No-reuse-streams (original) (raw)

Remi Forax forax at univ-mlv.fr
Fri Nov 23 03:52:53 PST 2012

On 11/23/2012 12:21 PM, Paul Sandoz wrote:

Hi,

There has been some discussions on the confusion of reusing streams when iterating and evaluating in addition to non-linear streams (where a two or more child streams share the same parent stream). e.g. iteration: Stream s1 = .. Stream s2 = s1.map(...); Iterator i1 = s1.iterator(); Iterator i2 = s2.iterator(); e.g. evaluation: Stream s1 = .. Stream s2 = s.map(...); Object i1 = s1.findFirst(); Object i2 = s2.findFirst(); e.g. non-linear Stream p = .. Stream c1 = p.map(...); Stream c2 = p.filter(...); There are many cases that will produce unexpected or confusing results when iterating or evaluating (and in the case of parallel evaluation it is trickier). So we made a hasty retreat to the strict position of no-reuse-streams: - Only one terminal operation may be performed. An IllegalStateException is thrown on subsequent terminal operations. - Stream.iterator() is a terminal operation (as will be spliterator() when added). - A parent stream may only have one child stream. An IllegalStateException is thrown if a child stream is created from a parent that already has a child. In all of the above examples the last line will result in an ISE being thrown. Thoughts?

I've discussed with Brian off list about these changes. I think it's the right semantics.

Should the following throw an ISE on the last line of the following? Stream s = .. Object i1 = s.findFirst(); Stream s1 = s.map(...); i.e. should we fail on the s.map(...) or just when a terminal operation occurs, if at all?

it should fail of s.map(...) fail early or fail earlier.

Paul.

Rémi

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