There's no specific Java course in combination with Mendix, learning the specific Mendix API isn't hard once you already know some Java. I don't know about a Dutch course, but starting with the basics at the official tutorial (https://docs.oracle.com/javase/tutorial/java/index.html) isn't a bad idea, it's a pretty decent one.
When you have a real scenario to work on, you will learn much faster just by experience and having to solve real problems.
There are some good books on the subject: top 9 books java And lynda.com is always a good source for the basics. In the Netherlands you can even do online courses with the LOI.
Regards,
Ronald