### Question:
StringUtils { public static String generateRandomAlphanumericString(Random rnd, int length) { checkNotNull(rnd); checkArgument(length >= 0); StringBuilder buffer = new StringBuilder(length); for (int i = 0; i < length; i++) { buffer.append(nextAlphanumericChar(rnd)); } return buffer.toString(); } private  StringUtils(); static String byteToHexString(final byte[] bytes, final int start, final int end); static String byteToHexString(final byte[] bytes); static byte[] hexStringToByte(final String hex); static String arrayAwareToString(Object o); static String arrayToString(Object array); static String showControlCharacters(String str); static String getRandomString(Random rnd, int minLength, int maxLength); static String getRandomString(Random rnd, int minLength, int maxLength, char minValue, char maxValue); static String generateRandomAlphanumericString(Random rnd, int length); static void writeString(@Nonnull String str, DataOutputView out); static String readString(DataInputView in); static void writeNullableString(@Nullable String str, DataOutputView out); static @Nullable String readNullableString(DataInputView in); static boolean isNullOrWhitespaceOnly(String str); @Nullable static String concatenateWithAnd(@Nullable String s1, @Nullable String s2); static String toQuotedListString(Object[] values);  }### Answer:
@Test public void testGenerateAlphanumeric() { String str = StringUtils.generateRandomAlphanumericString(new Random(), 256); if (!str.matches("[a-zA-Z0-9]{256}")) { fail("Not alphanumeric: " + str); } }