antlr - loop through nodes

Question

I have the following grammar (simplified):

filter   : eq | and;

eq       : 'eq' '(' property ',' value ')';

and      : 'and' '(' filter ',' filter (',' filter)* ')';

...

I am parsing simple prefix constructions like:

and(eq(name,john),eq(location,usa))

What is the best way to loop through the filter nodes of a and node? The generated class AndContext has a function filter(int index), but nothing more. I want to do something like:

AndContext c = ...;

for (int i = 0; i < c.numberOfFilters(); ++i) {
    FilterContext f = c.filter(i);
}

There is the function getChildCount(), but it returns the count of all nodes (including the terminal nodes representing (, ), etc.), and not just the interesting filter nodes.

Answer 1

Let's say your grammar looks like this:

grammar F;

eq       : 'eq' '(' property ',' value ')';
and      : 'and' '(' filter ',' filter (',' filter)* ')';
filter   : eq | and;
property : 'name' | 'location';
value    : 'john' | 'usa';

then you could extend the generated FBaseListener and override its enterAnd to get to the FilterContexts:

public class Main {

    public static void main(String[] args) throws Exception {
        FLexer lexer = new FLexer(new ANTLRInputStream("and(eq(name,john),eq(location,usa))"));
        FParser parser = new FParser(new CommonTokenStream(lexer));
        ParseTreeWalker.DEFAULT.walk(new AndListener(), parser.and());
    }
}

class AndListener extends FBaseListener {

    @Override
    public void enterAnd(@NotNull FParser.AndContext ctx) {
        for (FParser.FilterContext filterContext : ctx.filter()) {
            System.out.println("filterContext=" + filterContext.getText());
        }
    }
}

which will print:

filterContext=eq(name,john)
filterContext=eq(location,usa)