Runtime Error: java.lang.ClassCastException

Question

My project is to creat a postfix with boolean. I finished my code, it compiled great, than I got an java.lang.ClassCastException. So obviously I have a casting problem. Any help or hints in the right direction would be greatly appreciated.
     import java.util.*;    
import javax.swing.*;
public class KimbelPostfix
    public static void main(String[] args)
     String expression;
     String token;
     String operator;
     String myString = "true";
     StringTokenizer tokens;
     Boolean operand1, operand2, result = true;
     //create empty stack of pending boolean operands
     Stack operandStack = new Stack(); 
     expression = JOptionPane.showInputDialog( "Enter postfix expression" );
     tokens = new StringTokenizer(expression);
     while (tokens.hasMoreTokens())
         token = tokens.nextToken();
         if (!isOperator(token))   //operand
             new Boolean(myString);
             myString = token;
             operandStack.push(token); 
         //  printStack(operandStack);   //display stack at each step. Debugging
         else
         {   //token is an operator
             //pop two booleans off stack.
             //convert popped Object to boolean, extract boolean
             operand2 = ((Boolean)operandStack.pop()).booleanValue();
             operand1 = ((Boolean)operandStack.pop()).booleanValue();  //"first" one
          if (token.equals("!"))
              operandStack.push(token);
              result = operand1 && !operand2;
          else if (token.equals("&&"))
              result = operand1 && operand2;
          else if (token.equals("||"))
              result = operand1 || operand2;
          operandStack.push(new Boolean(result)); 
     //only object in stack is the answer
     result =  ((Boolean)operandStack.pop()).booleanValue();
     JOptionPane.showMessageDialog(null,expression+" = "+result);
     System.exit(0);
    static boolean isOperator(String t) {
         if (t.equals("!") || t.equals("&&") || t.equals("||"))
             return true;
         else
             return false;
    static void printStack( Stack s)
     System.out.print("Stack is: ");
        Iterator items = s.iterator();
        while (items.hasNext())
        System.out.print(items.next()+"  ");
        System.out.println();
     

Answer 1

When I run the program and enter "false false ||" I get the slightly more useful error message:
Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Boolean\\     at KimbelPostfix.main(KimbelPostfix.java:38)Now line 38 is the one in the else part of the while loop, that says:operand2 = ((Boolean)operandStack.pop()).booleanValue();Evidently what you were casting to a Boolean - operandStack.pop() - is a String. And you can't cast Strings to Booleans like that. So the questions are "what were you expecting to be on the stack?" and "what was actually on the stack?".
You already have a method that will help answer the second question. As regards the first, if you were expecting a Boolean on the stack you had better go back to whereever you push things onto the stack and see why it's a String not a Boolean.
Edited by: pbrockway2 on Dec 17, 2007 2:05 PM
I've just read the edit of your last post, and I agree with petes1234. No-one here is going to write the code for you. That's not trying to be discouraging or dismissive: just saying how things are.
There are signs of dispair in the myString variable, and the way you (attempt to) handle the unary ! operator. If you're tired and "fiddling around" to no productive effect your best bet would be to step away from the computer and do something else for a few hours.