java.io.InvalidClassException: local class incompatible:
client-server, deserialization, java, serialization
Solution
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 `serialVersionUID`s 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;
...
Problem
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; } ```