Handling exceptions from Future/FutureTask.get() : a question

Question

Hello.
While trying to understand the way (possible) exceptions from Future/FutureTask.get() method can be handled, I found the following example on how this could be done:
[http://www.javaconcurrencyinpractice.com/listings.html]
* StaticUtilities
* @author Brian Goetz and Tim Peierls
public class LaunderThrowable {
     * Coerce an unchecked Throwable to a RuntimeException
     * <p/>
     * If the Throwable is an Error, throw it; if it is a
     * RuntimeException return it, otherwise throw IllegalStateException
    public static RuntimeException launderThrowable(Throwable t) {
        if (t instanceof RuntimeException)
            return (RuntimeException) t;                // Line #1
        else if (t instanceof Error)
            throw (Error) t;                                    // Line #2
        else
            throw new IllegalStateException("Not unchecked", t);// Line #3
}And here is an example on how this handler is supposed to be used:
* Preloader
* Using FutureTask to preload data that is needed later
* @author Brian Goetz and Tim Peierls
public class Preloader
    ProductInfo loadProductInfo() throws DataLoadException {
        return null;
    private final FutureTask<ProductInfo> future =
        new FutureTask<ProductInfo>(new Callable<ProductInfo>() {
            public ProductInfo call() throws DataLoadException {
                return loadProductInfo();
    private final Thread thread = new Thread(future);
    public void start() { thread.start(); }
    public ProductInfo get()
    throws DataLoadException, InterruptedException
        try {
            return future.get();
        } catch (ExecutionException e) {
            Throwable cause = e.getCause();
            if (cause instanceof DataLoadException)
                throw (DataLoadException) cause;
            else
                throw LaunderThrowable.launderThrowable(cause);
    interface ProductInfo {  }
class DataLoadException extends Exception { }Everything is clear in general, except for one thing:
Why is the LaunderThrowable.launderThrowable() method re-throws Error and IllegalStateException (Line #2 and #3 above) and just returns RuntimeException (Line #1)? Especially taking into consideration that whatever is returned from launderThrowable() is re-thrown in any case...
Any ideas?
Thanks.

Answer 1

...Why is the LaunderThrowable.launderThrowable() method re-throws Error and IllegalStateException (Line #2 and #3 above) and just returns RuntimeException (Line #1)?this is explained in the end of paragraph [5.5.2. FutureTask|http://www.javaconcurrencyinpractice.com/]:
+"...When get throws an ExecutionException in Preloader, the cause will fall into one of three categories: a checked exception thrown by the Callable, a RuntimeException, or an Error. We must handle each of these cases separately, but we will use the launderThrowable utility method in Listing 5.13 to encapsulate some of the messier exception-handling logic. Before calling launderThrowable, Preloader tests for the known checked exceptions and rethrows them. That leaves only unchecked exceptions, which Preloader handles by calling launderThrowable and throwing the result. If the Throwable passed to launderThrowable is an Error, launderThrowable rethrows it directly; if it is not a RuntimeException, it throws an IllegalStateException to indicate a logic error. That leaves only RuntimeException, which launderThrowable returns to its caller, and which the caller generally rethrows..."+
...Especially taking into consideration that whatever is returned from launderThrowable() is re-thrown in any case...As far as I understand, assumption about re-throwing in any case is not quite correct. launderThrowable is intended to be a general utility, #) so it leaves a way for client to handle the returned value in a way different from re-throwing.
#) intended to be a general utility -- launderThrowable is used not only in Preloader, but in [other listings|http://www.google.com/search?q=site%3Ahttp%3A%2F%2Fwww.javaconcurrencyinpractice.com%2Flistings+launderThrowable] as well