Infix to Postfix calculation: having problems.. help?

Question

This is a program that when you input infix expression, it converts it to postfix expressions the evaluates the postfix..
I'm having difficulty in my main method. How do i loop it? when the user wants to another input an expression?..
import java.io.*;
import java.util.*;
import javax.swing.*;
public class Calculator {
     private static Stack operators = new Stack();
     private static Stack operands = new Stack();
     private static int precedence( char operator ) {
          if (operator == '+' || operator == '-' )
             return 1;
          else if (operator == '*' || operator == '/' || operator == '%')
              return 2;
          return 0;
     private static String convertToPostfix( String infix ) {
     StringTokenizer input = new StringTokenizer( infix );
     String symbol;
     String postfix = "";     
     while (input.hasMoreTokens()) {          
         symbol = input.nextToken();     
          if (Character.isDigit(symbol.charAt(0))){
              postfix = postfix + " " + (Integer.parseInt(symbol));     
          }else if (symbol.equals("(")){ 
               Character isOperator = new Character('(');
               operators.push( isOperator );
          }else if (symbol.equals(")")) {
               while (((Character)operators.peek()).charValue() != '(') {
                   postfix = postfix + " " + operators.pop();
               }operators.pop();
          } else {
               while (!operators.empty() && !(operators.peek()).equals("(") && precedence(symbol.charAt(0)) <= precedence(((Character)operators.peek()).charValue()))
                    postfix = postfix + " " + operators.pop();
                    Character isOperator = new Character(symbol.charAt(0));
                    operators.push( isOperator );
          while (!operators.empty())
               postfix = postfix + " " + operators.pop();
                    return postfix;
     private static int evaluate( String postfix ) {
     StringTokenizer input2 = new StringTokenizer( postfix );
     int value;
     String symbol;
     while (input2.hasMoreTokens()) {
         symbol = input2.nextToken();
          if (Character.isDigit(symbol.charAt(0))) {
              Integer isOperand = new Integer(Integer.parseInt(symbol));
               operands.push( isOperand );
          }else{
               int op2 = ((Integer)operands.pop()).intValue();
               int op1 = ((Integer)operands.pop()).intValue();
               int result = 0;
                    switch(symbol.charAt(0)){
                         case '*': result = op1 * op2;
                                        break;
                         case '+': result = op1 + op2;
                                        break;
                         case '-': result = op1 - op2;
                                        break;
                         case '/': result = op1 / op2;
                                        break;
                         case '%': result = op1 % op2;
                                        break;
                    Integer isOperand = new Integer(result);
                    operands.push( isOperand );
          value = ((Integer)operands.pop()).intValue();
          return value;
     public static void main(String args[]) throws IOException {
                 JOptionPane.showMessageDialog(null,  "Hello user! I hope you have a wonderful time using the
                    calculator.  = ]"  , "Welcome", JOptionPane.PLAIN_MESSAGE);     
              JOptionPane.showMessageDialog(null,  "Note: Enter Infix expressions with spaces in between.",
                    "Welcome", JOptionPane.PLAIN_MESSAGE);
              String infix = JOptionPane.showInputDialog("Enter Mathematical Expression Here: ");
              String output = "The expression in Postfix is: " +  convertToPostfix(infix);
              JOptionPane.showMessageDialog(null, output);
              String answer = "The answer to the equation:  " + evaluate(convertToPostfix(infix));
              JOptionPane.showMessageDialog(null, answer);
                   String options[] = {"Yes","No",}
               int option = JOptionPane.showOptionDialog(null,"Do you want evaluate an expression?", "Calculator",  
                   JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, null,options,options[0]); 
              JOptionPane.showMessageDialog(null,  "Thank you for using the calculator.\n Have a nice day!  = ]");
}

Answer 1

Don't waste people's time by posting your question in more than one forum, please.
Continued here: [http://forums.sun.com/thread.jspa?threadID=5333073]