Antlr4 - Is there a simple example of using the Pa

Antlr4 has a new class ParseTreeWalker. But how do I use it? I am looking for a minimal working example. My grammar file is 'gram.g4' and I want to parse a file 'program.txt'

Here is my code so far. (This assumes ANTLR has run my grammar file and created all of the gramBaseListener, gramLexer, etc etc):

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
import static org.antlr.v4.runtime.CharStreams.fromFileName;

public class launch{
public static void main(String[] args) {

    CharStream cs = fromFileName("gram.g4");  //load the file
    gramLexer lexer = new gramLexer(cs);  //instantiate a lexer
    CommonTokenStream tokens = new CommonTokenStream(lexer); //scan stream for tokens
    gramParser parser = new gramParser(tokens);  //parse the tokens

    // Now what??  How do I connect the above with the below? 

    ParseTreeWalker walker = new ParseTreeWalker();  // how do I use this to parse program.txt??
}}

I am using java but I assume it is similar in other languages.

The ANTLR documentation (http://www.antlr.org/api/Java/index.html) is short on examples. There are many tutorials on the internet but they are mostly for ANTLR version 3. The few using version 4 don't work or are outdated (for example, there is no parser.init() function, and classes like ANTLRInputStream are depreciated)

Thanks in advance for anyone who can help.

For each of your parser rules in your grammar the generated parser will have a corresponding method with that name. Calling that method will start parsing at that rule.

Therefore if your "root-rule" is named start then you'd start parsing via gramParser.start() which returns a ParseTree. This tree can then be fed into the ParseTreeWalker alongside with the listener you want to be using.

All in all it could look something like this (EDITED BY OP):

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
import static org.antlr.v4.runtime.CharStreams.fromFileName;

public class launch{
public static void main(String[] args) {

    CharStream cs = fromFileName("program.txt");  //load the file
    gramLexer lexer = new gramLexer(cs);  //instantiate a lexer
    CommonTokenStream tokens = new CommonTokenStream(lexer); //scan stream for tokens
    gramParser parser = new gramParser(tokens);  //parse the tokens

    ParseTree tree = parser.start(); // parse the content and get the tree
    Mylistener listener = new Mylistener();

    ParseTreeWalker walker = new ParseTreeWalker();
    walker.walk(listener,tree);
}}

************ NEW FILE Mylistener.java ************

public class Mylistener extends gramBaseListener {
        @Override public void enterEveryRule(ParserRuleContext ctx) {  //see gramBaseListener for allowed functions
            System.out.println("rule entered: " + ctx.getText());      //code that executes per rule
        }
    }

Of course you have to replace <listener> with your implementation of BaseListener

And just one small sidenode: In Java it is convention to start classnames with capital letters and I'd advise you to stick to that in order for making the code more readable for other people.