This Java API can be used to place blocks on a newly created or already existing Minecraft world. This small tool was originally created as part of (and is still used by) the CodeMetropolis project. I make the source publicly available to help new contributors add new functionality to CodeMetropolis, but feel free to use it in any other project. Please note that I created this years ago when I was learning the basics of programming so the code is probably full of obvious mistakes and should be refactored. I also want to highlight that I used a modified version of RegionFile implementation by Scaevolus so credit goes there as well.
DISCLAIMER: The BlockModifier API was only tested with Minecraft 1.8, still it might work with newer versions. (I am not aware of the recent changes in Minecraft's world format if there are any.)