gistfile1.java

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

public class ShowEth0PrimaryIp {
    public static final String ETH0 = "eth0";

    public static void main(String[] args) throws Exception {
        System.out.println(new ShowEth0PrimaryIp().getEth0PrimaryIp());
    }


    public InetAddress getEth0PrimaryIp() throws SocketException {
        List<InetAddress> tracker = new ArrayList<InetAddress>();

        NetworkInterface itf = NetworkInterface.getByName(ETH0);
        List<InetAddress> allAddrs = getInetAddresses(itf);
        for (InetAddress addr: allAddrs) {
            tracker.add(addr);
        }

        Enumeration<NetworkInterface> subItfs = itf.getSubInterfaces();
        while (subItfs.hasMoreElements()) {
            NetworkInterface subItf = subItfs.nextElement();
            List<InetAddress> subItfAddrs = getInetAddresses(subItf);
            for (InetAddress subItfAddr: subItfAddrs) {
                if (tracker.contains(subItfAddr)) {
                    System.out.println("Removing " + subItfAddr + " from all addrs");
                    tracker.remove(subItfAddr);
                }
            }
        }

        if (tracker.size() != 1) {
            System.out.println("Was hoping to whittle list down to 1, but didn't");
        }

        return tracker.get(0);
    }


    private List<InetAddress> getInetAddresses(NetworkInterface itf) throws SocketException {
        System.out.println("=== " + itf.getName() + " ===");
        List<InetAddress> myAddrs = new ArrayList<InetAddress>();
        Enumeration<InetAddress> addrs = itf.getInetAddresses();
        while (addrs.hasMoreElements()) {
            InetAddress addr = addrs.nextElement();
            if (
                addr != null
                && !addr.isLoopbackAddress()
                && (
                    itf.isPointToPoint() || !addr.isLinkLocalAddress()
                )
            ) {
                myAddrs.add(addr);
                System.out.println("found: " + addr);
            }
        }
        return myAddrs;
    }
}