java扫描文件并计算文件中包含的算式

怎么用java扫描指定txt，然后计算txt中的算式，比如d：/data.txt，包含算式
12+15-13
33*6 / 6

http://ethen.iteye.com/blog/1011402

http://blog.csdn.net/lip009/article/details/7768258

import java.io.File;
import java.util.Scanner;

public class T03Scanner {

public static void main(String args[]) throws Exception {

     File file=new File("d:\\data2.txt");   

        // create a scanner from the data file
        Scanner scanner = new Scanner(file);

        // 重复从文件中读取数据
        while (scanner.hasNext()) {
        int op1,op2,op3,result=0;
        String operator1 ="";
        String operator2="";


            // retrieve each data element

           op1 = scanner.nextInt();
            operator1 = scanner.next();
            op2 = scanner.nextInt();
            operator2=scanner.next();
            op3=scanner.nextInt();

            if (operator1.equals("+")&&operator2.equals("-"))
                result = op1 + op2-op3;
             if (operator1.equals("-")&&operator2.equals("+"))
                result = op1 - op2;
             if(operator1.equals("*")&&operator2.equals("/"))
                result=op1*op2/op3;
             if (operator1.equals("/")&&operator2.equals("+"))
                result=op1/op2+op3;
            System.out.println("result is " + result);
        }
        scanner.close(); // also closes the File
    }

}
ps(data2.txt)
12 + 15 -13
17 - 18 + 24
33 * 6 / 6
55 / 10 + 2
为什么这样不行