The java command can be used to execute class files contained in a jar file if the jar file has a proper manifest. All of the java apps I use on my PC are contained in jar files. Clicking on the jar file causes the java program to execute the class files contained in the jar file.
A properly written (no specific OS references) java app contained in a jar file should execute on any PC that has the java command installed.
That includes with OSs such as Linux and Mac and Windows.
possible to package an entire java application in a Jar file so that the Jar file is the entire installation package?
Yes.