To be simple, python is not fast enought for solving, java is not fast enough for coding. So my solution is build prototype with python and transfer the code to java.
The reason is simple, I'm not exactly a full time java engineer, as a matter of fact, I write python 80% of my time, all of this project's prototype is written in python in another project. I'm slowly transfering some of the code to java, for now the tree construction part is still in python.