Basically, I want to do two things:
Yes, it is possible.
Step 1: Compile the Code
Use ProcessBuilder or Runtime to construct a Process in which the Java compiler compiles their code. (Note that this requires that a Java compiler be available on the system at runtime).
Step 2: Invoke their Code
There are two ways to invoke their code. You can again use a ProcessBuilder or Runtime object to construct a process in which you execute their Java code. You can use the Process's getInputStream and getOutputStream functions to read from and write to the other process. An alternative is that you can use Class and the various reflection APIs to load their code and execute it directly within Java.