chapter4 of antlr4---Building a Calculator Using a Visitor

Calc.java

/***
 * Excerpted from "The Definitive ANTLR 4 Reference",
 * published by The Pragmatic Bookshelf.
 * Copyrights apply to this code. It may not be used to create training material, 
 * courses, books, articles, and the like. Contact us if you are in doubt.
 * We make no guarantees that this code is fit for any purpose. 
 * Visit http://www.pragmaticprogrammer.com/titles/tpantlr2 for more book information.
***/
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.ParseTree;

import java.io.FileInputStream;
import java.io.InputStream;

public class Calc {
    public static void main(String[] args) throws Exception {
        String inputFile = null;
        if ( args.length>0 ) inputFile = args[0];
        InputStream is = System.in;
        if ( inputFile!=null ) is = new FileInputStream(inputFile);
        ANTLRInputStream input = new ANTLRInputStream(is);
        LabeledExprLexer lexer = new LabeledExprLexer(input);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        LabeledExprParser parser = new LabeledExprParser(tokens);
        ParseTree tree = parser.prog(); // parse

        EvalVisitor eval = new EvalVisitor();
        eval.visit(tree);
    }
}

command.sh

antlr4 -no-listener -visitor LabeledExpr.g4 # -visitor is required!!!
javac Calc.java LabeledExpr*.java
java Calc t.expr

#answer
# 193
# 17
# 9

EvalVisitor.java

/***
 * Excerpted from "The Definitive ANTLR 4 Reference",
 * published by The Pragmatic Bookshelf.
 * Copyrights apply to this code. It may not be used to create training material, 
 * courses, books, articles, and the like. Contact us if you are in doubt.
 * We make no guarantees that this code is fit for any purpose. 
 * Visit http://www.pragmaticprogrammer.com/titles/tpantlr2 for more book information.
***/
import java.util.HashMap;
import java.util.Map;

public class EvalVisitor extends LabeledExprBaseVisitor<Integer> {
    /** "memory" for our calculator; variable/value pairs go here */
    Map<String, Integer> memory = new HashMap<String, Integer>();

    /** ID '=' expr NEWLINE */
    @Override
    public Integer visitAssign(LabeledExprParser.AssignContext ctx) {
        String id = ctx.ID().getText();  // id is left-hand side of '='
        int value = visit(ctx.expr());   // compute value of expression on right
        memory.put(id, value);           // store it in our memory
        return value;
    }

    /** expr NEWLINE */
    @Override
    public Integer visitPrintExpr(LabeledExprParser.PrintExprContext ctx) {
        Integer value = visit(ctx.expr()); // evaluate the expr child
        System.out.println(value);         // print the result
        return 0;                          // return dummy value
    }

    /** INT */
    @Override
    public Integer visitInt(LabeledExprParser.IntContext ctx) {
        return Integer.valueOf(ctx.INT().getText());
    }

    /** ID */
    @Override
    public Integer visitId(LabeledExprParser.IdContext ctx) {
        String id = ctx.ID().getText();
        if ( memory.containsKey(id) ) return memory.get(id);
        return 0;
    }

    /** expr op=('*'|'/') expr */
    @Override
    public Integer visitMulDiv(LabeledExprParser.MulDivContext ctx) {
        int left = visit(ctx.expr(0));  // get value of left subexpression
        int right = visit(ctx.expr(1)); // get value of right subexpression
        if ( ctx.op.getType() == LabeledExprParser.MUL ) return left * right;
        return left / right; // must be DIV
    }

    /** expr op=('+'|'-') expr */
    @Override
    public Integer visitAddSub(LabeledExprParser.AddSubContext ctx) {
        int left = visit(ctx.expr(0));  // get value of left subexpression
        int right = visit(ctx.expr(1)); // get value of right subexpression
        if ( ctx.op.getType() == LabeledExprParser.ADD ) return left + right;
        return left - right; // must be SUB
    }

    /** '(' expr ')' */
    @Override
    public Integer visitParens(LabeledExprParser.ParensContext ctx) {
        return visit(ctx.expr()); // return child expr's value
    }
}

LabeledExpr.g4

grammar LabeledExpr; // rename to distinguish from Expr.g4

prog:   stat+ ;

stat:   expr NEWLINE                # printExpr
    |   ID '=' expr NEWLINE         # assign
    |   NEWLINE                     # blank
    ;

expr:   expr op=('*'|'/') expr      # MulDiv
    |   expr op=('+'|'-') expr      # AddSub
    |   INT                         # int
    |   ID                          # id
    |   '(' expr ')'                # parens
    ;

MUL :   '*' ; // assigns token name to '*' used above in grammar
DIV :   '/' ;
ADD :   '+' ;
SUB :   '-' ;
ID  :   [a-zA-Z]+ ;      // match identifiers
INT :   [0-9]+ ;         // match integers
NEWLINE:'\r'? '\n' ;     // return newlines to parser (is end-statement signal)
WS  :   [ \t]+ -> skip ; // toss out whitespace

t.expr

193
a = 5
b = 6
a+b*2
(1+2)*3