Develop Reference

Why does my recursive method make my program throw an InvalidClassException?

In my program, I have a FacebookUser class that has an ArrayList of all of that user's friends. All of those friends may also have friends. I am using recursion to get a new ArrayList that contains all of the user's friends and all of the user's friends' friends and so on. I also serialized the FacebookUser class so I can save the data. Without the new recursive method, I can serialize without error and the data persists over executions. With the recursive method, though, I get an InvalidClassException. It shows that the exception is at the line that uses the ObjectInputStream to get the FacebookUser from memory.
I'm not understanding why the new method is causing the exception to be thrown from another method. I believe the problem lies only in the FacebookUser.java class (since getRecommendations is recursive method) but I thought it'd be best to also show the UserAccount.java class since the former extends the latter. Any help will be appreciated. Thank you.
Execution:
Exception in thread "main" java.io.InvalidClassException: FacebookUser; local class incompatible: stream classdesc serialVersionUID = 4110981517967863740, local class serialVersionUID = 5032562666539812166
at java.base/java.io.ObjectStreamClass.initNonProxy(Unknown Source)
at java.base/java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.base/java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.base/java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.base/java.io.ObjectInputStream.readObject0(Unknown Source)
at java.base/java.io.ObjectInputStream.readObject(Unknown Source)
at FacebookUser.readFile(FacebookUser.java:157)
at DriverClass.main(DriverClass.java:12)
FacebookUser.Java
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Scanner;
public class FacebookUser extends UserAccount implements Serializable{
private String passwordHint;
private ArrayList<FacebookUser> friends;
public FacebookUser(String username, String password) {
super(username, password);
friends = new ArrayList<FacebookUser>();
}
#Override
public void getPasswordHelp() {
System.out.println("Password Hint: " + passwordHint);
}
public void getPasswordHelp(String username) {
if (friends.size() == 0) {
System.out.println("There is no user with that username.");
} else {
for (int i = 0; i < friends.size(); i++) {
if (friends.get(i).toString().equals(username)) {
friends.get(i).getPasswordHelp();
break;
}
if (i == friends.size() - 1) {
System.out.println("There is no user with that username.");
break;
}
}
}
}
void setPasswordHint(String hint) {
passwordHint = hint;
}
void friend(FacebookUser newFriend) {
if (friends.contains(newFriend)) {
System.out.println("That person is already your friend.");
} else {
friends.add(newFriend);
}
}
void friend(String username) {
Scanner s = new Scanner(System.in);
if (friends.size() == 0) {
System.out.println("Please create a password: ");
String password = s.nextLine();
System.out.println("Please create a password hint: ");
String passHint = s.nextLine();
FacebookUser fbu = new FacebookUser(username, password);
fbu.setPasswordHint(passHint);
friends.add(fbu);
} else {
for (int i = 0; i < friends.size(); i++) {
if (friends.get(i).toString().equals(username)) {
System.out.println("That person is already your friend.");
break;
}
if (i == friends.size() - 1) {
System.out.println("Please create a password: ");
String password = s.nextLine();
System.out.println("Please create a password hint: ");
String passHint = s.nextLine();
FacebookUser fbu = new FacebookUser(username, password);
fbu.setPasswordHint(passHint);
friends.add(fbu);
break;
}
}
}
}
void defriend(FacebookUser formerFriend) {
if (friends.contains(formerFriend)) {
friends.remove(formerFriend);
} else {
System.out.println("That person is not your friend.");
}
}
void defriend(String username) {
Scanner s = new Scanner(System.in);
if (friends.size() == 0) {
System.out.println("That person is not your friend.");
} else {
for (int i = 0; i < friends.size(); i++) {
