Going home after having written this I realized what was the problem. In essence it is the difference between all free software and almost all free software. IOW the java environment included in Fedora Core 5 (which is the distro I have on my new laptop) has the free java-gcj and not the non-free Sun JDK.
I seem to remember reading about Sun going open source with the core Java API but I guess it’s not come through yet. Anyhow, following these excellent instructions I managed to install the Sun Java alongside the Gnu Java. My project, including the NumberFormatter objects, now compiles without errors.