How to get openssl rsa private key out of private_key.json

Hate to completely out myself as a java noob, but how on earth does one actually use this code? javac won’t compile it. It doesn’t run directly via the ‘java’ command (saved in a file and passed as an argument). Can those of us that aren’t java devs please get some hints?

Clues are much appreciated.

Thanks,
-C