### Question:
StringUtil { public static String camelToSnakeCase(final String str) { return camelToConstCase(str, true); } private  StringUtil(); static boolean isBlank(final String strValue); static String first2LowerCase(final String strValue); static String first2UpperCase(final String strValue); static String constToLowerCamelCase(final String str); static String constToUpperCamelCase(final String str); static String camelToConstCase(final String str); static String camelToSnakeCase(final String str); static boolean isUpperCamelCase(final String testString); static boolean isLowerCamelCase(final String testString); static boolean isSimpleLetterOrDigit(final char c); static String replace(final String str, final String oldStr, final String newStr); static void replace(final StringBuilder str, final String oldStr, final String newStr); static String format(final String msg, final Object... params);  }### Answer:
@Test public void testCamelToSnakeCase() { assertEquals("xxx_yyy_zzz", StringUtil.camelToSnakeCase("XxxYyyZzz")); assertEquals("xxx_yyy_zzz", StringUtil.camelToSnakeCase("xxxYyyZzz")); assertEquals("xxx_123", StringUtil.camelToSnakeCase("Xxx123")); assertEquals("xxx_123_y", StringUtil.camelToSnakeCase("Xxx123Y")); assertEquals("xxx_123y", StringUtil.camelToSnakeCase("Xxx123y")); }