Serialization problem(StreamCorrupedException) in java.math.RoundingMode between 1.4.2 and 6

When a CU tries to read an object serialized in jdk6 in jdk1.4.2,
StreamCorruptedException occurs.

CONFIGURATION :
JDK : JDK6u23
OS : Windows Vista business(SP2, Japanese)

REPRODUCE :
1) Compile the attaxhed x.java in jdk6u23
   and the attached y.java in jdk1.4.2 respectively.
2) Invoke the following command line. Then StreamCorruptedException appears.

C:\Users\licensees\serialization-142-6>java -showversion x out6in142
java version "1.6.0_23"
Java(TM) SE Runtime Environment (build 1.6.0_23-b04)
Java HotSpot(TM) Client VM (build 19.0-b09, mixed mode)


C:\Users\licensees\serialization-142-6>java -showversion y out6in142
java version "1.4.2_22"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_22-b02)
Java HotSpot(TM) Client VM (build 1.4.2_22-b02, mixed mode)

Exception in thread "main" java.io.StreamCorruptedException
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1301)
        at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1838)
        at java.io.ObjectInputStream.defaultReadObject(ObjectInputStream.java:452)
        at java.text.DecimalFormat.readObject(DecimalFormat.java:2507)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:331)
        at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:838)
        at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1740)
        at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:322)
        at y.main(y.java:7)