Spliterator flags as enum (was Initial java.util.Spliterator putback) (original) (raw)

Doug Lea dl at cs.oswego.edu
Thu Mar 28 19:06:17 UTC 2013

On 03/28/13 14:52, Joshua Bloch wrote:

Doug,

I don't get it. You can set and unset flags on your own EnumSet. Why isn't that sufficient?

There are a lot of problems. First, even though most spliterators will return the same set of characteristics each time, you can't just create one static one:

class MySpliterator { ... static final EnumSet cs = EnumSet.of(...); EnumSet characteristics() return cs; } }

... because you cannot risk that no one will modify.

Second, when inside streams, you'd have to create a new EnumSet Object across each stage, that somehow secretly extends the public Characteristics with non-public internal control flags. Which means either some slow conversion table or grabbing private EnumSet internals.

So it is both slow and painful. I tried to make it less so, knowing that people sometimes react hostilely to plain bit sets. But I'm sure that the current scheme is better than all I tried. (Ditto for Brian Goetz and Paul Sandoz). In fact, I think the current scheme is sorta nice in an absolute sense.

-Doug

Josh On Thu, Mar 28, 2013 at 11:45 AM, Doug Lea <dl at cs.oswego.edu_ _<mailto:dl at cs.oswego.edu>> wrote: On 03/28/13 13:18, Tim Peierls wrote: I can't find a discussion of why Spliterator flags are ints rather than enum. We started out with enums on (my) initial Spliterator side vs control flags internal to streams. The we had to somehow mesh these to work together. On the stream side, you need to set and unset various bits across stages. Clearly you can't do that to someone's EnumSet -- they will not expect you to modify it, but enforcing this makes it both unwieldy and sleaze-inducing (we'd have to grab underlying representation from EnumSet). Another way of saying this is that we needed an efficient propagate-by-value small-N bit set mechanism, and the only candidate was the traditional one. This amounts to the same reason that nio "interest" flags are done the same way. -Doug

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