Using a derived value in Forms

I have a derived value that comes from a custom Java class. How do I reuse the value in the form? eg: I want to use the value that comes from calculatedfld in Field1 and Field2.

<Field name='calculatedfld'>

<Display class='Text'>

<Property name='title' value='calculatedfld'/>

<Property name='size' value='30'/>

</Display>

<Default>

<invoke name='NextID' class='gencode'/>

</Default>

</Field>

[526 byte] By [nnkcvsa] at [2007-11-26 17:08:48]