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java 1.8 jar runs in Windows not Ubuntu, "could not find or load main class ..."

I have a jar file that runs fine from the Windows 10 command line, but is not working on my desktop Ubuntu 18.04 command line. The file was exported from Eclipse as a jar, and copied to Ubuntu. The project has about 30 classes over a half dozen packages. I've done some renaming in order to simplify things for this question.
This is the command used to run the jar:
java -jar myproject.jar
On Ubuntu, I get the error "Could not find or load main class com.a.b.LaunchThis"
Following are some of the things I've tried, based on suggestions from many similar posts. Sorry if this is a duplicate, but I couldn't find a workable answer.
I verified that the current Java is 1.8 using the commands:
$ update-alternatives --config java
and
$ java -version
openjdk version "1.8.0_252"
OpenJDK Runtime Environment (build 1.8.0_252-8u252-b09-1~18.04-b09)
OpenJDK 64-Bit Server VM (build 25.252-b09, mixed mode)
(I also verified that the program runs using java 8 on Windows.)
I ran the following check, which displays all the files, and confirmed that com/a/b/LaunchThis.class exists and is spelled exactly the same way.
java tj myproject.jar
The manifest seems correct.
Manifest-Version: 1.0
Main-Class: com.a.b.LaunchThis
I also tried unpacking the jar and running the program directly from the folder /myproject which contains both /com and META-INF. That should work, shouldn't it?
java com.a.b.LaunchThis
java com/a/b/LaunchThis
java com.a.b/LaunchThis
And from one level outside the myproject folder:
java myproject/com/a/b/LaunchThis
java myproject.com.a.b/LaunchThis
java myproject.com.a.b.LaunchThis
java myproject/com.a.b.LaunchThis
I am not totally clear on when the syntax requires . or /. The package command in "LaunchThis" which holds the main entry point is the following:
package com.a.b;
And in each of the above iterations I also tried including "-cp myproject.jar" as an option.
Always the exact same error message.
To be careful about not having a typo in the above, I first ran the ls command to make sure the file could be found (then edited this line to create the variants).
ls myproject.com.a.b.LaunchThis.class
Any suggestions as to what else to try?
For grins, running with Java11 gets this response:
Error: Could not find or load main class com.a.b.LaunchThis
Caused by: java.lang.NoClassDefFoundError: javafx/event/EventHandler
This makes some sense, because Java11 does not include JavaFX. But Java 8 does include EventHandler. It's been a part of Java since JavaFX 2.
UPDATE: I made two "Hello world" jars, one with and one without JavaFX on an Ubuntu installation of Eclipse running OpenJDK 8. To get the version with JavaFX to run, I downloaded the Oracle JDK 1.8, and linked to /lib/etc/jfxrt.jar as an "external jar" library. Both programs run in Eclipse but only the non-FX jar works. The FX jar gives exactly the same error message.
I think this pretty much establishes my problem to be one of not having JavaFX as part of OpenJDK 8.
My plan is to now try out two solutions: (1) copying the Oracle jfxrt.jar into the OpenJDK lib, (2) backloading OpenJFX 8.
Simply installing OpenJFX from repository loads OpenJFX 11, which is a little dated but will work with the OpenJDK 11.

The issue here is that the repository OpenJDK 8 on Ubuntu 18.04 does not include JavaFX. I'm guessing that the error message results from the fact that a JavaFX main() is located on a class that extends the JavaFX class Application.
How to fix this?
There are a number of things I tested.
First, one could use Oracle's JDK 1.8 instead. I ruled this out due to licensing issues.
Some sites suggested copying the jfxrt.jar from either the Oracle JDK 1.8 or from a working Java program that uses JavaFX and uses a self-contained JRE. I found that if I linked to the Oracle jfxrt.jar (located in the /jre/lib/ext folder) as an external jar, I could run a simple, "Hello world" javafx program in Eclipse. But I could not run the program after it was exported to a jar.
One answer on a related stackoverflow thread suggested copying over several additional files along with jfxrt.jar, which I tried. This did not work, so I'm not going to pass that on.
The solution that has been most successful came from a link provided in the comment by #dave_thomson_085, How do I get Java FX running with OpenJDK 8 on Ubuntu 18.04.2 LTS?.
This answer involves purging and reinstalling openjfx with an older version, and marking it to NOT be subjected to updates.
The code provided by Wolfgang Fahl (which he credits to Druidefix) follows:
apt purge openjfx
apt install openjfx=8u161-b12-1ubuntu2 libopenjfx-jni=8u161-b12-1ubuntu2 libopenjfx-java=8u161-b12-1ubuntu2
apt-mark hold openjfx libopenjfx-jni libopenjfx-java
Now, I can run and test jars containing Java8 using JavaFX that were built on my Windows Eclipse, on my Ubuntu desktop.
I do get a warning message which I haven't dealt with yet:
Gtk-Message: 13:25:40.829: Failed to load module "canberra-gtk-module"
But this is not preventing my programs from running.

