Checked exceptions in visitors

问题:

I am learning ANTLR4 and I have no previous experience with parser generators.

When I define my own visitor implementation, I have to override methods of the BaseVisitor (I am looking for instance at the EvalVisitor class at page 40 of the book). In case my method implementation possibly throws an exception, what should I do? I cannot use a checked exception since the original method has an empty throws clause. Am I expected to use unchecked exceptions? (this seems a bad Java design). For instance, assume that in the EvalVisitor class I want method visitId (page 41) to throw a user-defined exception, say UndefinedId, rather than returning 0. How should I write my code?

回答1:

You have two options:

1. Handle the exception inside the visitor method itself.

2. Wrap your checked exception in an unchecked exception. One possibility is ParseCancellationException, but you'll have to determine for yourself whether or not that makes sense in your application.

   try {
       ...
   } catch (IOException ex) {
       throw new ParseCancellationException(ex);
   }