public class Message {
  // the properties of a message
  private String sender;
  private String receiver;
  private String subject;
  private String body;

  // all property values are known at creation of the message; so initialize
  public Message (String s, String r, String sub, String b)
  {
    sender = s;
    receiver = r;
    subject = sub;
    body = b;
  }

  // any nice format of printing the names and the values of the properties will do
  public void printMsg()
  {
    System.out.println("Sender: " + sender);
    System.out.println("Receiver: " + receiver);
    System.out.println("Subject: " + subject);
    System.out.println("Message: " + body);
  }

  // what follows are basic getter methods

  public String getSender()
  {
    return sender;
  }

  public String getReceiver()
  {
    return receiver;
  }

  public String getSubject()
  {
    return subject;
  }

  public String getBody()
  {
    return body;
  }

}

import java.util.ArrayList;

  public class Userlist
  {

    private ArrayList<User> users;
    private int numUsers;

    public Userlist()
    {
      users = new ArrayList<User>();
    }

    public User findUser(String username)
    {
      for (User i : users)
      {
        if (i.userName.equals(username))
          return i;
      }
      return null;
    }

    public void addUser(User u)
    {
      if (findUser(u.userName) != null)
        System.out.println("User already exists");
      else
      {
       users.add(u);
       numUsers++;
      }
    }

    public int getNumUsers()
    {
       return this.numUsers;
    }

    public User getUser(int i)
    {
      if (i>=users.length)
        return null;
      else
        return users.get(i-1);
    }
  }

public class UserList implements Serializable
{
}

public class Message implements Serializable
{
}

FileOutputStream  fos = new FileOutputStream(filename);
ObjectOutputStream out = new ObjectOutputStream(fos);

UserList userList ; // assume this is the object to be serialized
out.writeObject(userList); // serializes the object

FileInputStream  fis = new FileInputStream(filename);
ObjectInputStream in = new ObjectInputStream(fis);
userList = (UserList) in.readObject();