Is there a way to check whether a stream is finite in Java?

Question

I know there is infinite stream in Java. Is there a way to check whether the stream is finite or not? Something like this method `isStreamFinite(Stream stream)`? ``` @Test public void testStreamFinity() { assertFalse(isStreamFinite(generateLong())); } private boolean isStreamFinite(Stream stream) { if (stream.count() < Long.MAX_VALUE) { return true; } return false; } private Stream generateLong() { return LongStream.generate(() -> new Random().nextLong()).boxed(); } ``` The construction of the Stream would leave some marks/tracks that we can use to trace back to check it? The checking method could work always as expected reliably? Update I was wrong about this when trying to solve a problem by detecting the `isFinite` of a stream. It seems it's not stable/reliable as the answers mentioned. I will refactor my solution in another around. Thank you for the help ~

Accepted Answer

There is no reliable way to do it. Even using the `estimateSize`, which is documented:

... or returns Long.MAX_VALUE if infinite, unknown, or too expensive to compute.

So simply relying on the fact that `Long.MAX_VALUE` will tell you if this stream is infinite or not - is simply wrong.

The other suggestion says that to use `SIZED`, which is again wrong:

```
IntStream.generate(() -> 1).limit(5)
```

You know that this is a `SIZED` stream - the implementation does not and will not report the sized flag.

Bottom-line: you can't in a reliable way. Theoretically this could be possible for some cases, like when you just use `generate` without limiting or short-circuiting the stream, the implementation could register a flag like `IS_INFINITE` or something, but this IMO would be useless, probably for 99% of the cases.

You can also think in reverse here, how about when you want to tell with certainty if this stream is finite? In such a case, `getExactSizeIfKnown()` would tell you that.

Is there a way to check whether a stream is finite in Java?

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