The Definitive ANTLR 4 Reference, 2nd Edition

Labeling Rule Alternatives for Precise Event Methods

To illustrate the event granularity problem, let’s try to build a simple calculator with a listener for the following expression grammar:

listeners/Expr.g4

grammar Expr;
s : e ;
e : e op=MULT e    // MULT is '*'
  | e op=ADD e     // ADD is '+'
  | INT
  ;

As it stands, rule e yields a fairly unhelpful listener because all alternatives of e result in a tree walker triggering the same enterE and exitE methods.

public interface ExprListener extends ParseTreeListener {
    void enterE(ExprParser.EContext ctx);
    void exitE(ExprParser.EContext ctx);
    ...
}

The listener methods would have to test to see which alternative the parser matched for each e subtree using ...