Top 50 Most Asked Java Full Stack with React Interview Questions

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.

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.

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.

-- 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);

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.

# 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.

@RestController @RequestMapping("/api/products") public class ProductController { private final ProductService service; public ProductController(ProductService service) { this.service = service; } @GetMapping("/{id}") public ProductDto get(@PathVariable Long id) { return service.get(id); } @PostMapping public ResponseEntity<ProductDto> create(@Valid @RequestBody ProductRequest req) { ProductDto saved = service.create(req); return ResponseEntity.created(URI.create("/api/products/" + saved.id())).body(saved); } @DeleteMapping("/{id}") @ResponseStatus(HttpStatus.NO_CONTENT) public void delete(@PathVariable Long id) { service.delete(id); } }

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.

List<Integer> nums = List.of(4, 2, 7, 2, 4, 9); Set<Integer> seen = new HashSet<>(); Set<Integer> duplicates = nums.stream() .filter(n -> !seen.add(n)) .collect(Collectors.toSet()); System.out.println(duplicates); // [2, 4] // Stateless alternative using counts: // nums.stream().collect(groupingBy(identity(), counting())) // .entrySet().stream().filter(e -> e.getValue() > 1)

@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.

@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);

@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.

Path path = Path.of("data.txt"); try (BufferedReader reader = Files.newBufferedReader(path)) { String line; while ((line = reader.readLine()) != null) { System.out.println(line); } } catch (IOException e) { log.error("Could not read file", e); } // The reader is closed automatically, even if an exception occurs, // because BufferedReader implements AutoCloseable.

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.