Non-serializable object error while working on Memcache

Hi everyone i was working with memcache, and when i compiled this code i got following errors.

2010-07-09 10:35:53.499 INFO net.spy.memcached.MemcachedConnection:  Added {QA sa=localhost/127.0.0.1:11211, #Rops=0, #Wops=0, #iq=0, topRop=null, topWop=null, toWrite=0, interested=0} to connect queue
2010-07-09 10:35:53.520 INFO net.spy.memcached.MemcachedConnection:  Connection state changed for sun.nio.ch.SelectionKeyImpl@7fdcde
Exception in thread "main" java.lang.IllegalArgumentException: Non-serializable object
 at net.spy.memcached.transcoders.BaseSerializingTranscoder.serialize(BaseSerializingTranscoder.java:86)
 at net.spy.memcached.transcoders.SerializingTranscoder.encode(SerializingTranscoder.java:135)
 at net.spy.memcached.MemcachedClient.asyncStore(MemcachedClient.java:301)
 at net.spy.memcached.MemcachedClient.set(MemcachedClient.java:691)
 at def.Tweet.main(Tweet.java:23)
Caused by: java.io.NotSerializableException: def.User
 at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1173)
 at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:343)
 at net.spy.memcached.transcoders.BaseSerializingTranscoder.serialize(BaseSerializingTranscoder.java:81)
 ... 4 more

I managed to fix it with converting User class toString but ı dont want to use toString method.I just want to use my class to add and get mwethods. How can i fix this problem? Here is the code i am using.

package def;

import java.io.IOException;
import java.io.Serializable;
import java.net.InetSocketAddress;

import net.spy.memcached.MemcachedClient;

public class Tweet {
  private static User user;
  private static Message message;
  private static Followers follower;

public static void main(String[] args) throws Exception {
 try{
     user = new User(1,"Mehmet Özer","asd","127.0.0.1","True","Onine");
  //String deneme = user.toString();
  MemcachedClient c=new MemcachedClient(new InetSocketAddress("localhost", 11211));