You can easily hot-swap classes in a running program as long as their interface doesn't change. I.e., you can tune your algorithm w/o restarting the server.

I went wow the first time I edited a program during a debugging session and Eclipse just recompiled the class and reloaded it into the program.

Yes, there details of getting this right (transferring the state), but nevertheless.. It'd be one heck of a job implementing this _correctly_ in a MT scenario for C and C++. JVM just does "magic" here.