@Test
    void testFilter() {
        assertEquals(1L, instance.filter(ELEMENT::equals).count());
        assertEquals(1L, instance.filter(ELEMENT::equals).filter(Objects::nonNull).count());
    }
Test of filter method, of class SingletonStream.
common-parent/singletonstream/src/test/java/com/speedment/common/singletonstream/SingletonStreamTest.java