I (and my colleagues) have worked on statistical, data analysis, and ML in Java since before data science was even a career field, and before Python became popular. In my opinion Java has better IDE support, more stable and proven libraries, and high performance.

Java does occasionally require that a person might have to implement their own code after reading a research paper, but I've always enjoyed that part of the job.

I've never understood Python's popularity except that I've heard some people say that it's used at Google.