To use the "java" command-line tool you need to install a JDK

Get Java for your Mac

Or go to Oracle.com to find their Java installer for the Mac. They have an SDK installer that you do not want. The plug-in is a JRE installer.

The Apple Java is a Legacy version needed especially if you use Adobe products.

Went from G5 to MacBook Air. Mojave 10.14.1. How do I get rid of this annoying pop up?

You either uninstall the crappy software that requires Apple's defunct Java 6 implementation, or install Apple's defunct Java 6. Download Java for OS X 2017-001

It should have been simple for the software you installed to ask for the most current, compatible Java version installed, but instead they coded their software to specifically require an ancient version developed by Apple.