IntStream iterate function with filter

Question

I was reading this book called Modern Java in Action and one part of code I could not comprehend.

      IntStream.iterate(0, n -> n + 4)
                .filter(n -> n < 100)
                .forEach(System.out::println);

Authors says that code will not terminate. The reason is that there’s no way to know in the filter that the numbers continue to increase, so it keeps on filtering them infinitely!

I did not get the reason. Could someone explain it why.

Answer 1

The int stream will generate a sequence of numbers that start from 0 and have step of 4. Then they will be filtered out. The Int keep generating because there is no terminal condition. It equivalent to

for(int n=0;;n=n+4){... Filter out }

Answer 2

Authors says that code will not terminate.

Yes, because this specific overload of iterate

static IntStream iterate(int seed,
                         IntUnaryOperator f)

Returns an infinite sequential ordered IntStream produced by iterative application of a function f to an initial element seed, producing a Stream consisting of seed, f(seed), f(f(seed)), etc.

returns an infinite stream and given it's an infinite stream it means it can only be terminated via certain operations.

Given the terminal operation in use here (forEach) is not short-circuiting it means that you require a "short-circuiting intermediate operation" to truncate the infinite stream in this specific case e.g. limit (JDK8),takeWhile (JDK9) et al.

The only short-circuiting intermediate operation in JDK8 is limit as it allows computations on infinite streams to complete in finite time.

The reason is that there’s no way to know in the filter that the numbers continue to increase, so it keeps on filtering them infinitely!

filter itself is not a short-circuiting intermediate operation hence cannot terminate a stream. filter's job is essentially to return a stream consisting of the elements of the stream that match the given predicate.

Conclusion: if one is using an infinite stream which does not have a short-circuiting terminal operation then it requires a short-circuiting intermediate operation to truncate the stream else the stream remains infinite.