[register]

The REPL in CL allows you to redefine running code, even code compiled with high optimization levels. This is a feature of the language rather than a feature of the REPL but it's in the REPL that such features shine. In the best case in Java you can redefine methods when running under a debugger, using a framework such as JRebel or leveraging an application server/framework ( Play for example). When changes are too radical you have to recompile and redeploy. In CL this is only rarely needed. In short a REPL in Java will never be able to reach the level of interactivity provided by a CL REPL simply because interactive development is not built in the language as in the case of CL.