@Test
    void mutableOnClose() {
        final AtomicInteger cnt = new AtomicInteger();
        instance.onClose(cnt::incrementAndGet);
        instance.close();
        assertEquals(1, cnt.get());
    }
Test of mapToInt method, of class SingletonStream.
common-parent/singletonstream/src/test/java/com/speedment/common/singletonstream/SingletonStreamTest.java