Stream parallel() / sequential() question. (original) (raw)

Brian Goetz brian.goetz at oracle.com
Thu Apr 4 13:28:57 PDT 2013

Parallel is clearly needed because, for many stream generators (such as intRange), they are created sequential and have to be explicitly flipped to parallel if you want parallel. The alternative to this was the API bloat of having 2x as many methods everywhere streams are generated. Saying generator().parallel() is pretty efficient at this point; it just flips a bit and returns the underlying stream. Collection is the notable exception, with two methods; this was a common enough case that the small additional API surface area (one new method) aided discoverability of parallel stream capability enough to warrant the additional method.

Sequential is somewhat less essential, but again costs little to include, and provides a way of saying "force this stream to execute sequentially", which is still useful.

On 4/4/2013 4:20 PM, Peter Levart wrote:

On 04/04/2013 04:05 PM, Brian Goetz wrote: This was simplified recently.

There is one sequential/parallel bit for the whole pipeline. The stream starts off with it set one way or the other. These calls overwrite it. The bit is only acted on when you actually start the computation (invoke the terminal operation.) Hi Brian, So why are sequential()/parallel() methods needed then after all? We have the Collection.stream() and Collection.parallelStream() where the shape is pre-determined at the begining by the user. Is there an API that constructs a Stream where the shape is chosen automatically? Regards, Peter On 4/4/2013 9:21 AM, Boaz Nahum wrote: When I invoked parallel() or sequential() how backward it goes ?

Let me explain, I wrote a simple Consumer that report how many different threads used to run it: source. parallel().peek(new ThreadReporter("Segement 1 parallel")). sequential().peek(new ThreadReporter("Segement 2 sequential")). parallel().peek(new ThreadReporter("Segement 3 parallel")). sequential().peek(new ThreadReporter("Segement 4 sequential")).forEach((t) -> {});

private static class ThreadReporter implements Consumer { @Override public void accept(Integer integer) { threads.put(Thread.currentThread(), true); } public void report() { System.out.println("Name +'" + name + "': " + threads.size() + " Thread(s)"); } }

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