Java - Getting MAC address of Linux system

You might have more than one network interface and I would not count on the interface's name. I suggest you to go over all the interfaces and look for one that has a MAC address. You can use this example as a base line:

try {

        Enumeration<NetworkInterface> networkInterfaces = NetworkInterface.getNetworkInterfaces();
        while(networkInterfaces.hasMoreElements())
        {
            NetworkInterface network = networkInterfaces.nextElement();
            System.out.println("network : " + network);
            byte[] mac = network.getHardwareAddress();
            if(mac == null)
            {
                System.out.println("null mac");             
            }
            else
            {
                System.out.print("MAC address : ");

                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(sb.toString());  
                break;
            }
        }
    } catch (SocketException e){

        e.printStackTrace();

    }

Just a small modification of the code of Guy :

public String searchForMac() throws SocketException {
    String firstInterface = null;        
    Map<String, String> addressByNetwork = new HashMap<>();
    Enumeration<NetworkInterface> networkInterfaces = NetworkInterface.getNetworkInterfaces();

    while(networkInterfaces.hasMoreElements()){
        NetworkInterface network = networkInterfaces.nextElement();

        byte[] bmac = network.getHardwareAddress();
        if(bmac != null){
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < bmac.length; i++){
                sb.append(String.format("%02X%s", bmac[i], (i < bmac.length - 1) ? "-" : ""));        
            }

            if(sb.toString().isEmpty()==false){
                addressByNetwork.put(network.getName(), sb.toString());
                System.out.println("Address = "+sb.toString()+" @ ["+network.getName()+"] "+network.getDisplayName());
            }

            if(sb.toString().isEmpty()==false && firstInterface == null){
                firstInterface = network.getName();
            }
        }
    }

    if(firstInterface != null){
        return addressByNetwork.get(firstInterface);
    }

    return null;
}

This code has been tested on Windows, Linux (Ubuntu) and Mac OS X with success.

Because the network can be null, I ignore all null cases and I also ignore empty addresses. I think that if we don't do this, we view the crash. I choose the first found address and it works but it may be wrong, so just test it.