Medium Java Full Stack with React Interview Questions

ExecutorService pool = Executors.newFixedThreadPool(4); try { List<Callable<Integer>> tasks = List.of( () -> fetchCount("orders"), () -> fetchCount("users")); List<Future<Integer>> results = pool.invokeAll(tasks); for (Future<Integer> f : results) { System.out.println(f.get()); } } finally { pool.shutdown(); } // On Java 21 you can use Executors.newVirtualThreadPerTaskExecutor() // for I/O-bound tasks.

sealed interface Payment permits Card, Upi {} record Card(String number, double amount) implements Payment {} record Upi(String vpa, double amount) implements Payment {} static String describe(Payment p) { return switch (p) { case Card c when c.amount() > 50000 -> "High value card"; case Card c -> "Card ending " + c.number().substring(12); case Upi u -> "UPI to " + u.vpa(); }; } // Sealed types make the switch exhaustive without a default branch (Java 21).

Map<String, Long> countByDept = employees.stream() .collect(Collectors.groupingBy( Employee::getDepartment, Collectors.counting())); // Average salary per department: Map<String, Double> avgSalary = employees.stream() .collect(Collectors.groupingBy( Employee::getDepartment, Collectors.averagingDouble(Employee::getSalary))); // Use TreeMap::new as the second argument to get sorted keys.

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.

products.sort( Comparator.comparing(Product::getCategory) .thenComparing(Product::getPrice, Comparator.reverseOrder()) .thenComparing(Product::getName)); // Null-safe variant for an optional field: Comparator<Product> byBrand = Comparator.comparing( Product::getBrand, Comparator.nullsLast(Comparator.naturalOrder()));

String text = "java is fun and java is fast"; Map<String, Long> freq = Arrays.stream(text.toLowerCase().split("\\s+")) .collect(Collectors.groupingBy( Function.identity(), LinkedHashMap::new, Collectors.counting())); System.out.println(freq); // {java=2, is=2, fun=1, and=1, fast=1}

@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; } }

# application.yml app: payment: timeout-ms: 3000 retry-count: 2 @ConfigurationProperties(prefix = "app.payment") @Validated public record PaymentProperties( @Min(500) int timeoutMs, @Max(5) int retryCount) {} @SpringBootApplication @EnableConfigurationProperties(PaymentProperties.class) public class App { } // Relaxed binding maps timeout-ms to timeoutMs automatically.

@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); } }

FROM eclipse-temurin:21-jdk AS build WORKDIR /app COPY . . RUN ./mvnw -q -DskipTests package FROM eclipse-temurin:21-jre WORKDIR /app RUN useradd -r appuser USER appuser COPY --from=build /app/target/*.jar app.jar EXPOSE 8080 ENTRYPOINT ["java", "-XX:MaxRAMPercentage=75", "-jar", "app.jar"] # Multi-stage build keeps the image small; runs as non-root.

public interface OrderRepository extends JpaRepository<Order, Long> { @Query(""" select distinct o from Order o join fetch o.items i where o.customer.id = :customerId """) List<Order> findWithItems(@Param("customerId") Long customerId); @EntityGraph(attributePaths = {"items", "customer"}) Optional<Order> findWithGraphById(Long id); }

@GetMapping("/api/products") public Page<ProductDto> list( @RequestParam(defaultValue = "0") int page, @RequestParam(defaultValue = "20") int size, @RequestParam(defaultValue = "price") String sort) { Pageable pageable = PageRequest.of(page, Math.min(size, 100), Sort.by(sort).ascending()); return repository.findByActiveTrue(pageable).map(ProductDto::from); } // Repository: Page<Product> findByActiveTrue(Pageable pageable);

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.

