Question

What is the effect of declaring a private static final long serialVersionUID = 1945670924947820279L; in a class that implements Serializable?

I remember that one may implement the class without specifying the long, or just 1L. What is the difference?

import java.io.Serializable;

public class KAS implements Serializable
{ 
    private static final long serialVersionUID = 1945670924947820279L;
}

Answer 1

If you do not specify a version, the JVM will use some internal rules to try to work out on its own whether or not the serialized object and the version in its own classloader are compatible. This can result in a situation where someone has an outdated jar but the deserialization still works, because the newer version of the class didn’t change it in a way that is picked up as an incompatibility. (Perhaps you fixed a bug in a method implementation but left all the fields alone.)

Alternately you can exploit it by doing something like setting the value to 1 and never changing it if you want people with outdated versions to still be able to use the data.