Is it possible to convert a custom exception inheriting from RuntimeException to the RuntimeException type?

Yes, a custom exception inheriting from RuntimeException is its subclass and can be cast to RuntimeException without any issues.

Example:

class MyException extends RuntimeException {}

try {
    throw new MyException();
} catch (RuntimeException e) {
    // Here e is a RuntimeException, although it is actually a MyException
    System.out.println("Caught a RuntimeException");
}

Thus, inheritance allows using custom exceptions as RuntimeException and casting them to this type.