Java Backend Snippets for Modern Development

List<String> names = users.stream()
    .filter(u -> u.getAge() > 18)
    .map(User::getName)
    .collect(Collectors.toList());

Map<Department, List<Employee>> byDept = employees.stream()
    .collect(Collectors.groupingBy(Employee::getDepartment));

User user = users.stream()
    .filter(u -> u.getId() == id)
    .findFirst()
    .orElseThrow(() -> new ResourceNotFoundException("User not found"));

double total = orders.stream()
    .mapToDouble(Order::getAmount)
    .sum();

@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<?> handleNotFound(ResourceNotFoundException ex) {
        return new ResponseEntity<>(new ErrorDetails(ex.getMessage()), HttpStatus.NOT_FOUND);
    }
}

@Bean
public RestTemplate restTemplate(RestTemplateBuilder builder) {
    return builder
        .setConnectTimeout(Duration.ofMillis(3000))
        .setReadTimeout(Duration.ofMillis(3000))
        .build();
}

@Component
public class ScheduledTasks {
    @Scheduled(fixedRate = 5000)
    public void reportCurrentTime() {
        System.out.println("The time is now " + LocalDateTime.now());
    }
}

@Service
public class EmailService {
    @Async
    public CompletableFuture<String> sendEmail(String to) {
        // logic
        return CompletableFuture.completedFuture("Sent");
    }
}

ExecutorService executor = Executors.newFixedThreadPool(10);
executor.submit(() -> {
    // task logic
});
executor.shutdown();

CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> "Hello");
CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> "World");

String result = future1.thenCombine(future2, (s1, s2) -> s1 + " " + s2).join();

public class Singleton {
    private static volatile Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) instance = new Singleton();
            }
        }
        return instance;
    }
}

String content = Files.readString(Path.of("file.txt"));

Files.writeString(Path.of("file.txt"), "Hello World");

ObjectMapper mapper = new ObjectMapper();
String json = mapper.writeValueAsString(myObject);
MyClass obj = mapper.readValue(json, MyClass.class);

Optional.ofNullable(user)
    .map(User::getAddress)
    .map(Address::getCity)
    .ifPresent(System.out::println);