I’m trying to connect from Base to Hsqldb on a Linux server.
Which JDBC driver class should I use ?
You don’t need to connect to HSQLDB. HSQLDB is a built database engine, built right into LibreOffice Base (in other words it’s already connected).
You use connect for things like MySQL which are not built into LibreOffice Base.
HSQL server requires a different connection the same as MySQL and others. It affords the ability to access data from multiple locations simultaneously as well as keeping data separated from .odb file. See HSQL manual (depending on version) for details.