How can I assess if my java developer is doing a good job?

Sounds like you have a bad apple. Customer complaints should almost never be excused by "they should know better" and these seem like actual functional errors.

I would encourage you to replace this person as soon as possible.

Happy to talk this through in a quick call

In general, make sure you get regular updates on progress. Besides that, I would recommend you adopt the Agile methodology for the software project or have the software delivered in iterations so that you can touch and feel parts of the application. It minimizes risk and everyone can ensure the requirements are getting met early. Define the first iteration and then it'll be a deliverable in like a couple of weeks or so. Then you can gauge how things are going. Attitude of developer is also important. You don't want someone who thinks they know more than you or is better than you. Changes will be harder to make with that attitude. There's lots of other things, but those are a few pointers

How are you today? I would like to start by asking if you have a QA process in place? Normally, with any application or website development, there is a final step of validating what has been developed before releasing it to the public. This ensures that there are no bugs or defects being released. You can do this yourself, hire a freelancer to do it for cheap or retain a professional service like applause.com. You can also do code reviews, to check for commonly accepted coding conventions. Hope this helps, let me know if I can be of assistance?