Problem with NotSerializableException on class implementing Serializable

Question

Hi,
Any idea as to why I get this exception, and what I can do to avoid it ?
java.io.NotSerializableException: util.NumberStore$UserI have this, relatively simple, class
class User implements Serializable
     private static long serialVersionUID = 1L;
     private String uid;
     private String nr;
     public User(){}
     public User( String uid, String nr ) {
          this.uid = uid;
          this.nr = nr;
     public String getUid()     {
          return uid;
     public String getNr() {
          return nr;
     public void setUid(String uid){
          this.uid = uid;
     public void setNr(String nr){
          this.nr = nr;
}When loading and saving it, I use these two methods:
     private void saveLocalList()
          FileOutputStream f = null;
          try {
               f = new FileOutputStream("./users.dat");
               ObjectOutputStream out = new ObjectOutputStream(f);
               for (User u: numbers)
                    out.writeObject(u);
               f.close();
          } catch (IOException e) {
               System.err.println("User info not saved:"+e.getLocalizedMessage());
               if (f != null)
                    try {
                         f.close();
                    } catch (IOException e1) {
     private boolean loadLocalList()
          FileInputStream f = null;
          try {
               f = new FileInputStream("./users.dat");
               ObjectInputStream input = new ObjectInputStream(f);
               numbers = new ArrayList<User>();
               User u = (User) input.readObject();
               while (u != null)
                    numbers.add(u);
                    u = (User) input.readObject();
               f.close();
               return true;
          } catch (FileNotFoundException e) {
               return false;
          } catch (IOException e) {
               e.printStackTrace();
               if (f != null)
                    try {
                         f.close();
                    } catch (IOException e1) {
                         // ignore close errors
               return false;
          } catch (ClassNotFoundException e) {
               if (f != null)
                    try {
                         f.close();
                    } catch (IOException e1) {
                         // ignore close errors
               return false;
     }Both the User class and the methods are found within the NumberStore class.

Answer 1

The User class indeed does not contain any references at all to the NumberStore classIf 'User' is nested inside 'NumberStore' and it isn't static, User does indeed have a hidden reference to the outer class NumberStore. That's how the syntax 'NumberStore.this' works.
I can say with absolute certainty that an object which was not marked as serializable was written to the users.dat file.Quite apart from the fact that the source code of ObjectOutputStream makes it utterly impossible, I don't see how you can possibly say that 'with absolute certainty'. For a start there's no information in the file to indicate what interfaces were and weren't implemented by the class of the serialized object at the time it was serialized.
I'm not absolutely positive about whether or not it bombed out on this occassion, or when trying to re-read it next time aroundNotSerializableException is only thrown when writing, not when reading, so that answers that.
Do you still have the file? I'd like to see exactly what was written. In any case a serialization file that copped any kind of IOException when being written should have been thrown away immediately, you're right about that.
BTW your reading code is wrong. You shouldn't be testing for null, unless you are deliberately writing a null as a sentinel value. You should be catching EOFException.