Exception yet no exception?

Question

Ello,
I've been trying to make a little program which uses multicasting to send various things over the network using the way as described in the tutorial. However, whenever I try to write my own implementation with multicasting, it always complains about unhandled exceptions. So after I add some exception handling, it all works... Except there is no sign of any exception actually being thrown at all, for example:
        try
            Address_DST = InetAddress.getByName( "230.0.0.1" );
        catch( UnknownHostException e )
            System.out.println("Caught Exception: " + e.getMessage());
        }The code snippet above prints out absolutely nothing, as if nothing had been caught at all. Yet if I remove both the try and catch blocks, it freaks out about an unhandles UnknownHostException. I suppose it would make sense, sine InetAddress.getByName() does check the IP and I don't think 230.0.0.1 doesn't resolve to anything, because it's in the multicast range... Then again, I don't know how clever the whole method itself is...
Anyways, I got it to work but I don't understand WHY it works... Which is quite annoying... Can anyone help me with this one?

Answer 1

The unhandled exception message comes from the
compiler. Your program's not running yet, but
the compiler knows that getByName can throw
UnknownHostExceptionAnd Java has rules that you have to account for every checked exception that could come your way--either by catching it or by declaring that you throw it.
Here's a quick overview of exceptions:
The base class for all exceptions is Throwable. Java provides Exception and Error that extend Throwable. RuntimeException (and many others) extend Exception.
RuntimeException and its descendants, and Error and its descendants, are called unchecked exceptions. Everything else is a checked exception.
If your method, or any method it calls, can throw a checked exception, then your method must either catch that exception, or declare that your method throws that exception. This way, when I call your method, I know at compile time what can possibly go wrong and I can decide whether to handle it or just bubble it up to my caller. Catching a given exception also catches all that exception's descendants. Declaring that you throw a given exception means that you might throw that exception or any of its descendants.
Unchecked exceptions (RuntimeException, Error, and their descendants) are not subject to those restrictions. Any method can throw any unchecked exception at any time without declaring it. This is because unchecked exceptions are either the sign of a coding error (RuntimeException), which is totally preventable and should be fixed rather than handled by the code that encounters it, or a problem in the VM, which in general can not be predicted or handled.