### Question:
Identifiers { protected static String mergeCharacterSets(String... s) { Set characters = new HashSet<Character>(); for (String characterSet : s) { characters.addAll(Arrays.asList(characterSet.split(""))); } return Joiner.on("").join(characters); }  static String makeRandomId(int l); static String makeRandomPassword(final int length); static String makeRandomPassword(final int length, String... passwordValidCharsPool); static String makeIdFromHash(long d); static String makeRandomJavaId(int l); static double randomDouble(); static long randomLong(); static boolean randomBoolean(); static int randomInt(); static int randomInt(int upbound); static byte[] randomBytes(byte[] buf); static byte[] randomBytes(int length); static String makeRandomBase64Id(int length); static String getBase64IdFromValue(long value); static String getBase64IdFromValue(long value, int length); static void appendBase64IdFromValueOfLength(long value, int length, StringBuffer sb); static void appendBase64IdFromValueOfLength(long value, int length, StringBuilder sb); static boolean isValidToken(String token, String validStartChars, String validSubsequentChars); static final String UPPER_CASE_ALPHA; static final String LOWER_CASE_ALPHA; static final String NUMERIC; static final String NON_ALPHA_NUMERIC; static final String JAVA_GOOD_START_CHARS; static final String JAVA_GOOD_NONSTART_CHARS; static final String JAVA_GOOD_SEGMENT_REGEX; static final String JAVA_GOOD_PACKAGE_OR_CLASS_REGEX; static final String JAVA_GOOD_BINARY_REGEX; static final String JAVA_GENERATED_IDENTIFIER_START_CHARS; static final String JAVA_GENERATED_IDENTIFIERNONSTART_CHARS; static final String BASE64_VALID_CHARS; static final String ID_VALID_START_CHARS; static final String ID_VALID_NONSTART_CHARS; static final String PASSWORD_VALID_CHARS; }### Answer:
@Test public void testCharMerge() { String characters = Identifiers.mergeCharacterSets("abc", "bcd", "ghjj"); Assert.assertEquals(characters.indexOf('b'), characters.lastIndexOf('b')); }