public static String substringAfter(String s, String splitter) {
        final int indexOf = s.indexOf(splitter);
        return indexOf >= 0 ? s.substring(indexOf + splitter.length()) : null;
    }
@Test
    void testSubstringAfterSplitterMultiChar() {
        assertThat(substringAfter("this is a test", " is ")).isEqualTo("a test");
        assertThat(substringAfter("this is a test", " was ")).isNull();
    }