The whole concept of serialization works on versioning. If you save a class object using one version of the class but attempt to deserialize using a newer or different version of the class deserialization might fail.

When you class structure can change in between you serialize the instance and go again to de-serialize it. Changed structure of class will cause JVM to give exception while de-serializing process.This problem can be solved only by adding a unique serial version id to class. It will prevent the compiler to throw the exception by telling that both classes are same, and will load the available instance variables only.

The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender’s class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named “serialVersionUID” that must be static, final, and of type long

 static final long serialVersionUID = 69L;

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