Extract network card address

For JDK1.6 or bigger:

import java.io.*;
import java.net.*;
import java.util.*;

public class ListNetsEx
{
    public static void main(String args[]) 
      throws SocketException 
    {
        Enumeration nets = 
            NetworkInterface.getNetworkInterfaces();
        for (NetworkInterface netint : Collections.list(nets))
            displayInterfaceInformation(netint);
    }

    static void displayInterfaceInformation(NetworkInterface netint) 
      throws SocketException 
    {
        System.out.println("Display name: " 
           + netint.getDisplayName());
        System.out.println("Hardware address: " 
           + Arrays.toString(netint.getHardwareAddress()));
    }
}  

JDK1.5 or less
One way, without using JNI, is to launch an external utility and interpret the output.

In Windows, “arp -a” will return the MAC addresses of all adapters that have TCP/IP bound to them and have recently (default < 5 mins) resolved an IP address. Otherwise, in NT/2K/XP, “ipconfig /all” will return all relevant network info for IP-bound adapters, including the MAC address (displayed as “physical address”).

In Linux, “ifconfig” does something similar.

Example for Windows:

import java.net.InetAddress;
import java.io.InputStream;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.text.ParseException;
import java.util.StringTokenizer;


public final class NetworkUtils {

  private final static int MACADDR_LENGTH = 17;
  private final static String WIN_OSNAME = "Windows";
  private final static String WIN_MACADDR_REG_EXP =
   "^[0-9A-F]{2}-[0-9A-F]{2}-[0-9A-F]{2}-[0-9A-F]{2}-[0-9A-F]{2}-[0-9A-F]{2}$";
  private final static String WIN_MACADDR_EXEC = "ipconfig /all";


  public final static String getMacAddress() throws IOException {
    String os = System.getProperty("os.name");
    try {
      if (os.startsWith(WIN_OSNAME)) {
         return winMacAddress(winIpConfigCommand());
      }
      // other OS left as an exercise !
      //   LINUX -->    else if (os.startsWith("Linux")) { ...
      //                ...
      //                Process p = Runtime.getRuntime().exec("ifconfig");
      //   MAC OSX -->  else if(os.startsWith("Mac OS X")) { ...
      //                ...
      //                Process p = Runtime.getRuntime().exec("ifconfig");
      else {
         throw new IOException("OS not supported : " + os);
      }
    }
    catch(ParseException e) {
      e.printStackTrace();
      throw new IOException(e.getMessage());
    }
  }

  private final static String winMacAddress(String ipConfigOutput) 
        throws ParseException {
    String localHost = null;
    try {
      localHost = InetAddress.getLocalHost().getHostAddress();
    }
    catch(java.net.UnknownHostException ex) {
      ex.printStackTrace();
      throw new ParseException(ex.getMessage(), 0);
    }

    StringTokenizer tokenizer = 
        new StringTokenizer(ipConfigOutput, "\n");
    String lastMacAddress = null;

    while(tokenizer.hasMoreTokens()) {
      String line = tokenizer.nextToken().trim();

      // see if line contains IP address
      if (line.endsWith(localHost) && lastMacAddress != null) {
         return lastMacAddress;
      }

      // see if line contains MAC address
      int macAddressPosition = line.indexOf(":");
      if(macAddressPosition <= 0) continue;

      String macAddressCandidate = line.substring(macAddressPosition + 1).trim();
      if (winIsMacAddress(macAddressCandidate)) {
         lastMacAddress = macAddressCandidate;
         continue;
      }
    }

    ParseException ex = new ParseException
       ("cannot read MAC address from [" + ipConfigOutput + "]", 0);
    ex.printStackTrace();
    throw ex;
  }


  private final static boolean winIsMacAddress(String macAddressCandidate) {
    if (macAddressCandidate.length() != MACADDR_LENGTH)    return false;
    if (!macAddressCandidate.matches(WIN_MACADDR_REG_EXP)) return false;
    return true;
  }


  private final static String winIpConfigCommand() throws IOException {
    Process p = Runtime.getRuntime().exec(WIN_MACADDR_EXEC);
    InputStream stdoutStream = new BufferedInputStream(p.getInputStream());

    StringBuffer buffer= new StringBuffer();
    for (;;) {
       int c = stdoutStream.read();
       if (c == -1) break;
          buffer.append((char)c);
    }
    String outputText = buffer.toString();
    stdoutStream.close();
    return outputText;
  }


  public final static void main(String[] args) {
    try {
      System.out.println("MAC ADDRESS");
      System.out.println("  OS          : " 
         + System.getProperty("os.name"));
      System.out.println("  IP/Localhost: " 
         + InetAddress.getLocalHost().getHostAddress());
      System.out.println("  MAC Address : " 
         + getMacAddress());
    }
    catch(Throwable t) {
      t.printStackTrace();
    }
  }
}

Done! Happy coding!