Replying to my own problem with a partial solution. Perhaps it will
help someone else.

I now have most of my favorite Java sites working OK.

The problem was my "Hosts" file ( /etc/hosts ).

I had been using one on my Windows PCs to block advertising sites. I
transferred it to the Ubuntu box and appended it to the /etc/hosts
file generated during installation. Worked just fine during ordinary
web-surfing.

Maybe my "Hosts" file was malformed, maybe it was just too big. Java
sites started working again when I reverted /etc/hosts back to the
"installation" version.