@Test public void testKeep() { Assert.assertEquals( Lists.newArrayList( FunctionalIterable.create(Arrays.asList("1", "2", "3")) .keep( new Function<String, Integer>() { @Override public Integer apply(String input) { if ("2".equals(input)) { return null; } return Integer.parseInt(input); } } ) ), Arrays.asList(1, 3) ); }