@Test
    void collect3Arg() {
        assertEquals(singletonList(ELEMENT), instance.collect(ArrayList::new, List::add, ArrayList::addAll));

        assertThrows(IllegalStateException.class, () -> instance.collect(ArrayList::new, List::add, ArrayList::addAll));

    }
Test of mapToInt method, of class SingletonStream.
common-parent/singletonstream/src/test/java/com/speedment/common/singletonstream/SingletonStreamTest.java