Java | Фишки и трюки

class Cache {
  private static int sum = 0;
    
  static {
    initializeIfNecessary();
  }
    
  public static int getSum() {
    initializeIfNecessary();
    return sum;
  }
    
  private static boolean initialized = false;
    
  private static synchronized void initializeIfNecessary() {
    if (!initialized) {
      for (int i = 0; i < 5; i++)
        sum += i;
      initialized = true;
    }
  }
}

class Client {
  public static void main(String[] args) {
    System.out.println(Cache.getSum());
  }
}

import org.apache.poi.ss.usermodel.*;

public class ExcelWriter {
    public static void main(String[] args) {
        Workbook wb = new XSSFWorkbook();
        Sheet sheet = wb.createSheet("Sheet1");

        Row headerRow = sheet.createRow(0);
        Cell headerCell = headerRow.createCell(0);
        headerCell.setCellValue("Name");

        Row dataRow = sheet.createRow(1);
        Cell dataCell = dataRow.createCell(0);
        dataCell.setCellValue("John Doe");

        try (FileOutputStream fileOut = new FileOutputStream("workbook.xlsx")) {
            wb.write(fileOut);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

@RepositoryRestResource(path = "users")
public interface UserRepository extends JpaRepository<User, Long> {
    
}

GET /users - Получить всех пользователей
POST /users - Создать нового пользователя
GET /users/{id} - Получить пользователя по ID
PUT /users/{id} - Обновить пользователя
DELETE /users/{id} - Удалить пользователя

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpGet httpGet = new HttpGet("http://example.com");
CloseableHttpResponse response = httpClient.execute(httpGet);

httpGet.setHeader("Content-Type", "application/json");

StringEntity entity = new StringEntity("{\"key\": \"value\"}");
httpPost.setEntity(entity);

int statusCode = response.getStatusLine().getStatusCode();

String responseBody = EntityUtils.toString(response.getEntity());

class Point {
  private final int x, y;
  private final String name;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
    name = makeName();
  }

  protected String makeName() {
    return "[" + x + "," + y + "]";
  }

  public final String toString() {
    return name;
  }
}

public class ColorPoint extends Point {
  private final String color;

  ColorPoint(int x, int y, String color) {
    super(x, y);
    this.color = color;
  }

  protected String makeName() {
    return super.makeName() + ":" + color;
  }

  public static void main(String[] args) {
    System.out.println(new ColorPoint(4, 2, "red"));
  }
}

File file = new File("test.txt");
String content = FileUtils.readFileToString(file, StandardCharsets.UTF_8);
System.out.println(content);

InputStream input = new FileInputStream("input.txt");
OutputStream output = new FileOutputStream("output.txt");
IOUtils.copy(input, output);
input.close();
output.close();

File directory = new File("C:/my_folder");
String[] extensions = {"txt", "xml"};
List<File> files = (List<File>) FileUtils.listFiles(directory, extensions, true);
for (File file : files) {
    System.out.println(file.getName());
}

String filename = "example.txt";
String extension = FilenameUtils.getExtension(filename);
System.out.println(extension);

public class Boom {
  private Boom internalInstance = new Boom();

  public Boom() throws Exception {
    throw new Exception("Boom!");
  }

  public static void main(String[] args) {
    try {
      Boom b = new Boom();
      System.out.println("Surprise");
    } catch (Exception e) {
      System.out.println("I caught!");
    }
  }
}

@Mapper
public interface CarMapper {

    CarDto carToCarDto(Car car);

    List<CarDto> carsToCarDtos(List<Car> cars);
}

@Mappings({
    @Mapping(source = "make", target = "manufacturer"),
    @Mapping(source = "numberOfSeats", target = "seatCount")
})
CarDto carToCarDto(Car car);

@Mapping(source = "make", target = "manufacturer")
CarDto carToCarDto(Car car);

@InheritInverseConfiguration
Car carDtoToCar(CarDto carDto);

@IterableMapping(elementTargetType = CarDto.class)
List<CarDto> carsToCarDtos(List<Car> cars);

SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
Session session = sessionFactory.openSession();

Employee employee = session.get(Employee.class, 1); // получение сотрудника с id = 1
System.out.println(employee.getName());

Employee newEmployee = new Employee();
newEmployee.setName("John Doe");
newEmployee.setDepartment("IT");
session.save(newEmployee);

Employee employee = session.get(Employee.class, 1);
employee.setDepartment("HR");
session.update(employee);

Employee employee = session.get(Employee.class, 1);
session.delete(employee);