if (friends.get(i).toString().equals(username)) {
System.out.println("Password for " + username + ": ");
String passw = s.nextLine();
if (friends.get(i).getPassword().equals(passw)) {
friends.remove(i);
break;
} else {
System.out.println("Incorrect Password");
break;
}
}
if (i == friends.size() - 1) {
System.out.println("That person is not your friend.");
break;
}
}
}
}
ArrayList<FacebookUser> getRecommendations(FacebookUser friend) {
ArrayList<FacebookUser> recFriends = new ArrayList<FacebookUser>();
for (int i = 0; i < friend.getFriends().size(); i++) {
for (int j = 0; j < recFriends.size(); j++) {
for (int k = 0; k < recFriends.size(); k++) {
if (recFriends.get(j).equals(recFriends.get(k))) {
recFriends.remove(j); // This removes any duplicate friends so there won't be an infinite loop.
break;
}
}
}
recFriends.addAll(friend.getFriends().get(i).getRecommendations(friend)); // This adds the friends' friends' friends and so on into the ArrayList.
}
return recFriends;
}
ArrayList<FacebookUser> getFriends() {
ArrayList<FacebookUser> friendsCopy = new ArrayList<FacebookUser>();
for (int i = 0; i < friends.size(); i++) {
friendsCopy.add(friends.get(i));
}
Collections.sort(friendsCopy, new Comparator<FacebookUser>() {
#Override
public int compare(FacebookUser fb1, FacebookUser fb2) {
return fb1.toString().compareToIgnoreCase(fb2.toString());
}
});
return friendsCopy;
}
public static void writeToFile(FacebookUser facebookUser) throws IOException {
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("FacebookUser.bin"));
oos.writeObject(facebookUser); // This saves the data.
}
public static FacebookUser readFile() throws IOException, ClassNotFoundException {
ObjectInputStream ois = new ObjectInputStream(new FileInputStream("FacebookUser.bin"));
FacebookUser facebookUser = (FacebookUser) ois.readObject(); // This reads in the data which is called in the DriverClass class line 12.
return facebookUser;
}
}
UserAccount.java
import java.io.Serializable;
public abstract class UserAccount implements Serializable{
private String username;
private String password;
private boolean active;
public UserAccount(String username, String password) {
this.username = username;
this.password = password;
active = true;
}
public boolean checkPassword(String password) {
if (password.equals(this.password)) {
return true;
} else {
return false;
}
}
public void deactivateAccount() {
active = false;
}
public String getPassword() {
return password;
}
public String toString() {
return username;
}
public boolean checkActive() {
if (active == true) {
return true;
} else {
return false;
}
}
public abstract void getPasswordHelp();
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((username == null) ? 0 : username.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
UserAccount other = (UserAccount) obj;
if (username == null) {
if (other.username != null)
return false;
} else if (!username.equals(other.username))
return false;
return true;
}
}
DriverClass.java
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
public class DriverClass {
public static void main(String[] args) throws IOException, ClassNotFoundException {
int choice = 0;
String username;
FacebookUser fu0;
try {
fu0 = FacebookUser.readFile();
} catch (FileNotFoundException e) {
fu0 = new FacebookUser("Robert", "password1");
}
while (choice != 5) {
System.out.println(
"1. List Users \n2. Add a User \n3. Delete a User \n4. Get Password Hint \n5. Get Recommendations \n6. Quit");
Scanner s = new Scanner(System.in);
choice = s.nextInt();
switch (choice) {
case 1:
System.out.println(fu0.getFriends());
break;
case 2:
System.out.println("Username: ");
s.nextLine();
username = s.nextLine();
fu0.friend(username);
break;
case 3:
System.out.println("Username: ");
s.nextLine();
username = s.nextLine();
fu0.defriend(username);
break;
case 4:
System.out.println("Username: ");
s.nextLine();
username = s.nextLine();
fu0.getPasswordHelp(username);
break;
case 5:
System.out.println(fu0.getRecommendations(fu0));
break;
case 6:
FacebookUser.writeToFile(fu0);
break;
}
}
}
}

