Concat applied after filter produces unexpected result

[kotucz]

Applying concat() on stream that is result of filter() produces incorrect result.
Below is new test that reproduces the issue:

@Test
public void testConcatOfFilter() {
    final PrintConsumer consumer = new PrintConsumer();
    final Stream<Integer> first = Stream.ofRange(0, 5)
        .filter(it -> true);
    final Stream<Integer> second = Stream.ofRange(5, 10)
        .filter(it -> true);
    Stream.concat(first, second)
        .forEach(consumer);
    assertEquals("0123456789", consumer.toString());
}

org.junit.ComparisonFailure: 
Expected :0123456789
Actual   :13null56789

The issue is that hasNext() implementation in filter() is not idempotent - when called multiple times, value is skipped.
In concat() implementation the (filter() hasNext()) is called once in hasNext() and once in next(), which would be ok, if filter() hasNext() did not have the side-effect.

[aNNiMON]

Fixed! Thank you so much, @kotucz

[kotucz]

Thanks for the quick fix!

Just one note - this implementation assumes that hasNext() is called at least once before next().
So just calling next() could break something (but this probably will never happens inside the library).
For inspiration the same thing is in guava Iterators.filter() (which is not a lightweight dependency though).

[aNNiMON]

Fixed with 80885bb