hodoku.jar on Ubuntu 18.04

I have used hodoku.jar successfully on Ubuntu 16.04, my prior version.

Now I cannot get it to run. I have the two OpenJDF Java Runtime installed (8 & 11). I have the .jar set to execute, but it doesn't. I have also tried the Hodoku user manual suggested

java -Xmx512m -jar hodoku.jar

command. And also tried it with the option -Xmx1024m.

I really like this game and would like to get it running again.
Any advice is appreciated.

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