Compiling JNI application on Mac OSX - No Java Runtime Present

I have a C++ application that uses JNI to create a Java virtual machine. I have the latest version of Java installed - JDK 8-144. If I run "java -version" it comes up and tells me that Java Hotspot 8 is the JVM. I have JAVA_HOME defined in the .profile file, and it works -- JAVA_HOME points to the JDK8-144 folder.
My application is including jni.h from the latest JDK. Our code creates a JVM with the JNI_VERSION_1_8 version flag. On Windows, this compiles and works perfectly.
The problem is that when we compile this code on the Mac, and use the JVM Framework for linking, we end up with an executable that, when run, says that there's "No Java Runtime present" and a dialog comes up that says we need to install the Version 6 JVM. This happens if we run the application from either the Bash terminal or from XCode.
I've researched this and there are other people who have had this identical issue. Everyone seems to suggest that JAVA_HOME needs to be pointing to the correct place, and mine is. Again, java -version works correctly. The whole thing is quite puzzling.
Does anyone know the right way to solve this? I've tried two different Macs with identical results. They are both running the latest version of macOS Sierra.
Thanks!
Dan

How to compile a java project with a terminal/cmd

I got tired of using Eclipse because it was too "plain" and "old style". So I moved on to a program called Sublime Text 3 and I used a package named "material theme" which made the program look flat and all nice and stuff.
But I wanted to code Java in that but I don't know how to compile stuff without using an IDE and it was not easy to find out but I found out that you could do it through the terminal/cmd of your operating system.
But what I don't understand is how I could actually use the javac command which means Java Compile and I searched and I didn't find out because I couldn't understand anything that was posted on either YouTube or Stack Overflow because I'm new to this whole terminal/cmd Java Compiling thing.

You're almost there buddy!
Just be sure you have the Java Development Kit (JDK) installed in your system. The JDK provides you the command javac -which you need to compile your .java program files.
The javac command is not that friendly as you think it is. You have to let it know where to find the java file you want to compile.
For example you saved Main.java inside C:\Projects\Java you must compile it this way javac C:\Projects\Java\Main.java.
Then you can run your program this way too java C:\Projects\Java\Main
As mentioned here... You can find a more detailed explanation to the official tutorial.

You need to install java JDK which you can find here
Then you need to right-click on my computer->properties->advanced system settings->Environment variables -> Click on path -> New
Then add the path to the bin folder inside the install folder for JDK.
Then in cmd you can compile with
javac *.java
inside the directory of your code

Install JDK
(Make sure your PATH variable is set up)
Use SublimeText's Build System, which allows you to compile open java documents with the Ctrl + B command.

You did not install JDK. You need to install JDK which has javac command.
Installing JDK on MAC instructions
Installing JDK on Windows instructions
After installing JDK(Java Development Kit), you can run the command javac -version on command line and let it show the version that is installed.

Can't run java through vim

I'm trying to learn java, and since I'm using 'javac' and 'java' all day, I'm using vim as my editor of choice. But I'm having a problem getting this system to work: I can't even get a minimal "Hello World" program to run! Within vim, I can use :!javac % to compile the current file just fine. However, with a file Test.java, any attempt to call :!java Test yields an UnsupportedClasVersionError, even though the same command works fine outside of vim. I'm completely stumped.
I'm using java7 on windows 7 (64bit) with cmd.exe as my shell.

The Java you invoke is an older version than the Javac you invoke.
You most likely have a Java 6 JRE installed (for browser plugins etc) at system level and a Java 7 JDK installed which you've added at the end of your path.
Either tell Javac to produce Java 6 compatible class files (with -target) or put the JDK in the front of your path.

Unable to run Rational Rhapsody due to "Unable to compile" error

I have problems with running the code in rhapsody and my error message is:
"
Building ------------ MainDefaultComponent.class ------------
Executing: "C:\Users\olle\IBM\Rational\Rhapsody\7.5.3\Share\etc\jdkmake.bat" DefaultComponent.bat build
Unable to compile
Build Done
"
Do anybody know how to make my program to successfully compile?

The error is because Rhapsody can't find a java compiler. If you want to generate Java code using Rhapsody you need a JDK installed before you install Rhapsody, not just the JRE.
What JDK did you have installed before you installed Rhapsody, and where did you tell the Rhapsody installer it was located?
Your rhapsody.ini file (C:\Users\olle\IBM\Rational\Rhapsody\7.5.3\rhapsody.ini)will contain a line starting JavaLocation=. This needs to match your JDK directory, e.g. C:\Program Files\Java\jdk1.6.0_24
This value is also hard-coded into jdkmake.bat; you can see this if you open it up in a text editor. So if you update your JDK after installing Rhapsody the build script doesn't pick up the path to the newly installed java compiler, javac.
Putting the path to your javac into jdkmake.bat lines 2 and 3 will fix the issue. Note that you'll need to use 8.3 filename style if there are spaces in the path to your javac.

I got similar errors installing rational rhapsody developer and then installing stuff like MingW, Visual C++ , etc.. the easiest way for me was to do in "control Panel->Software->Rhapsody->Change -> Repair/reinstall" rhapsody after it, so all internal necessary settings were performed by rhapsody itself. hope this helps.