Have you had projects that compiled without a problem in JBuilder 5, only to
have them fail to work in JBuilder 6?  Have you had problems with classing 
being built, only to have the JVM say that the classloader can't find the 
class?  Here's a workaround I developed that seems to have solved these  
problems for me.

First in your \jbuilder6\bin\jbuilder.config file add the following line:

vmparam -Duse.old.bmj

Next, start JBuilder 6.  Open your project.  From the main menu select Project
| Project Properties.  Choose the 'Build' tab.  Be sure two options are 
CHECKED:

- Check stable packages.
- Make stable packages.

Doing this seems to have fixed all build errors for me.