No-reuse-streams

[David M. Lloyd]

On 11/23/2012 05:21 AM, 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 agree that we should throw an exception in such cases, but it should 
be a specific exception type (e.g. StreamUsedException) which derives 
from ISE.  It seems to me that this will be a common mistake and having 
easy Google-ability of the exception name (ala 
ConcurrentModificationException) as well as problem-specific JavaDoc 
will be a good thing.

> 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?

I agree with you and Rémi - this should fail.


-- 
- DML