Antlr4中的Antr3规则重写

grammar Example;

options {
  output=AST;
}

tokens {
  PLUS = '+' ;
  MINUS = '-' ;
}

expr
  : term ( (PLUS^ | MINUS^) term )*
  ;

term
  : INT
  ;

INT
  : '0'..'9'+
  ;

grammar Example;

expr
  : term ( (PLUS | MINUS) term )*
  ;

term
  : INT
  ;

PLUS: '+';
MINUS: '-';
INT: [0-9]+;
WS: [ \t\r\n]+ -> skip;

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;

public class ExampleVisitor extends ExampleBaseVisitor<Integer> {
    @Override
    public Integer visitExpr(ExampleParser.ExprContext ctx) {
        int left = visit(ctx.term(0));
        for (int i = 1; i < ctx.term().size(); i++) {
            if (ctx.getChild(2 * i - 1).getText().equals("+")) {
                left += visit(ctx.term(i));
            } else {
                left -= visit(ctx.term(i));
            }
        }
        return left;
    }

    @Override
    public Integer visitTerm(ExampleParser.TermContext ctx) {
        return Integer.parseInt(ctx.INT().getText());
    }

    public static void main(String[] args) throws Exception {
        CharStream input = CharStreams.fromString("3 + 5 - 2");
        ExampleLexer lexer = new ExampleLexer(input);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        ExampleParser parser = new ExampleParser(tokens);
        ParseTree tree = parser.expr();
        ExampleVisitor visitor = new ExampleVisitor();
        int result = visitor.visit(tree);
        System.out.println("Result: " + result);
    }
}