Java Algorithms Parse postfix arithmetic expressions

import java.util.Stack;

class ParsePost {
   private Stack<Integer> theStack = new Stack<>();
   private String input;

   public ParsePost(String s) {
      input = s;//www.  j  a v  a 2s .  c  om
   }
   public int doParse() {
      char ch;
      int j;
      int num1, num2, interAns;

      for (j = 0; j < input.length(); j++) // for each char,
      {
         ch = input.charAt(j); // read from input
         System.out.println(theStack);
         if (ch >= '0' && ch <= '9') // if it's a number
            theStack.push((int) (ch - '0')); // push it
         else // it's an operator
         {
            num2 = theStack.pop(); // pop operands
            num1 = theStack.pop();
            switch (ch) // do arithmetic
            {
            case '+':
               interAns = num1 + num2;
               break;
            case '-':
               interAns = num1 - num2;
               break;
            case '*':
               interAns = num1 * num2;
               break;
            case '/':
               interAns = num1 / num2;
               break;
            default:
               interAns = 0;
            } // end switch
            theStack.push(interAns); // push result
         }
      }
      interAns = theStack.pop(); // get answer
      return interAns;
   }
}

public class Main {
   public static void main(String[] args) {
      int output;
      ParsePost aParser = new ParsePost("345+*612+/-");
      output = aParser.doParse();
      System.out.println("Evaluates to " + output);
   }
}