests and integration tests?

• Unit tests verify individual components (classes, methods) in isolation, without external dependencies (databases, external services). The goal is to ensure correctness of the smallest testable parts of the code.
• Integration tests verify the interaction between multiple components or systems. They involve dependencies and check that different parts of the application work together correctly.

Comparison table:

Aspect	Unit tests	Integration tests
Scope	Single component	Interaction of components/systems
Isolation	High (mocks, stubs)	Low (use real dependencies)
Speed	Fast	Slower
Complexity	Low	Higher
Error localization	Precise (points to specific component)	Harder (can be in any component)
Example	Testing logic of a single method	Testing database save via service

Example:

// Unit test example
public class CalculatorTest {
    @Test
    public void testAdd() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
    }
}

// Integration test example
public class UserServiceIntegrationTest {
    @Autowired
    private UserService userService;

    @Test
    public void testCreateUser() {
        User user = new User("john", "password");
        userService.save(user);
        assertNotNull(userService.findById(user.getId()));
    }
}