@Test
public void testIsKeyIn() {

    final Map<String, Integer> refMap = new LinkedHashMap<>();
    refMap.put("one", 1);
    refMap.put("two", 2);
    refMap.put("three", 3);
    String[] filtered = Stream.of("one", "two", "tree", "four")
            .filter(isKeyIn(refMap).negate()).toArray(String[]::new);
    assertArrayEquals("filtered should contain tree and four", new String[]{"tree", "four"}, filtered);
}