You can get rid of most of these simply by not extending these mostly
concrete classes (okay, difficult for JApplet, but you can limit the
number of subclasses, Throwable is quite useful too...).

When an object is serialised, the class information includes this UID.
If you choose not to provide one, then a value is derived using some
function applied to the public attributes of the class. The message just
mean that that function will need to be applied to you class if an
instance is serialised. You can supply one by adding a field:

   private static final long serialVersionUID = ...some long number...L;

You can choose anything. Preferably it should not duplicate the number
of another class. One way of creating it is to run serialver on the
class, and that gives the number which would be assigned anyway. If the
way the class is serialised changes, then so should the serialVersionUID.

The Bad Thing is that a change in public attributes of a class may
change the generated UID, and therefore prevent new code loading older
serialised data, or vice versa.

The Bad Thing that may happen if you do supply one is that you really do
change the serialised format, but not the UID. Hence a complete mess
when it comes to deserialisation.

It is a pain if you want a custom exception, say, and have no intention
of going anywhere near serialization. I guess you could get a revision
control system to embed a version number in the constant.

Tom Hawtin

On Sun, 11 Dec 2005 14:00:53 -0500, "Rhino"
<no.offline.contact.please@nospam.com> wrote, quoted or indirectly
quoted someone who said :

see http://mindprod.com/jgloss/serialization.html#SERIALVERSIONUID

An instance of a serializable class can be serialized: this means that its
state can be saved to a stream.
When you save an instance of a serializable class Test, you can reload the
instance into memory at a later point in time. The field serialVersionUID is
used to verify whether the class Test hasn't changed since you saved the
instance.

You can generate a number for a .class file: try
"$JAVA$/bin/serialver.exe -show" where $JAVA$ is your java-path.

Then put the generated value in the corresponding .java file like this:
private final long serialVersionUID = generatedValue;

Whenever you change some instance fields of the class, the generated value
will change (and you are supposed to adapt the value of serialVersionUID).
This way, when you try to load an old stream into an instance of the new
(adapted) class, an Exception will be thrown because serialVersionUID of the
saved stream doesn't match serialVersionUID of the adapted class.

Question to the community: why doesn't Java automatically save some
hashvalue when streaming an instance of a serializable class instead of
relying on a 'manual' value like serialVersionUID?