Why should I learn Java if Python have libraries offset it shortfall?

I am studying Python and R to work in Data, and my mentor said that I should learn Java. I think it is regards to Machine Learning, but Python has an extensive libraries that helps offset it short fall. The problem that I can never finish a crash course book on Python is it's speed, but I read that NumPy and Pandas help make it faster. So my question is, what benefits are there to learn Java for Data Science …

data datascience java learn libraries machine machine learning mentor python studying think work