Transientタグを付ける

import java.io.*;

import java.util.*;



public class Logon implements Serializable {

    /**

     * 

     */

    private static final long serialVersionUID = 1L;

    private Date date = new Date();

    private String username;

    private transient String password;



    Logon(String name, String pwd) {

        username = name;

        password = pwd;

    }



    public String toString() {

        String pwd = (password == null) ? "(n/a)" : password;

        return "logon info: 
 " + "username: " + username + "
 date: "

                + date.toString() + "
 password: " + pwd;

    }



    public static void main(String[] args) {

        Logon a = new Logon("Hulk", "myLittlePony");

        System.out.println("logon a = " + a);

        try {

            ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream(

                    "Logon.out"));

            o.writeObject(a);

            o.close();

            // Delay:

            int seconds = 5;

            long t = System.currentTimeMillis() + seconds * 1000;

            while (System.currentTimeMillis() < t)

                ;

            // Now get them back:

            ObjectInputStream in = new ObjectInputStream(new FileInputStream(

                    "Logon.out"));

            System.out.println("Recovering object at " + new Date());

            a = (Logon) in.readObject();

            System.out.println("logon a = " + a);

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}