getHardwareAddress（）在JDK 6中返回null

Question

我正在尝试编写一种方法来获取计算机的mac地址。在几台计算机上测试过：

public String mac() throws UnknownHostException, SocketException {
            InetAddress ip = InetAddress.getLocalHost();
    NetworkInterface network = NetworkInterface.getByInetAddress(ip);
    byte[] mac = network.getHardwareAddress();
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < mac.length; i++) {
        sb.append(String.format("%02X%s", mac[i], (i < mac.length - 1) ? "-" :""));     
    }
            System.out.println("MAC: " + sb.toString());
            return(sb.toString());
}

它在运行JDK 7的计算机上执行它的操作。但是，在JDK 6上， byte [] mac 返回null，从而导致nullPointerException。任何人都知道为什么？