I have the following classes:

class A implements Serializable {
  String field1;
  String field2;
  // a lot more fields here...
}

class B extends A {
  int someOtherField;
}

where B is my class and A is in a package I don't own. In my environment, an object of class B is serialized and will later be deserialized in a different environment, where only class A is in the classpath and not class B, thus an error will occur, that class B is not found.

To solve this problem, I could add a copy constructor to A as follows and instead of serializing an object b, I would serialize new A(b):

A(A other) {
  this.field1 = other.field1;
  this.field1 = other.field1;
  // and so on
}

My problem now is that I can only change B, but not A. So, is there any way I can serialize a B and deserialize it as an A without access to the source code of A?