Back in college I had an assignment to build some kind of CRM-like system, and the professor made a point that we used Java applets, because that was the future (https://en.wikipedia.org/wiki/Write_once,_run_anywhere was all the rage).

I proposed instead learning PHP and using it for that purpose, but was laughed out of the room. Developing this was a PITA and I feel vindicated that applets faded away soon after, and PHP became part of the web-app "standard" (the LAMP acronym was started to be used around then). But I never learned PHP.