Java Full Stack with React Online Assessment Questions & Answers

@SpringBootApplication @EnableScheduling public class App { } @Component public class CleanupJob { private final SessionRepository repo; public CleanupJob(SessionRepository repo) { this.repo = repo; } @Scheduled(cron = "0 0 2 * * *", zone = "Asia/Kolkata") public void removeExpiredSessions() { repo.deleteByExpiresAtBefore(Instant.now()); } } // With several instances, use ShedLock so only one runs the job.

function ProductList() { const [products, setProducts] = useState([]); const [loading, setLoading] = useState(true); const [error, setError] = useState(null); useEffect(() => { const controller = new AbortController(); fetch('/api/products', { signal: controller.signal }) .then(res => { if (!res.ok) throw new Error('HTTP ' + res.status); return res.json(); }) .then(data => setProducts(data.content)) .catch(err => { if (err.name !== 'AbortError') setError(err.message); }) .finally(() => setLoading(false)); return () => controller.abort(); }, []); if (loading) return <p>Loading...</p>; if (error) return <p>Error: {error}</p>; return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>; }

-- Find duplicates SELECT email, COUNT(*) AS cnt FROM users GROUP BY email HAVING COUNT(*) > 1; -- Keep the lowest id, delete the rest (MySQL / PostgreSQL) DELETE FROM users u USING users k WHERE u.email = k.email AND u.id > k.id; -- PostgreSQL syntax -- Then add: ALTER TABLE users ADD CONSTRAINT ux_email UNIQUE (email);

@RestControllerAdvice public class GlobalExceptionHandler { @ExceptionHandler(ResourceNotFoundException.class) public ProblemDetail handleNotFound(ResourceNotFoundException ex) { return ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, ex.getMessage()); } @ExceptionHandler(MethodArgumentNotValidException.class) public ProblemDetail handleValidation(MethodArgumentNotValidException ex) { ProblemDetail pd = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST); pd.setProperty("errors", ex.getBindingResult().getFieldErrors() .stream().map(e -> e.getField() + ": " + e.getDefaultMessage()).toList()); return pd; } }

@Bean PasswordEncoder passwordEncoder() { return new BCryptPasswordEncoder(12); } public void register(RegisterRequest req) { User user = new User(req.email(), encoder.encode(req.password())); userRepository.save(user); } public boolean checkLogin(String raw, User user) { return encoder.matches(raw, user.getPasswordHash()); } // BCrypt adds a random salt and is deliberately slow. // Never store plain text or use MD5/SHA-1 for passwords.

SELECT e.name AS employee, e.salary, m.name AS manager, m.salary AS manager_salary FROM employees e JOIN employees m ON e.manager_id = m.id WHERE e.salary > m.salary; -- A self join treats the same table as two roles.

Optional<Double> second = employees.stream() .map(Employee::getSalary) .distinct() .sorted(Comparator.reverseOrder()) .skip(1) .findFirst(); second.ifPresentOrElse( s -> System.out.println("Second highest: " + s), () -> System.out.println("Not enough distinct salaries")); // distinct() handles ties at the top salary.

@ExtendWith(MockitoExtension.class) class ProductServiceTest { @Mock ProductRepository repo; @InjectMocks ProductService service; @Test void throwsWhenProductMissing() { when(repo.findById(1L)).thenReturn(Optional.empty()); assertThrows(ResourceNotFoundException.class, () -> service.getProduct(1L)); verify(repo).findById(1L); } }

-- Using a subquery SELECT MAX(salary) AS second_highest FROM employees WHERE salary < (SELECT MAX(salary) FROM employees); -- Using DISTINCT with OFFSET SELECT DISTINCT salary FROM employees ORDER BY salary DESC LIMIT 1 OFFSET 1; -- The first query returns NULL when there is no second salary.

@Entity @Table(name = "products") public class Product { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; @Column(nullable = false, length = 120) private String name; @Column(nullable = false, precision = 10, scale = 2) private BigDecimal price; @ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "category_id") private Category category; protected Product() {} // required by JPA }

function cartReducer(state, action) { switch (action.type) { case 'add': return { count: state.count + 1 }; case 'remove': return { count: Math.max(0, state.count - 1) }; case 'reset': return { count: 0 }; default: throw new Error('Unknown action ' + action.type); } } function Cart() { const [state, dispatch] = useReducer(cartReducer, { count: 0 }); return ( <div> <span>Items: {state.count}</span> <button onClick={() => dispatch({ type: 'add' })}>+</button> <button onClick={() => dispatch({ type: 'remove' })}>-</button> </div> ); }