Declare method to throw an exception and subclass of this exception

Question

Is it meaningful to declare a method to throw an exception and a subclass of this exception, e.g. IOException and FileNotFoundException?

I guess that it is used in order to handle both exceptions by a caller method differently. However, is it possible to handle both exceptions if the method throws only the most generic i.e IOException?

Answer 1

However, is it possible to handle both exceptions if the method throws only the most generic i.e IOException?

Absolutely. You can still catch them separately:

try {
  methodThrowingIOException();
} catch (FileNotFoundException e) {
  doSomething();
} catch (IOException e) {
  doSomethingElse();
}

So it makes no difference to what the caller can do if the method declares both - it's redundant. However, it can emphasize exceptions that you might want to consider. This could be done better in Javadoc than just the throws declaration.

Answer 2

Is it meaningful to declare a method to throw an exception and a subclass of this exception, e.g. IOException and FileNotFoundException?

Usually not - most IDEs I know of even issue warnings for such declarations. What you can and should do is to document the different exceptions thrown in Javadoc.

However, is it possible to handle both exceptions if the method throws only the most generic i.e IOException?

Yes it is, you just need to ensure that the catch blocks are in the right order, i.e. more specific first. Catch blocks are evaluated in the order they are defined, so here

try {
  ...
} catch (FileNotFoundException e) {
  ...
} catch (IOException e) {
  ...
}

if the exception thrown is a FileNotFoundException, it will be caught by the first catch block, otherwise it will fall to the second and dealt with as a general IOException. The opposite order would not work as catch (IOException e) would catch all IOExceptions including FileNotFoundException. (In fact, the latter would result in a compilation error IIRC.)