How to safely consume Java Streams safely without isFinite() and isOrdered() methods?

Question

There is the question on whether java methods should return Collections or Streams, in which Brian Goetz answers that even for finite sequences, Streams should usually be pr

Answer 1

After looking at things a bit (some experimentation and here) as far as I see, there is no way to know definitely whether a stream is finite or not.

More than that, sometimes even it is not determined except at runtime (such as in java 11 - IntStream.generate(() -> 1).takeWhile(x -> externalCondition(x))).

What you can do is:

1. You can find out with certainty if it is finite, in a few ways (notice that receiving false on these does not mean it is infinite, only that it may be so):

   1. stream.spliterator().getExactSizeIfKnown() - if this has an known exact size, it is finite, otherwise it will return -1.

   2. stream.spliterator().hasCharacteristics(Spliterator.SIZED) - if it is SIZED will return true.

2. You can safe-guard yourself, by assuming the worst (depends on your case).

   1. stream.sequential()/stream.parallel() - explicitly set your preferred consumption type.

   2. With potentially infinite stream, assume your worst case on each scenario.

      1. For example assume you want listen to a stream of tweets until you find one by Venkat - it is a potentially infinite operation, but you'd like to wait until such a tweet is found. So in this case, simply go for stream.filter(tweet -> isByVenkat(tweet)).findAny() - it will iterate until such a tweet comes along (or forever).
      2. A different scenario, and probably the more common one, is wanting to do something on all the elements, or only to try a certain amount of time (similar to timeout). For this, I'd recommend always calling stream.limit(x) before calling your operation (collect or allMatch or similar) where x is the amount of tries you're willing to tolerate.

After all this, I'll just mention that I think returning a stream is generally not a good idea, and I'd try to avoid it unless there are large benefits.