[stcredzero]

Unit tests are a pretty good poor-mans REPL in languages that don't have REPLs.

Java IDEs, especially eclipse, are amazing at fast, incremental compilation. You make one change, and then only recompile what you need to.

I always found this lugubrious compared to the experience in many Smalltalks. The Java vs. Smalltalk USENET flame wars in the 90's were partly motivated by the downgrade in developer UX caused by the taking of the Smalltalk runtime model and deliberately crippling it.

Granted, Java has made amazing progress since then, as have IDEs generally. Is there a non-Smalltalk environment that lets one "code in the debugger" to the same extent as Smalltalk? I know of Clojure and the kind of experience shown with LightTable. (To elaborate, to "code in the debugger" in Smalltalk is very nimble and powerful. It's kind of like the turn-based version of LightTable but with pausing in the debugger and manual tinkering/time-travel with the context stack.)

[sreque]

The JVM is less dynamic in its ability to reload code, but the payoff is that java code can be 10-100x faster than smalltalk code. I personally think the performance gain is worth it. Hotspot has some limited support for hot code reloading, which is a pre-requisite to being able to "code in your debugger", but it's limited enough that I don't bother with it. Alternatively, you can code in a dynamic language on the JVM such as JRuby or Clojure and have full support for "coding in the debugger". These languages are also a lot slower than Java.