Java Full Stack Developer Interview Questions & Answers

public interface ProductSummary { Long getId(); String getName(); BigDecimal getPrice(); } public interface ProductRepository extends JpaRepository<Product, Long> { List<ProductSummary> findByCategoryName(String category); @Query("select new com.shop.dto.PriceDto(p.name, p.price) from Product p where p.price < :max") List<PriceDto> cheaperThan(@Param("max") BigDecimal max); } // Projections select fewer columns and avoid loading full entities.

@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 LoginForm({ onLogin }) { const [form, setForm] = useState({ email: '', password: '' }); const [errors, setErrors] = useState({}); const handleChange = e => setForm({ ...form, [e.target.name]: e.target.value }); const handleSubmit = e => { e.preventDefault(); const errs = {}; if (!/^\S+@\S+\.\S+$/.test(form.email)) errs.email = 'Enter a valid email'; if (form.password.length < 8) errs.password = 'Minimum 8 characters'; setErrors(errs); if (Object.keys(errs).length === 0) onLogin(form); }; return ( <form onSubmit={handleSubmit}> <input name="email" value={form.email} onChange={handleChange} /> {errors.email && <span>{errors.email}</span>} <input name="password" type="password" value={form.password} onChange={handleChange} /> {errors.password && <span>{errors.password}</span>} <button type="submit">Login</button> </form> ); }

-- 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);

@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 }

.grid { display: grid; grid-template-columns: 1fr; gap: 16px; padding: 0 16px; } @media (min-width: 640px) { .grid { grid-template-columns: repeat(2, 1fr); } } @media (min-width: 1024px) { .grid { grid-template-columns: repeat(4, 1fr); } } /* Mobile-first: base styles for small screens, then widen. */

@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); } }

const products = [ { name: 'Pen', price: 20, qty: 10, inStock: true }, { name: 'Bag', price: 900, qty: 0, inStock: false }, { name: 'Book', price: 250, qty: 4, inStock: true }, ]; const total = products .filter(p => p.inStock) .map(p => p.price * p.qty) .reduce((sum, value) => sum + value, 0); console.log(total); // 1200

-- 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.

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> ); }

@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.