Dan Creswell wrote:
When access large disk records and pulling these into memory, you typically
only take a small chunk, process it and move on, so the behaviour is more
stream like, rather than iterator, I created an interface called
ResultStream (yes it supports Generics, but beware the compilation
boundaries that haven't been checked by the compiler), result stream is
terminated by a null value.
That's generally similar to how MatchSet works under the covers....
<snip>
package org.apache.river.api.util;
/**
* This interface is similar to an Enumerator, it is designed to return
* results incrementally in loops, however unlike an Enumerator, there is no
* check first operation as implementors must return a null value after
* the backing data source has been exhausted. So this terminates like a
stream
* by returning a null value.
*
* @author Peter Firmstone
*/
public interface ResultStream<T> {
/**
* Get next T, call from a loop until T is null;
* @return T unless end of stream in which case null is returned.
*/
public T get();
So this overall, isn't far from where MatchSet goes. For those following
along, remoteness issues are such that we can't reliably return null to
indicate "end of stream". One may never reach the end of the stream if
there's a failure at the back-end. One could counter that with a "wait for
the failure to go away" scenario but that assumes the failure does go away
and it might not (and whilst you're figuring all that out your thread is
blocked and can't say much to your upstream users).
In essence, then returning null means "we successfully reached the end of
the stream". And we need some other mechanism to say "we didn't reach the
end of the stream for some reason". Typically the mechanism is an
exception....
Yeah, you'd need to throw an IOException, I've pondered adding it, so it
can be used over remote connections, that way another interface can
implement both Remote and ResultStream.
Thanks, I think that was all the convincing I needed.
ResultStream was originally created for Parallel iteration, multiple
threads calling get, you can't check first, you have to check after ;)
Cheers,
Peter.
