@Test
    void toArrayGenerator() {
        assertEquals(1, instance.toArray(String[]::new).length);

        assertThrows(IllegalStateException.class, () -> instance.toArray(String[]::new));
    }
Test of mapToInt method, of class SingletonStream.
common-parent/singletonstream/src/test/java/com/speedment/common/singletonstream/SingletonStreamTest.java