When I tried to serialize a class that has a serializable class in it (java.awt.Font) I get this exception:

java.io.NotSerializableException: sun.java2d.T1FontObject
        at java.io.ObjectOutputStream.outputObject(ObjectOutputStream.java:704)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:238)
        at java.io.ObjectOutputStream.outputClassFields(ObjectOutputStream.java:1250)
        at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:339)
        at java.awt.Font.writeObject(Font.java:527)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.io.ObjectOutputStream.invokeObjectWriter(ObjectOutputStream.java:1269)
        at java.io.ObjectOutputStream.outputObject(ObjectOutputStream.java:747)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:238)
        at java.io.ObjectOutputStream.outputClassFields(ObjectOutputStream.java:1250)
        at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:339)
        at java.io.ObjectOutputStream.outputObject(ObjectOutputStream.java:751)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:238)
        at Test.main(Test.java:17)

The underlying 2d java FontObject must not implements Serializable.

import java.io.*;
import java.awt.*;

public class Test implements Serializable {
    Font font;

    Test() {
font = new Font("Helvetica", Font.BOLD, 20);
    }

    public static void main(String args[]) {
ByteArrayOutputStream bo = null;

try {
bo = new ByteArrayOutputStream();
ObjectOutputStream so = new ObjectOutputStream(bo);
so.writeObject(new Test());
so.flush();
} catch (Exception e) {
e.printStackTrace();
System.exit(1);
}

try {
ByteArrayInputStream bi =
new ByteArrayInputStream(bo.toByteArray());
ObjectInputStream si = new ObjectInputStream(bi);
si.readObject();
} catch (Exception e) {
e.printStackTrace();
System.exit(1);
}
    }
}