how to use antlr4 visitor

I am a beginner of antlr. I was trying to use visitor in my code and following the instruction on the net. However, I found out that the visitor was not entering the method I create at all. May anyone tell me what I did wrong?

This is my visitor:

import java.util.LinkedList;
import org.antlr.v4.runtime.misc.NotNull;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author Sherwood
 */
public class ExtractMicroBaseVisitor extends MicroBaseVisitor<Integer> {
    //LinkedList<IR> ll = new LinkedList<IR>();
    //MicroParser parser;
    //System.out.println("11");

    @Override 
    public Integer visitPgm_body(@NotNull MicroParser.Pgm_bodyContext ctx){
        System.out.println(ctx.getText());
        return 467;
    }

    @Override
    public Integer visitProgram(@NotNull MicroParser.ProgramContext ctx){
        System.out.println("11");
        return 456;
    }

}

As you can see, the stdout should print 11 when the method "visitProgram" was entered. But the output screen gave me nothing (a null type).

This is my main code:

import java.io.IOException;
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
/**
 *
 * @author Sherwood
 */
public class Compiler {

    /**
     * @param args the command line arguments    
     */
    public static void main(String[] args) {
        // TODO code application logic here 
        //SymbolTable table = new SymbolTable();
        try {
            ANTLRFileStream reader = new ANTLRFileStream("TestCases/step4_testcase3.micro");
            MicroLexer lexer  = new MicroLexer((CharStream)reader);
                    TokenStream tokens = new CommonTokenStream(lexer);
                    MicroParser parser = new MicroParser(tokens);
                    parser.setErrorHandler(new MyErrorStrategy());
                    ParseTree tree = parser.program();

                    //ParseTreeWalker walker = new ParseTreeWalker(); // create standard walker
                    //ExtractMicroBaseListener extractor = new ExtractMicroBaseListener(parser);
                    //walker.walk(extractor, tree); // initiate walk of tree with listener
                    ExtractMicroBaseVisitor visitor = new ExtractMicroBaseVisitor();
                    int t = visitor.visit(tree);
                    //for(String str : extractor.table.checkDuplicate()){
                    //    System.out.println("SHADOW WARNING " + str);
                    //}
                    //System.out.println(extractor.table.checkDuplicate().toString());
                    //System.out.println(extractor.table.toString());
                    //System.out.println("Accepted");
    }catch (IOException e) { 
                System.out.println("Not Accepted");
            }catch(IllegalArgumentException e){
                System.out.println(e.getMessage());
            }
    }
}

This is my grammar file(partially):

grammar Micro;

options {
  language = Java;
 }

//Program 
program           : ('PROGRAM' id 'BEGIN' pgm_body 'END')
; 

id                : IDENTIFIER;
pgm_body          : (decl func_declarations);
decl          : (string_decl_list | var_decl_list)* ;

You have to call super if you want ANTLR4 to visit children. Like this:

@Override 
public Integer visitPgm_body(@NotNull MicroParser.Pgm_bodyContext ctx){
    super.visitPgm_body(ctx);
    System.out.println(ctx.getText());
    return 467;
}

@Override
public Integer visitProgram(@NotNull MicroParser.ProgramContext ctx){
    super.visitProgram(ctx);
    System.out.println("11");
    return 456;
}

You have to think about where to put your logic: before super or after.