Reverse Polish notation with stacks confusion

[jigglywiggly]

Ok just to give you an idea if you don't know what reverse polish notation is:
(5+3)/2 In reverse polish notation is
5
3
+
2
/


But that's not what I came here, I can figure the way to do that myself.


But what I am confused is this:

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
        at CalcStack.solve(CalcStack.java:21)
        at CalcStackDriver.main(CalcStackDriver.java:19)

Basic driver

import java.util.*;
public class CalcStackDriver {

 
    public static void main(String[] args) {
     Stack j = new Stack();
     Object firstthing ="2";
     Object secondthing ="3";
     Object thirdthing ="/";
     
     
    
     
     j.push(thirdthing);
     j.push(secondthing);
     j.push(firstthing);

     CalcStack m = new CalcStack(j);
       m.solve();

    }

}

And where the computation is done

import java.util.*;

public class CalcStack {
    Stack theStack;
    double finalvalue=0;


    public CalcStack(Stack j) {
    theStack=j;
    int counter=0;

    }

    public double solve()  {
 while(!(theStack.empty())) {


   
            
            double d=0;
            int j = ((Integer) theStack.pop()).intValue(); //here is where the error points too
            int m = ((Integer) theStack.pop()).intValue();
            String s = ((String) theStack.pop()).toString();

            if(s.equals("+")) {
                d=j+m;
                finalvalue=d;
                theStack.push(d);
            }

            if(s.equals("*")) {
                d=j*m;
                finalvalue=d;
                theStack.push(d);
            }
            if(s.equals("/")) {
                d=j/m;
                finalvalue=d;
                theStack.push(d);
            }
           //(3+2)/2
            //3
            //2
            //+
            //2      5/2
            ///


        }
          return finalvalue;

        }
 
      



    }

It's says I can't convert a String to an int, but I'm a bit confused why not? This doesn't seem like a hard problem, I don't think I am thinking straight. Oh and don't comment on my algorithim, it's just a basic thing to see if it works.

[r035198x]

You pushed Strings and tried to pop integers.
Push integers.

[mrmatt1111]

int i = Integer.parseInt(theStack.pop() ); // will parse the string to integer

[jigglywiggly]

Says I can't do that because it can't find parseInt in Object.