How to implement visitor if not all functions are supported for each Element?

Question

I have two Visitor classes in my code Visitor1 and Visitor2 that execute functions on the classes ElementA and ElementB (both implement the interface Visitable that defines accept(Visitor)). I do not want to support calling visit from Visitor1 on ElementB.

I thought of two different ways:

Throw an exception when Visitor1.visit(ElementB) is called:

public class Visitor1 extends Visitor {
    @Override
    public void visit(ElementA element) {
        //...
    }

    @Override
    public void visit(ElementB element) {
        throw new UnsupportedOperationException();
    }
}

Implement two different Visitor classes and one accept method per visitor:

public ElementA implements Visitable1, Visitable 2 {
    public void accept(Visitor1 v) {
        //...
    }

    public void accept(Visitor2 v) {
        //...
    }
}

public ElementB implements Visitable2 {
    public void accept(Visitor2 v) {
        //...
    }
}

How should I implement this the best? The first solution contradicts the Interface Segregation Principle and the second solution contradicts the visitor pattern, as the visitors cannot be as easily extendable.

Typically, you'd apply visitors to a polymorphic element structure (like a tree), where you don't know the concrete type of each element in advance (or, more precisely, the visiting operation doesn't know the type in advance). As you traverse the tree, what do you want to happen when you come across an unsupported element? Your first option will throw an exception that you can't really do anything about; it requires the caller to know that the element structure has no ElementB nodes. 1/2 — Filip Milovanović
– Filip Milovanović, Commented Dec 16, 2021 at 16:09
Your second approach, although somewhat confusing and complicated, might work, but you still have the problem of traversal - say you pass Visitor1 to the root: in order for this to work, you either have to keep Visitable1 and Visitable2 child nodes separate, so that you can traverse only the children that implement Visitable1, or you have to check the type. But, have you considered implementing visit(ElementB element) as a no-op (similar to null object pattern)? Perhaps that makes sense, and it could make things simpler? 2/2 — Filip Milovanović
– Filip Milovanović, Commented Dec 16, 2021 at 16:10

Doc Brown · Accepted Answer · 2021-12-17 06:36:33Z

Here is a straightforward solution, expressing the case literally: implement ElementA.accept in terms of the base class Visitor, and ElementB.accept in terms of Visitor2:

public ElementA implements Visitable {
    public void accept(Visitor v) {
        //...
    }
}

// no derivation from Visitable here, which would require
// to provide accept(Visitor v) 
public ElementB { 
    public void accept(Visitor2 v) {
        //...
    }
}

This avoids the issue of two very similar ElementA.accept methods, which leads to the extendability problem mentioned in your question.

However, this solution will not work if the intention is to have some generic code which calls visitable.accept(someVisitor) without knowing beforehand if visitable is of type ElementA or ElementB (both derived from Visitable), and also not knowing if someVisitor is of type Visitor1 or Visitor2 (both derived from Visitor) . If that's the case, there is no way to prevent getting a combination of objects Visitor1 with ElementB at runtime without loosing the genericity. Hence you need to decide how you want the calling code to behave in case someone passes the forbidden combination in there:

maybe you want the behaviour from solution 1, where the forbidden combination throws an exception (so the caller can react accordingly)
maybe you want nothing to happen - similar to solution 1, but with Vistor1.visit(ElementB element) implemented empty (so the caller does not have to care for)
or maybe you don't need any generic caller. Then you can give up the genericity, make one function which calls ElementA.accept with arbitrary visitors, and a second one which calls ElementB.accept only with visitors of type Visitor2 (using my suggestion from above).

So look at the requirements of the caller - ask yourself: how will you are going to use the visitors (why do you need them at all), and which degree of genericity are you trying to achieve?

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