When a Java program is executed, one or more class loaders locate and load all the classes that are needed to run the program.

JEP 238: Multi-Release JAR Files extends the JAR file format to allow multiple, Java-release-specific versions of class/resource files to coexist in the same archive.

Since I have done most of my coding for classes (and almost exclusively in Java), I have always just run my programs through the compiler and never had to worry about actually setting it up to run ...