When using Maven to load the Firmata4j library you'll be required to use version 2.3.8 and you'll need to load dependencies in a particular order, as per my earlier blog page. It's manageable but a little cumbersome. That said, most students can handle it.
Instead, you can try to use version 2.3.9. I've loaded the source on this GitHub page. But you can also just bypass it and use the compiled Java JAR file from here. Then, instead of importing the library through Maven, choose "import from Java" instead and choose this JAR file.
Consider using 2.3.9's JAR for your lab and project work, especially if you're working on developing with new sensors, like the DHT20. Let me know how it goes.