ObjectInputStream.GetFields: Useful in readObject()?

Question

The following is an excerpt from the User Guide description of ObjectInputStream.GetField class (http://java.sun.com/j2se/1.3/docs/guide/serialization/spec/input.doc2.html):
��Using readFields to access the serializable fields does not change the format of the stream. It only provides an alternate API to access the values which does not require the class to have the corresponding non-transient and non-static fields for each named serializable field. The serializable fields are those declared using serialPersistentFields or if it is not declared the non-transient and non-static fields of the object. When the stream is read the available serializable fields are those written to the stream when the object was serialized. If the class that wrote the stream is a different version not all fields will correspond to the serializable fields of the current class. The available fields can be retrieved from the ObjectStreamClass of the GetField object.�
Based on this description, I assumed that this API could be used within an object�s readObject() method to retrieve fields with names/types that differ from the current object. This assumption appears to be false.
In my tests with 1.3 and 1.4.01, the serialization code makes a number of comparisons between the class being deserialized, and the local (vm loaded) version of the class. These comparisons include class cast compatibility, serialVersionUID compatibility, and field name/type compatibility. If any of the test fail, an appropriate exception is thrown. These tests are performed before the call is made to the object�s readObject method. (In 1.4_01, these tests are performed by ObjectSteamClass.initNonProxy().)
In the following example, Foo (version 2) cannot successfully deserialized Foo (version 1). The serialization mechanism with throw a field type mismatch on �field1� before calling readObject(). Foo (version 2b) will fail as well. This time, because of an incompatible serialVersionUID.
// Version 1
public class Foo implements Serializable
    private static final long serialVersionUID = 1L;
    protected int field1 = 123;
// Version 2
public class Foo implements Serializable
    private static final long serialVersionUID = 1L;
    protected String field1 = 123;
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException
        ObjectInputStream.GetField fields = in.readFields();
        field1 = Integer.toString(fields.get(�field1�,  0));
// Version 2b
public class Foo implements Serializable
    private static final long serialVersionUID = 2L;
    protected String field1 = 123;
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException
        ObjectInputStream.GetField fields = in.readFields();
        field1 = Integer.toString(fields.get(�field1�,  0));
}It appears ObjectInputStream.readFields() cannot be used within readObject() as the documentation describes. Am I missing something?
Thanks, Craig.

Answer 1

Oops. In version 2 and 2b, the following line:
protected String field1 = 123;should read:
protected String field1 = "123";