The exception suggests that the SerialVersionUID the JVM generated differs for some instances of the FacebookUser object. If you make your class implement Serializable, it is highly recommended that you explicitly declare the SerialVersionUID variable to avoid exceptions like these.
So in the FacebookUser class you should declare a variable like this:
private static final long serialVersionUID = 42L;
See this answer for more details.

Java Obtaining the First Element and Then Removing that First Element Printing the Rest of the Elements

Here is my code:
import java.util.LinkedList;
import java.util.Scanner;
import java.util.InputMismatchException;
import java.util.*;
class Customer {
public String lastName;
public String firstName;
public Customer() {
}
public Customer(String last, String first) {
this.lastName = last;
this.firstName = first;
}
public String toString() {
return firstName + " " + lastName;
}
}
class HourlyCustomer extends Customer {
public double hourlyRate;
public HourlyCustomer(String last, String first) {
super(last, first);
}
}
class GenQueue<E> {
private LinkedList<E> list = new LinkedList<E>();
public void enqueue(E item) {
list.addLast(item);
}
public E dequeue() {
return list.poll();
}
public boolean hasItems() {
return !list.isEmpty();
}
public boolean isEmpty()
{
return list == null;
}
public E removeFirst(){
return list.removeFirst();
}
public E getFirst(){
return list.getFirst();
}
public int size() {
return list.size();
}
public void addItems(GenQueue<? extends E> q) {
while (q.hasItems()) list.addLast(q.dequeue());
}
}
public class something {
public static void main(String[] args){
while(true){
Scanner sc = new Scanner(System.in);
String input1;
String input2;
int choice = 1000;
GenQueue<Customer> empList;
empList = new GenQueue<Customer>();
GenQueue<HourlyCustomer> hList;
hList = new GenQueue<HourlyCustomer>();
do{
System.out.println("================");
System.out.println("Queue Operations Menu");
System.out.println("================");
System.out.println("1,Enquene");
System.out.println("2,Dequeue");
System.out.println("0, Quit\n");
System.out.println("Enter Choice:");
try{
choice = sc.nextInt();
switch(choice){
case 1:
do{
System.out.println("\nPlease enter last name: ");
input1 = sc.next();
System.out.println("\nPlease enter first name: ");
input2 = sc.next();
hList.enqueue(new HourlyCustomer(input1, input2));
empList.addItems(hList);
System.out.println("\n"+(input2 + " " + input1) + " is successful queued");
System.out.println("\nDo you still want to enqueue?<1> or do you want to view all in queue?<0> or \nBack to main menu for dequeueing?<menu>: ");
choice = sc.nextInt();
}while (choice != 0);
System.out.println("\nThe customers' names are: \n");
while (empList.hasItems()) {
Customer emp = empList.dequeue();
System.out.println(emp.firstName + " " + emp.lastName + "\n" );
}
// int choose;
// do{
//
//
// System.out.println("\nGo back to main?<1=Yes/0=No>: \n");
// choose = sc.nextInt();
// }while (choose != 1);
break;
case 2:
if (empList.isEmpty()) {
System.out.println("The queue is empty!");
}
else{
System.out.println("\nDequeued customer: " +empList.getFirst());
empList.removeFirst();
System.out.println("\nNext customer in queue: " +empList.getFirst()+"\n");
}
break;
case 0:
System.exit(0);
default:
System.out.println("Invalid choice");
}
}
catch(InputMismatchException e){
System.out.println("Please enter 1-5, 0 to quit");
sc.nextLine();
}
}while(choice != 0);
}
}
}
Case 2 shows no error but when I run it on the IDE it shows:
Exception in thread "main" java.util.NoSuchElementException at
java.util.LinkedList.getFirst(Unknown Source) at
GenQueue.getFirst(something.java:44) at
something.main(something.java:113)
Any idea why this happen? and how to fix it? Your help will be very much appreciated, from python I just went Java today, newbie here.

