Java exception explicit casting vs implicit casting

Question

Came across this piece of code: -

public <T> T call(final Callable<T> callable) {
    try {
         return callable.call();
    } catch (Exception exception) {
         if (exception instanceof RuntimeException) {
             throw (RuntimeException) exception; // Line 6
         } else {
             throw new RuntimeException(exception); // Line 8
         }    
    }
}

What is the need for doing a (RuntimeException) exception at line 6?
What is the difference between the exceptions being thrown at line 6 v/s line 8. Aren't they doing the same thing?

(You could better write the code as `} catch (RuntimeException exc) { throw exc; } catch (Exception exc) { throw new RuntimeException(exc); }`. — Tom Hawtin - tackline, Oct 10 '20 at 20:53

score 1 · Accepted Answer · answered Oct 10 '20 at 20:51

The code there is to transform a checked exception into a unchecked exception. In Java, checked exceptions have to be declared on a method with the throws keyword, while unchecked exceptions don't need to be declared on the method. Exception is the base class for all exceptions while RuntimeException (which is a subclass of Exception) is the base class for all unchecked exceptions.

The code on line 6 is to make the compiler happy. As Exception is a checked exception, by casting it to RuntimeException the compiler won't enforce the exception to be declared on the method with a throws. Line 8 wrapes the checked exception into a unchecked exception.

Java exception explicit casting vs implicit casting

1 Answers1