We have C sample code with DLL file from vendor, hope it could help you to develop you Java.
The key press on the handset is different with USB keyboard, it’s like keypad matrix and using GPIO to scan/read the status of key press.
But it support USB HID function call.
I attached the chipset (Cmedia CM119) datasheet for your reference.
Should you have any question on it, feel free to let me know for detail.
Basically we have two option . Use the C code and transfer it to Java
or Write Java Code (with jni and provided DLL).