So instead of "java notes" and "design notes" you add the "java" and "design" tag, etc., so you can approach your notes in any direction: you can find "java" notes and then narrow the result sets to design notes (so java+design), and vice versa.
And you can add links too between the nodes, so you can also walk them in any order you define.
Existing tools can do this already, it's just a matter of using them.
Is there any more to zettelkasten? Its author invented the same thing on paper notes, but digital tools provide this naturally.