Why is this IntStream.mapToObj mapper not evaluated with Stream.count?

Question

I use an InStream.range with a mapToObj mapper as a substitute for-loop to initialize a database for a test by creating some records, which then I want to count to verify that the correct number has been created.

I have noticed that the Stream.count terminal operation returns the correct count, but when I use the count method from my persistence layer it returns 0. The mapper callback is not being called and thus no records are created.

Here is a sample that demonstrates the issue:

public class IntStreamTest {

    private final List<String> db = new ArrayList<>();

    @Test
    public void mapToStringAndCount() {
        long count = IntStream
            .range(1000, 1010)
            .mapToObj(this::mapToString)
            // .filter(Objects::nonNull)
            .count();
        Assert.assertEquals(10L, count);
        Assert.assertEquals(count, countString());
    }

    private long countString() {
        return db.size();
    }

    private String mapToString(int i) {
        String string = String.format("string-%d", i);
        System.out.printf("%s%n", string);
        db.add(string);
        return string;
    }
}

When I use the intermediate operation Stream.filter after IntStream.mapToObj everything works fine. Also the the records are created when is use Stream.collect after IntStream.mapToObj or IntStream.forEachinstead ofIntStream.mapToObj`.

Why does IntStream behaves like this?

$ java -version
openjdk version "11.0.4" 2019-07-16
OpenJDK Runtime Environment (build 11.0.4+11-post-Ubuntu-1ubuntu219.04)
OpenJDK 64-Bit Server VM (build 11.0.4+11-post-Ubuntu-1ubuntu219.04, mixed mode, sharing)