This gives a couple of options for the install of the IDE. If you are going to be learning Java in Helsinki's Object-Oriented programming with Java then they will point you at a download of Netbeans 8.2 that relies of the Java Development Kit (JDK) 8, whereas if you want the latest and greatest Apache Netbeans environment you will probably want Apache Netbeans 10 with JDK 11 support. Basically a choice between latest and greatest and tried and true.
I have got 8.2 installed at the moment so I am going to refer to another blog for install instructions. It refers to Ubuntu not KDE neon but as KDE neon runs on the Ubuntu 18.04 base the instructions are the same.