Android系统MulticastSocket编程的有关问题

package xiaobenben.muticast;import java.io.IOException;import java.net.DatagramPacket;import java.net.InetAddress;import java.net.MulticastSocket;import java.util.Scanner;import android.app.Activity;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.EditText;import android.widget.TextView;public class Muticast extends Activity {    /** Called when the activity is first created. */   private Button button_ok;   private TextView textview;   private Button button_begin;   private EditText edittext;    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        button_ok = (Button)findViewById(R.id.button_ok);        button_begin = (Button)findViewById(R.id.button_begin);        textview = (TextView)findViewById(R.id.textview_text);        edittext = (EditText)findViewById(R.id.editview);        textview.setText("hi,xiaobenben!");        button_begin.setOnClickListener(new Button.OnClickListener(){            public void onClick(View v) {                // TODO Auto-generated method stub                try {                    System.out.println("init() start");                    new Multicast().init();                } catch (IOException e) {                    // TODO Auto-generated catch block                    e.printStackTrace();                }            }                    });    }        class Multicast implements Runnable{        private static final String BROADCAST_IP = "230.0.0.1";        public static final int BROADCAST_PORT = 30000;        private static final int DATA_LEN = 4096;        private MulticastSocket socket = null;        private InetAddress broadcastAddress = null;        private Scanner scan = null;        byte[] inBuff = new byte[DATA_LEN];        private DatagramPacket inPacket = new DatagramPacket(inBuff , inBuff.length);        private DatagramPacket outPacket = null;        private String str;        public void init()throws IOException            {            try                {                socket = new MulticastSocket(BROADCAST_PORT);                broadcastAddress = InetAddress.getByName(BROADCAST_IP);                socket.joinGroup(broadcastAddress);                socket.setLoopbackMode(false);                outPacket = new DatagramPacket(new byte[0] , 0 ,broadcastAddress , BROADCAST_PORT);                new Thread(this).start();                scan = new Scanner(edittext.getText().toString());                while(scan.hasNextLine())                    {                    byte[] buff = scan.nextLine().getBytes();                    outPacket.setData(buff);                    socket.send(outPacket);                    }                }            finally            {            socket.close();            }            }        public void run(){            try            {            while(true)                {                                socket.receive(inPacket);                                System.out.println("聊天信息：" + new String(inBuff , 0 , inPacket.getLength()));                textview.setText("聊天信息：" + new String(inBuff , 0 , inPacket.getLength()));                System.out.println(inPacket.getAddress().getHostAddress());                }            }                catch (IOException ex)            {            ex.printStackTrace();            try                {                if (socket != null)                    {                                        socket.leaveGroup(broadcastAddress);                                        socket.close();                    }                System.exit(1);                 }            catch (IOException e)            {            e.printStackTrace();            }            }        }    }}