I've seen this idea in cls recently, and now in this article on O'Reilley: Prototype in (Smalltalk, Python), then build in Java.
I like programming problems where you think, "There has to be something really interesting over there, but I can't see it clearly." All you can do is move one step over there, with a small bit of code, and start exploring to see it more clearly. And maybe it actually wasn't there, it was over here. Or it had a different shape than you thought initially. Maybe it wasn't interesting at all, and you didn't waste a lot of time. The danger of planning is that you plan for the contingencies you know about, but by definition you don't plan for things you don't know you'll encounter. So when you do encounter an unexpected event in your programming endeavor, you have to fix many interfaces and change multiple method signatures. If you've already committed to your original plan and that's no longer where you're going, then you have a problem. I'm not particularly worried by the fact that people say you can prototype more easily in Python, but eventually the Java version makes it easier to build a robust large system. You can prototype in Python. Once you've explored the space more, you can do the planning and design that the Java version requires. If you start writing in Java knowing as little as you did when you started writing the Python version, you'll waste way more time exploring than actually building the system you'll eventually build.Here's a hot tip - just build the system in Smalltalk or Python. If you skip the (unproductive) porting step, how much further along would you be?