NoSuchElementException is thrown by LinkedList.getFirst when the list is empty.
You should handle that particular case before calling that method, for example:
if (!empList.hasItems()) {
System.out.println("The queue is empty!");
} else {
System.out.println("First in queue: " +empList.getFirst());
... // rest of code
}

Matching the index of username and index of password in java

I want to match the username "carl" to the password "0001", as well as the other elements, but it's not working. Can anyone explain me how to match each elements of "username" to the elements of "password"? Thank you very much!
This is my code:
class log{
BufferedReader br= new BufferedReader(new InputStreamReader(System.in));
String [] username ={"carl", "may", "joseph", "oliver", "ashley"};
String []password={"0001", "0002", "0003", "0004", "0005"};
public void login() throws IOException{
System.out.print("\nUSERNAME: ");
uname=br.readLine();
System.out.print("PASSWORD: ");
pword=br.readLine();
for (j = 0; j <= 4; j++) {
if (username[j].equals(password[j])) {
uname=username[j];
pword=password[j];
}
}
while((!uname.equals(password[j]))&&(!pword.equals(username[j]))){
System.out.println("Invalid username/password.");
System.out.print("\nUSERNAME: ");
uname=br.readLine();
System.out.print("PASSWORD: ");
pword=br.readLine();
}
public void main(String[] args)throws IOException {
log b=new log();
b.login();
}

You can use OOP (Object Oriented Programming) to make it easier in sovling your problem:
Here is my sample solution:
User class:
class User {
private String username;
private String password;
public User(String username, String password) {
this.username = username;
this.password = password;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
#Override
public boolean equals(Object o) {
User user2 = (User) o;
if (this.username.equals(user2.getUsername()) && this.password.equals(user2.getPassword()))
return true;
return false;
}
}
Login class:
public class Login {
private User[] users = new User[]{
new User("carl", "0001"),
new User("may", "0002"),
new User("joseph", "0003"),
new User("oliver", "0004"),
new User("ashley", "0005"),
};
public static void main(String args[]) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.print("\nUSERNAME: ");
String uname = br.readLine();
System.out.print("PASSWORD: ");
String pword = br.readLine();
boolean success = new Login().login(new User(uname, pword));
if (success)
System.out.println("The user account is valid, login");
else
System.out.println("The username and/or password is invalid.");
}
public boolean login(User user) {
for (int i = 0; i < users.length; i++) {
if (users[i].equals(user))
return true;
}
return false;
}
}

I think you should modify your method in following way
public void login() throws IOException {
System.out.print("\nUSERNAME: ");
uname = br.readLine();
System.out.print("PASSWORD: ");
pword = br.readLine();
while (!isValid(uname, pword)) {
System.out.println("Invalid username/password.");
System.out.print("\nUSERNAME: ");
uname = br.readLine();
System.out.print("PASSWORD: ");
pword = br.readLine();
}
}
//checks if the entered username and password are correct
public boolean isValid(String uname, String pword) {
for (int j = 0; j <username.length; j++) {
if (username[j].equals(uname)) {
if (!password[j].equals(pword))
return false;
else
return true;
}
}
return false;
}

I think it it better when you work with an extra Class User.
The class looks like this:
class User {
private String name;
private String pw;
public User (String name, String pw) {
this.name = name;
this.pw = pw;
}
public String getName() {
return this.name;
}
public String getPw() {
return this.pw;
}
}
In your program logic you have a list with all users:
ArrayList<User> userList
Now you have change the login algorithm:
class log{
BufferedReader br= new BufferedReader(new InputStreamReader(System.in));
ArrayList<User> userList = ...; //Here you have to add all users first
boolean loginCheck = false;
while(loginCheck == false) {
System.out.print("\nUSERNAME: ");
uname = br.readLine();
System.out.print("PASSWORD: ");
pword = br.readLine();
for (int i = 0; i < userList.size(); i++) {
if (userList.get(i).getName.equals(uname) && userList.get(i).getPw.equals(pword)) {
loginCheck = true;
break;
}
}
}
}
public void main(String[] args)throws IOException {
log b=new log();
b.login();
}
You can make the algorithm faster if you change the check for the user input into this:
for (int i = 0; i < userList.size(); i++) {
if (userList.get(i).getName.equals(uname)) {
if (userList.get(i).getPw.equals(pword)) {
loginCheck = true;
break;
} else {
break;
}
}
}

