[bhaak]

The Java VM has hot code swapping builtin (IIRC limited to method bodies but the commercial JRebel drops that limitation).

Start your program in your IDE's debugger (or point it to your application server where your program is running) and you can change the code immediately, rerun your methods, change variables, inspect the objects, etc.

It's more than a simple REPL, it's a full-blown debugger and it comes really close to what you have in a SmallTalk image where your code, data, and environment is all the same.