No-reuse-streams (original) (raw)

Paul Sandoz paul.sandoz at oracle.com
Fri Nov 23 03:21:56 PST 2012

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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?

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?

Paul.

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