Standard FTP Login java class

Is there a defined standard class for dealing with FTP login data? Something that already handles storing, sharing? Since I am relatively new to this, maybe someone else gave this topic already a thorough thinking. Could not find anything in Apache Commons so far...
For now I use my own class, it is not very fulfilling:
public class FTPLogin {
String hostaddress;
int port;
String name;
String password;
public FTPLogin() {
}
public FTPLogin(String hostaddress, int port, String name,
String password) {
this.hostaddress = hostaddress;
this.port = port;
this.name = name;
this.password = password;
}
#Override
public String toString() {
return hostaddress + ";" + port + ";" + name + ";" + password;
}
}

There is no Standard, I prefer ftp4j

import java.util.Scanner;
class users
{
String username;
String password;
String[][] accounts = {{"Karthik", "k123"},{"Ram", "r123"}};
users(String user, String pass)
{
username = user;
password = pass;
}
public boolean auth()
{
if((username == accounts[0][0]) && (password == accounts[0][1]))
{
return true;
}
else if((username == accounts[1][0]) && (password == accounts[1][1]))
{
return true;
}
else
{
return false;
}
}
public static void main(String[] args)
{
Scanner sc=new Scanner(System.in);
System.out.println("Enter name:");
String s=sc.next();
System.out.println("Enter pid:");
int i=sc.nextInt();
users obj=new users(s,i);
System.out.println(obj.auth());
}
}

import java.util.Hashtable;
import java.util.Scanner;
class Login
{
public boolean checkValue(String name,String Password)
{
Hashtable<String,String> map=new Hashtable<String,String>();
map.put(name,Password);
//System.out.println(map);
if(name.equals("abcd") && Password.equals("1234"))
{
return true;
}
else if(name.equals("Mnop") && Password.equals("5678"))
{
return true;
}
else if(name.equals("xyzx") && Password.equals("7890"))
{
return true;
}
else
{
return false;
}
}
public static void main(String[] args)
{
Scanner sc=new Scanner(System.in);
System.out.println("Enter a name s:");
String s=sc.next();
System.out.println("Enter a password p:");
String p=sc.next();
Login obj=new Login();
boolean status=obj.checkValue(s,p);
System.out.println(status);
}
}

