Do you mean that you want to, while your program is running, edit its source by hand and have the changes be used? You can do that from within your
IDE via the JVM's hotswap facility, but this is a bit limited. Frameworks such as OSGi aim to make this feature work all the time, but the simple answer is, at the moment, no.
If you want that, go for a language/runtime that explicitly supports it, rather than adds it in as an afterthought, such as Common Lisp.