JOSM works anyway, but says that soon it will no longer support the 11 version, and I would like to prevent blocks…
Hallo. I use JOSM on Ubuntu 20.04.6LTS for a long time without problems. For some time now launching JOSM it says that my Java version 11 is too old and need to upgrade to 17. But looking to "java -version" it already finds the 17 version.
Hallo 
As far as I know, you can specify the location of Java as a command line option in the console when starting JOSM: Help/CommandLineOptions – JOSM
Another option would be to check and, if necessary, correct the PATH variable in the system. See also: How to set Java environment path in Ubuntu - Stack Overflow
How do you start JOSM?
Which Java version does the Help → About dialog of JOSM show?
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And if all else fails, try the flatpak.
Do you need java11 at all? Maybe, you could just uninstall it.
Though for better help than just guessing to find the needle in the haystack, we need more information:
How did you install JOSM?
Have you tried the Ubuntu package?
Curiously there’s a ticket open about i think PMD switches which are said to be aimed at 11, and needed updating when upgrading to 17.
That said, i use OpenWebStart as front to start and keep JOSM updated. Always have latest stable and it pulled in 21. Works great on Windows reading that OWS also is available for linux and mac
edit: ticket link Changeset 19307 – JOSM
While an Java upgrade is begged for, not yet a must.