### Question:
YamlConfigSource extends MapConfigSource { static String keyOf(String prefix, String chunk) { if (prefix == null) { return chunk; } return prefix + "." + chunk; } YamlConfigSource(String name, Map<String, ?> tree, int defaultOrdinal); static YamlConfigSource of(URL url, int defaultOrdinal);  }### Answer:
@Test public void testKeyOfNoPrefix() { String result = YamlConfigSource.keyOf(null, "taco"); assertThat(result).isEqualTo("taco"); }
@Test public void testKeyOfWithPrefix() { String result = YamlConfigSource.keyOf("cheesy", "taco"); assertThat(result).isEqualTo("cheesy.taco"); }