为什么在声明Mac变量时得到nullPointerExecption

Question

我正在研究为本地计算机获取本地tcp / ip配置的类。但是，在声明byte [] mac时。我得到一个nullpointerexecption。所以我正在访问具有空值的东西。但我看不出问题是什么。我想知道是否与我在两种不同方法中使用networkinterface类有关。 错误：

java.lang.NullPointerException
    at jlanscan.LocalHost.getMacAddress(LocalHost.java:67)

班

package jlanscan;

import java.net.Inet4Address;
import java.net.InetAddress;
import java.net.InterfaceAddress;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.net.UnknownHostException;
import java.io.*;
import java.net.*;
import java.util.*;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * class to Get LocalHost TCP configuration: ip, subnet, router
 * @author jorge
 */
public class LocalHost {
    InetAddress localhost = null;
    String ipV6;
    String ipV4;
    String hostname;
    String iFace;
    String macAddr;
    Enumeration<NetworkInterface> nets = null;        

    public LocalHost() throws UnknownHostException, SocketException {
        hostname = "localhost";


        nets = NetworkInterface.getNetworkInterfaces();
        Collections.list(nets).forEach((netint) -> {
            try {
                if(!netint.isLoopback()) {
                    displayInterfaceInformation(netint);
                }
            } catch (SocketException ex) {
                Logger.getLogger(LocalHost.class.getName()).log(Level.SEVERE, null, ex);
            }
            try {
                getMacAddress();
            } catch (SocketException ex) {
                Logger.getLogger(LocalHost.class.getName()).log(Level.SEVERE, null, ex);
            } catch (UnknownHostException ex) {
                Logger.getLogger(LocalHost.class.getName()).log(Level.SEVERE, null, ex);
            }
        });
    }

    public void displayInterfaceInformation(NetworkInterface netint) {    
        Enumeration<InetAddress> inetAddresses = netint.getInetAddresses();
        for (InetAddress inetAddress : Collections.list(inetAddresses)) {
            if(inetAddress.getHostAddress().contains("%")) {
                String[] iface = inetAddress.getHostAddress().split("%");
                ipV6 = iface[0];
                iFace = iface[1];                
            }
            ipV4 = inetAddress.getHostAddress();
//            System.out.println(inetAddress.getHostAddress());
        }
    }   

    public void getMacAddress() throws SocketException, UnknownHostException {
        localhost = InetAddress.getLocalHost();
        NetworkInterface ni = NetworkInterface.getByInetAddress(localhost);
        byte[] mac = ni.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) ? "-" : ""));        
        }
        macAddr = sb.toString();

    }

    public InetAddress getLocalhost() {
        return localhost;
    }

    public String getIpAddressV4() {
        return ipV4;
    }

    public String getHostname() {
        return hostname;
    }

    @Override
    public String toString() {
        return "Localhost: interface: " + iFace +" ipV4: " + ipV4 +" ipV6: "+ ipV6 +" hostname: "+ hostname +" mac: "+macAddr;
    }        


}