Question

I created client and server and then added a class in client side for serializing purposes, then simply just went to the folder of the client in my hard drive and copy paste it to the server correponding location, both classname.class and classname.java respectively.

It worked well in my own laptop but when I wanted to continue my work on other system , when I opened the projects folders and after client tries to connect to the server, the following error appears:

Exception in thread "main" java.io.InvalidClassException: projectname.clasname; local class incompatible: stream classdesc serialVersionUID = -6009442170907349114, local class serialVersionUID = 6529685098267757690
    at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:562)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1582)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1495)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1731)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)

What is going on? Is it because I ran the program with an older version of the IDE?

EDIT

import java.io.Serializable;
import java.net.URL;

public class KeyAdr implements Serializable {
  private static final long serialVersionUID = 6529685098267757690L;

  public URL adr;
  public String key;
}

Answer 1

If a class does not explicitly define a private static final long serialVersionUID in the code it will be autogenerated, and there is no guarantee that different machines will generate the same id; it looks like that is exactly what happened.
Also if the classes are different in any way (using different versions of the class) the autogenerated serialVersionUIDs will also be different.

From the Serializable interface’s docs:

If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class–serialVersionUID fields are not useful as inherited members. Array classes cannot declare an explicit serialVersionUID, so they always have the default computed value, but the requirement for matching serialVersionUID values is waived for array classes.

You should define a serialVersionUID in the class definition, e.g.:

class MyClass implements Serializable {
    private static final long serialVersionUID = 6529685098267757690L;
    ...