Dynamically running a .class file in the same project or .jar

Here's a link of what I'm trying to do. You can say that I'm trying to do a simple IDE

I made a new thread because I'm now encountering new problems..

As I've read from searching about this topic. One way to do this is

Process process = Runtime.getRuntime().exec("java -cp Project.jar helloworld");

My question is, which java does .exec uses and is there a way to do this without defining the directory?
I mean, let's say I have the .class files inside .jar located at the resources directory.
Can I do it something like this..

Process process = Runtime.getRuntime().exec("java -cp Project.jar src\Project\resources\helloworld");

Or is there a better way to do this?