-- src/main/resources/db/migration/V3__add_sku_to_products.sql ALTER TABLE products ADD COLUMN sku VARCHAR(40); UPDATE products SET sku = CONCAT('SKU-', id) WHERE sku IS NULL; ALTER TABLE products ALTER COLUMN sku SET NOT NULL; CREATE UNIQUE INDEX ux_products_sku ON products (sku); # application.yml # spring.jpa.hibernate.ddl-auto: validate # Flyway runs pending V-scripts in order at startup.

public record RegisterRequest( @NotBlank @Size(max = 60) String name, @NotBlank @Email String email, @Pattern(regexp = "^[6-9]\\d{9}$", message = "Invalid mobile") String mobile, @Size(min = 8, max = 64) String password) {} @PostMapping("/api/auth/register") public ResponseEntity<Void> register(@Valid @RequestBody RegisterRequest req) { authService.register(req); return ResponseEntity.status(HttpStatus.CREATED).build(); } // Needs spring-boot-starter-validation. Errors raise // MethodArgumentNotValidException, mapped to 400 by the advice.

@Bean RestClient inventoryClient(RestClient.Builder builder) { var factory = new SimpleClientHttpRequestFactory(); factory.setConnectTimeout(2000); factory.setReadTimeout(3000); return builder.baseUrl("http://inventory-service") .requestFactory(factory).build(); } public StockDto getStock(String sku) { return inventoryClient.get() .uri("/api/stock/{sku}", sku) .retrieve() .body(StockDto.class); } // RestClient (Boot 3.2+) replaces RestTemplate for new code; // use WebClient for reactive stacks.

SELECT department_id, name, salary FROM ( SELECT department_id, name, salary, DENSE_RANK() OVER ( PARTITION BY department_id ORDER BY salary DESC) AS rnk FROM employees ) ranked WHERE rnk <= 3 ORDER BY department_id, rnk; -- DENSE_RANK keeps ties; use ROW_NUMBER for exactly 3 rows.

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.

SELECT order_date, SUM(amount) AS daily_sales, SUM(SUM(amount)) OVER ( ORDER BY order_date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW ) AS running_total FROM orders WHERE order_date >= DATE '2026-10-01' AND order_date < DATE '2026-11-01' GROUP BY order_date ORDER BY order_date;

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>; }

export function useFetch(url) { const [state, setState] = useState({ data: null, loading: true, error: null }); useEffect(() => { let ignore = false; setState(s => ({ ...s, loading: true })); fetch(url) .then(r => r.ok ? r.json() : Promise.reject(new Error(r.statusText))) .then(data => { if (!ignore) setState({ data, loading: false, error: null }); }) .catch(error => { if (!ignore) setState({ data: null, loading: false, error }); }); return () => { ignore = true; }; }, [url]); return state; } // Usage: const { data, loading, error } = useFetch(`/api/orders/${id}`);

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> ); }

function debounce(fn, delay = 300) { let timer; return function (...args) { clearTimeout(timer); timer = setTimeout(() => fn.apply(this, args), delay); }; } const search = debounce(term => { fetch(`/api/products?q=${encodeURIComponent(term)}`); }, 400); input.addEventListener('input', e => search(e.target.value)); // Only the last call within 400 ms actually runs.

async function loadDashboard() { const results = await Promise.allSettled([ fetch('/api/orders').then(r => r.json()), fetch('/api/stats').then(r => r.json()), fetch('/api/alerts').then(r => r.json()), ]); const [orders, stats, alerts] = results.map(r => r.status === 'fulfilled' ? r.value : null); return { orders, stats, alerts }; } // Promise.all rejects on the first failure; // allSettled lets the page show partial data.

const original = { user: { name: 'Asha' }, tags: ['a'], joined: new Date() }; // Modern built-in (handles Date, Map, Set, cycles): const copy = structuredClone(original); // Spread is only a shallow copy: const shallow = { ...original }; shallow.user.name = 'Ravi'; // also changes original.user.name // JSON.parse(JSON.stringify(obj)) loses Dates, functions, // undefined and fails on circular references.