Core Java Refresher: java.util.Optional<T>

Optional<T> is a generic class introduced in Java 8 to represent a value that may or may not be present, reducing the risk of NullPointerException.

Key Points

• Acts as a container for a value of type T or no value (empty).
• Encourages explicit handling of absent values.
• Commonly used in functional programming and modern APIs.

Key Methods

1. public T orElseThrow()

   • Returns the value if present; otherwise, throws NoSuchElementException.

   • Example:

     Optional<String> optional = Optional.ofNullable(null);
     String value = optional.orElseThrow(); // Throws NoSuchElementException
     
     

2. public T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X extends Throwable

   • Returns the value if present; otherwise, throws an exception created by the Supplier.

   • Example:

     Optional<String> optional = Optional.ofNullable(null);
     String value = optional.orElseThrow(() -> new IllegalArgumentException("Value not found!"));
     
     

Supplier Functional Interface

• Definition: A functional interface with a single abstract method (SAM): public T get().

• Used to supply an object, such as a custom exception in orElseThrow.

• Example in a service layer:

  public class RestaurantService {
      private final RestaurantDao restaurantDao;
  
      public RestaurantService(RestaurantDao restaurantDao) {
          this.restaurantDao = restaurantDao;
      }
  
      public Restaurant findRestaurantById(Long restaurantId) {
          return restaurantDao.findById(restaurantId)
                  .orElseThrow(() -> new MyCustomException("Restaurant not found !!!!!"));
      }
  }
  
  

Custom Exception Example (As Provided, Well-Formatted)

public class UserService {
    private final UserDao userDao;

    public UserService(UserDao userDao) {
        this.userDao = userDao;
    }

    public User findUserById(Long userId) {
        return userDao.findById(userId).orElseThrow(() -> new ResourceNotFoundException("Invalid User Id"));
    }
}

public class ResourceNotFoundException extends RuntimeException {
    public ResourceNotFoundException(String message) {
        super(message);
    }
}

1. Spring Boot in More Detail

Spring Boot is a framework that simplifies Spring application development by providing auto-configuration, standalone applications, and production-ready features.

Core Features