Start existing Java-program in VS Code

Question

I want to start an existing Java-program. I use Ubuntu 18.04, VS Code and installed openjdk-11-jdk. I installed the java extension pack as adviced on this site : https://code.visualstudio.com/docs/languages/java.

I created a new project and substituted the created src folder by the src folder of the program I want to run. I also created a workspace, containing the whole project. The project contains several possible entry points. I chose one and clicked on the displayed "play"-button. It starts to compile but fails with the message: "java.lang.NoClassDefFoundError: GameBoard". GameBoard is a class that is indeed needed and located in the upper folder but still inside the src folder of the whole project. It also shows a warning massage that ".classpath" was incomplete. But the file exists and contains a link to the src folder as it should be, I suppose. One time it also displayed the error message, that a function in the main-class was twice defined which is clearly not the case. There are instructions how to start said program and I think I followed them: https://github.com/WolfgangKonen/GBG/wiki/Install-and-Configure They are for eclipse thow but that shouldn't really matter, I suppose.

So could somebody tell me, what I am doing wrong? I'm a bit out of ideas. I really just want to start the program and quickly test it and nothing more. Command line would be perfectly ok, as well. I don't have much experience with setting up java projects in general.

EDIT: I created the project within VS Code. The created project is called .project and seems to be an eclipse-project-file. This was the only way I found to create a project from within the IDE. Maven seems to be a command line tool. The parameter structure seems a bit complex at first glance, but I might have a look at this tomorrow, if there is no quicker solution.

Those are the contents of the .project file and the .classpath file respectively:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<projectDescription>
  <name>GBG</name>
  <comment/>
  <projects>
    </projects>
  <buildSpec>
    <buildCommand>
      <name>org.eclipse.jdt.core.javabuilder</name>
      <arguments>
            </arguments>
    </buildCommand>
  </buildSpec>
  <natures>
    <nature>org.eclipse.jdt.core.javanature</nature>
  </natures>
</projectDescription>

<?xml version="1.0" encoding="UTF-8"?>
<classpath>
    <classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-10"/>
    <classpathentry kind="src" path="src"/>
    <classpathentry kind="output" path="bin"/>
</classpath>