Editing JavaAPI (section)

==Serialization==
When deserializing an object of class X, Java must establish that the incoming data is sufficiently compatible with the local class X definition. This is accomplished by comparing the so-called stream-unique identifiers (SUIDs) of the incoming and local class definitions. If the two SUIDs do not match, deserialization fails. If you don't do anything, that is manually add and maintain an attribute 
 public static final long serialVersionUID = 3L
to your class, the SUID is computed as a hash of various class elements: class name, implemented interfaces, declared nonstatic, nontransient fields, declared nonprivate methods, and so on.