I am using Eclipse 2020-06 (I have made a fresh install of Eclipse instead of upgrading it from an earlier version of Eclipse) and JDK 8 (1.8.0_261). My OS is Windows 10.
JAVA_HOME is set to C:\Program Files\Java\jdk1.8.0_261
When I download "Codename One" plugin from Eclipse Marketplace it shows the version as 6.0 and it says "Installed" at the end of the installation process.
However, when I go to "Help -> Eclipse IDE -> Installation Details" under Eclipse, CodenameOneFeature version is listed as: 1.0.0.201409151325
When I select CodenameOneFeature from this list and hit Update, it says "No updates found". So basically I cannot force it to update to 6.0.
I can confirm that I have the earlier version installed also from the setting of the Codename One projects I create: when I go to Properties -> Java Compiler, the compliance level is set to 1.5 (also 1.5 is used in build.xml file).
BTW, I have also tried installing Eclipse 2019-06 (instead of 2020-06) and/or installing JDK 10 (instead of JDK 8) and/or modifying the eclipse.ini (by adding -vm option) so that it uses the version of JDK I want (instead of setting JAVA_HOME). But no combination of those has solved the problem.
Updated answer:
I found the problem. I broke the update site with a commit a couple of weeks ago. This should be fixed in a couple of hours once server caches refresh.
Original answer:
With Eclipse at this time we only support JDK 8. We're experiencing issues in updating the eclipse plugin to the latest version. Once installed I think you can also use JDK 11 but it doesn't matter since we don't support JDK 11 features.
Once a reasonably new version of the plugin is installed you can press update in Codename One Settings and it will update our libraries to the latest version equivalent to the other IDEs.