DDariusA Open the terminal and enter java -version
Then tell us what it says.
Mine says this
bigrammy@bigrammy ~ $ java -version
openjdk version "1.8.0_252-solus"
OpenJDK Runtime Environment (build 1.8.0_252-solus-ga)
OpenJDK 64-Bit Server VM (build 25.252-bga, mixed mode)
If yours says the same then the system is using java 8.
If that is the case and you installed java 11 then you need to switch from 8 > 11
to do that you need to change the system links to point to java 11.
So in terminal enter this ls /usr/lib64/openjdk-11/bin/
if you see this output
jaotc javadoc jdeprscan jinfo jps jstatd rmiregistry
jar javap jdeps jjs jrunscript keytool serialver
jarsigner jcmd jfr jlink jshell pack200 unpack200
java jconsole jhsdb jmap jstack rmic
javac jdb jimage jmod jstat rmid
Then we are in business and can continue.
Now still in terminal enter this
cd /usr/bin/
Now enter the following line by line
sudo ln -sf /usr/lib64/openjdk-11/bin/java java
sudo ln -sf /usr/lib64/openjdk-11/bin/javac javac
sudo ln -sf /usr/lib64/openjdk-11/bin/java javadoc
sudo ln -sf /usr/lib64/openjdk-11/bin/javap javap
sudo ln -sf /usr/lib64/openjdk-11/bin/jar jar
sudo ln -sf /usr/lib64/openjdk-11/bin/jarsigner jarsigner
Now enter java -version
Hopefully your now showing java 11.
Now enter cd
to where your .jar is located example: If your.jar was in Downloads you would enter
cd ~/Downloads
then enter java -jar your.jar
To switch back to java 8
cd /usr/bin/
Now enter the following line by line
sudo ln -sf /usr/lib64/openjdk-8/bin/java java
sudo ln -sf /usr/lib64/openjdk-8/bin/javac javac
sudo ln -sf /usr/lib64/openjdk-8/bin/java javadoc
sudo ln -sf /usr/lib64/openjdk-8/bin/javap javap
sudo ln -sf /usr/lib64/openjdk-8/bin/jar jar
sudo ln -sf /usr/lib64/openjdk-8/bin/jarsigner jarsigner
Now enter java -version
Hopefully your now showing java 8.
Good Luck.