code: public static byte[] getOverride() {
          String overrideMac = System.getProperty(OVERRIDE_MAC_PROP);
          byte[] macBytes = null;
          if (overrideMac != null) {
              ByteArrayOutputStream out = new ByteArrayOutputStream();
              String[] rawBytes = overrideMac.split(":");
              if (rawBytes.length == 6) {
                  try {
                      for (String b : rawBytes) {
                          out.write(Integer.parseInt(b, 16));
                      }
                      macBytes = out.toByteArray();
                  } catch (NumberFormatException e) {
                      // do nothing
                  }
              }
          }
          return macBytes;
      }
docs: Return the overridden MAC address, if the system property
  OVERRIDE_MAC_PROP has been set. If the value of the property isn't a
  valid MAC address or it was not set, then null is returned.