class Strings {

    public static String toLowerCase(final String s) {
        if (s == null) {
            return null;
        } else {
            return s.toLowerCase(Locale.ENGLISH);
        }
    }

}

class StringsTest {

    @Test
    public void testToLowerCase() {


        final String nullString = null;
        final String emptyString = "";
        final String blankString = " ";
        final String string = "A";

        assertThat(Strings.toLowerCase(nullString), is(nullValue()));
        assertThat(Strings.toLowerCase(emptyString), is(""));
        assertThat(Strings.toLowerCase(blankString), is(" "));
        assertThat(Strings.toLowerCase(string), is("a"));
    }

}