In Java, you can throw exceptions when an Optional is empty using the orElseThrow method. This method accepts a Supplier that provides the exception to be thrown if the Optional is empty.

Here’s the syntax and an example:

package org.kodejava.util;

import java.util.Optional;

public class Main {
   public static void main(String[] args) {
      Optional<String> optionalValue = Optional.empty();

      // Throws an exception if Optional is empty
      String value = optionalValue.orElseThrow(() -> new IllegalArgumentException("Value is not present"));

      System.out.println(value);
   }
}

Explanation:

1. orElseThrow():
   • This method is used to retrieve the value from an Optional.
   • If the Optional is empty (Optional.empty()), it throws the exception provided by the Supplier.
2. Key Points:
   • orElseThrow takes a lambda expression or method reference as its argument. This lambda (or Supplier) returns the exception to be thrown.
   • Example of a custom exception:

   Optional<String> optionalValue = Optional.empty();
   
   String value = optionalValue.orElseThrow(() -> new MyCustomException("Custom message"));
   

3. Method Reference Example:
   If the exception has a default constructor, you can use a method reference:

   Optional<String> optionalValue = Optional.empty();
   
   String value = optionalValue.orElseThrow(MyCustomException::new);
   

4. When Optional is not empty:
   If the Optional contains a value (i.e., Optional.of("value")), the value is retrieved instead of throwing an exception.

   Optional<String> optionalValue = Optional.of("Hello");
   
   String value = optionalValue.orElseThrow(() -> new IllegalArgumentException("Value is not present"));
   
   System.out.println(value); // Prints "Hello"
   

Custom Exception Example:

If you want to define your own custom exception:

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

And use it with orElseThrow:

Optional<String> optionalValue = Optional.empty();

String value = optionalValue.orElseThrow(() -> new MyCustomException("Custom exception message"));

Using orElseThrow is a clean and concise way to handle empty Optional values by throwing appropriate exceptions.