FULL PRODUCT VERSION :
java version "1.4.2_04"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_04-b05)
Java HotSpot(TM) Client VM (build 1.4.2_04-b05, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows 2000 [Version 5.00.2195]

A DESCRIPTION OF THE PROBLEM :
De-serializing an instance of this class throws a NullPointerException:

    import java.io.*;
    import java.util.*;

    private class MyClass implements Serializable
    {
        private String m_name = "anything";
        private Set m_set = new HashSet ();

        public MyClass ()
        {
            m_set.add (this);
        }

        public int hashCode ()
        {
            return m_name.hashCode ();
        }
    }

I think this happens because the VM attempts to de-serialize m_set first and in doing so calls hashCode() on each of its members. However, the instance of MyClass, contained in m_set, has not yet been fully de-serialized since this involves de-serializing m_set first. Thus, m_name is still null when hashCode() is called, causing the Exception.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Serialize and then De-serialize an instance of MyClass using the program provided.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I was expecting to see  MyClass De-serialize completely and wihout error.
ACTUAL -
A NullPointerException was thrown from the line:

           return m_name.hashCode ();

m_name was null.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.NullPointerException
	at com.bentley.test.MyClass.hashCode(TestSerialization.java:67)
	at java.util.HashMap.hash(HashMap.java:261)
	at java.util.HashMap.put(HashMap.java:379)
	at java.util.HashSet.readObject(HashSet.java:277)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:324)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:838)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1746)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1845)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1769)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:324)
	at com.bentley.test.TestSerialization.input(TestSerialization.java:46)
	at com.bentley.test.TestSerialization.main(TestSerialization.java:23)
Exception in thread "main"

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
/*--------------------------------------------------------------------------------------+
 |
 |    $RCSfile: TestSerialization.java,v $
 |   $Revision: 1.0 $
 |       $Date: 2004/08/24 14:31:30 $
 |
 |  $Copyright: (c) 2004 Bentley Systems, Incorporated. All rights reserved. $
 |
 +--------------------------------------------------------------------------------------*/

package com.bentley.test;

import java.io.*;
import java.util.*;


public class TestSerialization
{
    public static void main (String[] args) throws Exception
    {
        MyClass obj = new MyClass ();
        byte[] serialized = output (obj);
        Serializable deserialized = input (serialized);
    }

    public static byte[] output (Serializable obj) throws Exception
    {
        ByteArrayOutputStream obytes = new ByteArrayOutputStream ();
        ObjectOutputStream ostream = new ObjectOutputStream (obytes);

        ostream.writeObject (obj);
        ostream.flush ();
        byte[] bytes = obytes.toByteArray ();

        obytes.close ();
        ostream.close ();

        return bytes;
    }

    public static Serializable input (byte[] bytes) throws Exception
    {
        ByteArrayInputStream ibytes = new ByteArrayInputStream (bytes);
        ObjectInputStream istream = new ObjectInputStream (ibytes);

        Serializable obj = (Serializable)istream.readObject ();

        ibytes.close ();
        istream.close ();

        return obj;
    }
}

class MyClass implements Serializable
{
    private String m_name = "anything";
    private Set m_set = new HashSet ();

    public MyClass ()
    {
        m_set.add (this);
    }

    public int hashCode ()
    {
        return m_name.hashCode ();
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
I have not found a good one yet, other than to avoid overriding the hashCode method.
###@###.### 2004-12-14 10:26:52 GMT