What happens if a method throws an exception that was not specified in the method declaration with “throws”

Question

I\'ve never used the \"throws\" clause, and today a mate told me that I had to specify in the method declaration which exceptions the method may throw. However, I\'ve been u

Answer 1

It can happen, even with checked exceptions. And sometimes it can break logging.

Suppose a library method uses this trick to allow an implementation of Runnable that can throw IOException:

class SneakyThrowTask implements Runnable {

    public void run() {
        throwSneakily(new IOException());
    }

    private static RuntimeException throwSneakily(Throwable ex) {
        return unsafeCastAndRethrow(ex);
    }

    @SuppressWarnings("unchecked")
    private static X unsafeCastAndRethrow(Throwable ex) throws X {
        throw (X) ex;
    }

}

And you call it like this:

public static void main(String[] args) {
    try {
        new SneakyThrowTask().run();
    } catch (RuntimeException ex) {
        LOGGER.log(ex);
    }
}

The exception will never be logged. And because it's a checked exception you cannot write this:

public static void main(String[] args) {
    try {
        new SneakyThrowTask().run();
    } catch (RuntimeException ex) {
        LOGGER.log(ex);
    } catch (IOException ex) {
        LOGGER.log(ex); // Error: unreachable code
    }
}