Downgrade OpenJDK 11 to 8

I had the same problem in Xubuntu 18.04 with Eclipse 2018-12 (4.10.0), using LibGDX. It was working fine, but probably some update in the system (or to OpenJDK specifically) started this problem. In addition Gradle Tasks weren’t showing up in the Gradle Window.

I solved the issue by removing theses packages: default-jre, default-jdk, default-jre-headless, default-jdk-headless, all of which were “pointing at” openjdk-11.

Then I installed openjdk-8-jre, openjdk-8-jdk, openjdk-8-jre-headless and openjdk-8-jdk-headless.

And I changed eclipse.ini‘s vm section to:
-vm /usr/lib/jvm/java-8-openjdk-amd64/bin

And now everything is working fine, and Gradle tasks are showing again.