[jcparkyn]

> Java is perfectly capable of expressing an enum

I think this is a bit of an exaggeration, unless you're talking about very modern versions of Java (sealed interfaces and better pattern matching). Java has no good way to attach different data types to each enum variant (a la ADTs) except inheritance, and until recently switching over class types was a PITA.

[delusional]

Enums were added in java 5 and I'm pretty sure you could switch on them from day one. From that point you just need a single class (call it TreeNode) that holds a value of that enum (say the node type) and some fields, and you can navigate it and switch over the node type without any sort of visitors.

"But what if I have a leaf node. Wont that have a child pointer, even though it doesn't need it" Sure it will. Who cares. You don't need to make it harder than it has to be. All the new stuff just enables you to get more type safety, it was never necessary.

[quantified]

You're getting downvoted but I don't think you're getting the benefit of understanding the disagreements. The benefit of visitor is for when you need multiple different TreeNode classes with different aspects, either different member variables or different methods/implementations themselves. In that case, the unused child ptrs are the smaller of the class modeling difficulties. For example, if you had a single base TreeNode class with the enum, after switching on the enum you need to explicitly downcast to the type. The dual-dispatch approach bakes this into the visit-accept protocol.

The last time I worked on a complex syntax tree structure, I used the approach you suggested because _editing_ a Visitor suite is a pain in the rear. Only terms that had special payload data (numbers, pointers to metadata objects, etc.) need downcasting.