Move array of doubles from Python to Java
Saving a file to disk to exchange the data between different applications sounds like a hacky approach to me. 
Depending on your structure and complexity, I would consider implementing a messaging queue (i.e. redis) or a document database (i.e. mongo or prefered alternative) with respective clients to do the data exchange between the apps.
On the data structure itself, I would choose json or csv for the task. If you need human readability or strict structure, json is your tool. If the data is meant only for the machine to read, csv takes less space to store the same amount of data.
I have a Python script that calculates a few, quite small, NumPy arrays and I have a Java service on a separate machine that needs to use these arrays. 
These arrays sometimes need to be recalculated and then used afterwards by the Java service. What is the best way to dump a NumPy array to the disk and load it in Java as float[][]? 
I know that I could use JSON to do this (Python script dumps the NumPy array to a JSON file and Java service recovers float[][] from this), but is there any other, "preferred" way?