### Question:
UUID { public static String uuid(final int len) { return uuid(len, CHARS.length); }  static String uuid(final int len); static String uuid(final int len,
                              final int radix); static String uuid();  }### Answer:
@Test public void testUUID() { for (int j = 0; j < TEST_COUNT; j++) { final String uuid = UUID.uuid(); for (int i = 0; i < uuid.length(); i++) { final char currentChar = uuid.charAt(i); boolean unexpected; if (i == 0) { unexpected = '_' != currentChar; } else if (i == 9 || i == 14 || i == 19 || i == 24) { unexpected = '-' != currentChar; } else { unexpected = EXPECTED_CHARS.indexOf(uuid.charAt(i)) < 0; } if (unexpected) { fail("Unexpected character: '" + currentChar + "' at position: " + i + " in UUID = " + uuid); } } } }