DeSerialized Object containing ArrayList has all null values

Hopefully someone can provide some insight on this issue. I have created an instance of an object that contains an ArrayList of information (username, password, password hint). I am trying to serialize the object. It looks like it is serializing properly, but when I restart the project to deserialize, it returns null values in the ArrayList. Why is it returning null values for the ArrayList objects?
Driver Class:
import java.io.Serializable;
public class TestingDriver implements Serializable{
private static final long serialVersionUID = 12345L;
private static TestingAccount users = new TestingAccount();
public static void main(String[] args) {
int foreverLoop = 0;
users = DeSerialize.main();
while (foreverLoop < 1) {
int selection = users.displayMainMenu();
if (selection == 1) {
users.listUsers();
}
else if (selection == 2) {
users.addUser();
}
else if (selection == 3) {
users.deleteUser();
}
else if (selection == 4) {
users.getPasswordHint();
}
else if (selection == 5) {
Serialize.main(users);
System.exit(0);
}
else {
System.out.println("That option does not exist. Please try again.");
}
}
}
}
TestingUser Class (objects of this class will populate the ArrayList):
import java.io.Serializable;
public class TestingUser extends UserAccount implements Serializable, Comparable <TestingUser> {
private static final long serialVersionUID = 12345L;
public TestingUser(String username, String password, String passwordHint) {
super(username, password, passwordHint);
}
public TestingUser() {
}
#Override
public void getPasswordHelp() {
System.out.println("");
System.out.println("Password hint: " + passwordHint);
System.out.println("");
}
#Override
public int compareTo(TestingUser otherAccount) {
if (this.username.compareToIgnoreCase(otherAccount.username) < 0) {
return -1;
}
else if (this.username.compareToIgnoreCase(otherAccount.username) > 0) {
return 1;
}
else {
return 0;
}
}
}
TestingAccount class (calling this class creates an object that contains the ArrayList):
import java.io.Serializable;
import java.util.ArrayList;
import java.util.Scanner;
public class TestingAccount implements Serializable {
private static final long serialVersionUID = 12345L;
public ArrayList<TestingUser> userList;
private String username;
private String password;
private String passwordHint;
public TestingAccount() {
userList = new ArrayList<TestingUser>();
}
public void listUsers() {
for (int i=0; i<this.userList.size(); i++) {
System.out.println(this.userList.get(i));
}
}
#SuppressWarnings("resource")
public void addUser() {
Scanner input = new Scanner(System.in);
System.out.println("Please enter a username: ");
username = input.next();
TestingUser tempAccount = new TestingUser(username, null, null);
if (this.userList.contains(tempAccount) == true) {
System.out.println("This user already exists.");
}
else {
System.out.println("Please enter a password: ");
password = input.next();
System.out.println("Please enter a password hint: ");
passwordHint = input.next();
tempAccount.password = password;
tempAccount.passwordHint = passwordHint;
this.userList.add(tempAccount);
System.out.println("Account " + tempAccount.username + " has been added.");
}
}
#SuppressWarnings("resource")
public void deleteUser() {
Scanner input = new Scanner(System.in);
System.out.println("Please enter the username to be deleted: ");
username = input.next();
TestingUser tempAccount = new TestingUser(username, null, null);
if (this.userList.contains(tempAccount) == true) {
int actIndex = this.userList.indexOf(tempAccount);
System.out.println("Please enter the password: ");
password = input.next();
tempAccount.password = password;
boolean passwordGood = this.userList.get(actIndex).CheckPassword(tempAccount);
int accountIndex = this.userList.indexOf(tempAccount);
tempAccount = this.userList.get(accountIndex);
if (passwordGood == true) {
this.userList.remove(actIndex);
System.out.println("The account has been deleted.");
}
else {
System.out.println("The password is not correct.");
}
}
else {
System.out.println("The account does not exist.");
}
}
#SuppressWarnings("resource")
public void getPasswordHint() {
Scanner input = new Scanner(System.in);
System.out.println("Please enter a username: ");
username = input.next();
TestingUser tempAccount = new TestingUser(username, null, null);
if (this.userList.contains(tempAccount) == true) {
int actIndex = this.userList.indexOf(tempAccount);
tempAccount = this.userList.get(actIndex);
System.out.println("The password hint isL: " + tempAccount.passwordHint);
}
else {
System.out.println("The account does not exist.");
}
}
#SuppressWarnings({ "resource" })
public int displayMainMenu() {
int selection = 0;
Scanner input = new Scanner(System.in);
System.out.println("");
System.out.println("System Menu:");
System.out.println("1. List Users");
System.out.println("2. Add User");
System.out.println("3. Delete User");
System.out.println("4. Get Password Hint");
System.out.println("5. Quit");
System.out.println("");
System.out.println("What would you like to do?");
selection = input.nextInt();
return selection;
}
}
Serialize class:
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
public class Serialize {
public static void main(TestingAccount users) {
try {
ObjectOutputStream oos = new ObjectOutputStream(
new FileOutputStream("serialize"));
oos.writeObject(users);
oos.close();
} catch (FileNotFoundException e1) {
System.err.println("File not found.");
} catch (IOException e2) {
System.err.println("Unable to serialize.");
e2.printStackTrace();
}
}
}
Deserialize class:
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.ObjectInputStream;
public class DeSerialize {
public static TestingAccount main() {
TestingAccount deSerialize = null;
try {
ObjectInputStream ois = new ObjectInputStream(
new FileInputStream("serialize"));
deSerialize = (TestingAccount) ois.readObject();
ois.close();
} catch (FileNotFoundException e1) {
System.err.println("Unable to open file.");
} catch (IOException e2) {
System.err.println("Could not de-serialize.");
e2.printStackTrace();
} catch (ClassNotFoundException e3) {
System.err.println("Could not cast to class TestingAccount.");
}
return deSerialize;
}
}

It looks like UserAccount isn'tSerializable. So when you serialize the derived TestingUser class, none of the UserAccount data gets serialized. See Object Serialization Specification #2.1.13(a). TestingUser doesn't have any instance state of its own to serialize.
The solution is to make UserAccount implement Serializable.
Not sure why this comes as a surprise.