Notch used a special programming framework to get their. Many game engines support similar features; the ability to modify the game world while its running, and can even do that with C++. The problem is you don't get it for free (framework transparent hot swapping isn't good enough), and its only because of special compromises made in the framework that you can get liveness at all.

Intellij keeps telling me it has the feature. Although I've never managed to use it on purpose.