Using a Switch statement for Infix to Prefix Expressions

Question

I am stuck on the numeric and operator portion of the switch statement...I have the problem also figured out in an if/else if statement and it works fine, but the requirements were for the following algorithm:
while not end of expression
switch next token of expression
case space:
case left parenthesis:
skip it
case numeric:
push the string onto the stack of operands
case operator:
push the operator onto the stack of operators
case right parenthesis:
pop two operands from operand stack
pop one operator from operator stack
form a string onto operand stack
push the string onto operand stack
pop the final result off the operand stack
I know that typically case/switch statement's can only be done via char and int's. As I said I am stuck and hoping to get some pointers. This is for a homework assignment but I am really hoping for a few pointers. I am using a linked stack class as that was also the requirements. Here is the code that I have:
   import java.io.*;
   import java.util.*;
   import java.lang.*;
/*--------------------------- PUBLIC CLASS INFIXTOPREFIX --------------------------------------*/
/*-------------------------- INFIX TO PREFIX EXPRESSIONS --------------------------------------*/
    public class infixToPrefix {
      private static LinkedStack operators = new LinkedStack();
      private static LinkedStack operands = new LinkedStack();
        // Class variable for keyboard input
      private static BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
         // Repeatedly reads in infix expressions and evaluates them
       public static void main(String[] args) throws IOException {
      // variables
         String expression, response = "y";
      // obtain input of infix expression from user
         while (response.charAt(0) == 'y') {
            System.out.println("Enter a parenthesized infix expression.");          // prompt the user
            System.out.println("Example: ( ( 13 + 2 ) * ( 10 + ( 8 / 3 ) ) )");
            System.out.print("Or as: ((13+2)*(10+(8/3))):  ");
            expression = stdin.readLine();     // read input from the user
         // output prefix expression and ask user if they would like to continue          
            System.out.println("The Prefix expression is: " + prefix(expression));     // output expression
            System.out.println("Evaluate another? y or n: ");          // check with user for anymore expressions
            response = stdin.readLine();     // read input from user
            if (response.charAt(0) == 'n') {          // is user chooses n, output the statement
               System.out.println("Thank you and have a great day!");
            }     // end if statement
         }     // end while statement
      }     // end method main
   /*------------- CONVERSION OF AN INFIX EXPRESSION TO A PREFIX EXPRESSION ------------*/ 
   /*--------------------------- USING A SWITCH STATEMENT ------------------------------*/
       private static String prefix(String expression) {
            // variables
         String symbol, operandA, operandB, operator, stringA, outcome;
           // initialize tokenizer
         StringTokenizer tokenizer = new StringTokenizer(expression, " +-*/() ", true);     
         while (tokenizer.hasMoreTokens()) {
            symbol = tokenizer.nextToken();     // initialize symbol     
            switch (expression) {
               case ' ':
                  break;     // accounting for spaces
               case '(':
                  break;     // skipping the left parenthesis
               case (Character.isDigit(symbol.charAt(0))):      // case numeric
                  operands.push(symbol);                                   // push the string onto the stack of operands
                  break;
               case (!symbol.equals(" ") && !symbol.equals("(")):     // case operator
                  operators.push(symbol);                                             // push the operator onto the stack of operators
                  break;
               case ')':
                  operandA = (String)operands.pop();     // pop off first operand
                  operandB = (String)operands.pop();     // pop off second operand
                  operator = (String)operators.pop();     // pop off operator
                  stringA = operator + " " + operandB + " " + operandA;          // form the new string
                  operands.push(stringA);
                  break;
            }     // end switch statement
         }     // end while statement
         outcome = (String)operands.pop();     // pop off the outcome
         return outcome;     // return outcome
      }     // end method prefix
   }     // end class infixToPrefixAny help would be greatly appreciated!

Answer 1

so, i did what flounder suggested:
         char e = expression.charAt(0);
         while (tokenizer.hasMoreTokens()) {
            symbol = tokenizer.nextToken();     // initialize symbol     
            switch (e) {
               case ' ':
                  break;     // accounting for spaces
               case '(':
                  break;     // skipping the left parenthesis
               case '0':
               case '1':
               case '2':
               case '3':
               case '4':
               case '5':
               case '6':
               case '7':
               case '8':
               case '9':
                  operands.push(symbol);     // push the string onto the stack of operands
                  break;                               // case numeric
               case '+':
               case '-':
               case '*':
               case '/':
                  operators.push(symbol);     // push the operator onto the stack of operators
                  break;                               // case operator
               case ')':
                  operandA = (String)operands.pop();     // pop off first operand
                  operandB = (String)operands.pop();     // pop off second operand
                  operator = (String)operators.pop();     // pop off operator
                  stringA = operator + " " + operandB + " " + operandA;          // form the new string
                  operands.push(stringA);
                  break;
               default:
            }     // end switch statement
         }     // end while statement
         outcome = (String)operands.pop();     // pop off the outcome
         return outcome;     // return outcomeafter this, I am able to compile the code free of errors and I am able to enter the infix expression, however, the moment enter is hit it provides the following errors:
Exception in thread "main" java.lang.NullPointerException
     at LinkedStack$Node.access$100(LinkedStack.java:11)
     at LinkedStack.pop(LinkedStack.java:44)
     at infixToPrefix.prefix(infixToPrefix.java:119)
     at infixToPrefix.main(infixToPrefix.java:59)
Any ideas as to why? I am still looking through seeing if I can't figure it out, but any suggestions? Here is the linked stack code:
    public class LinkedStack {
   /*--------------- LINKED LIST NODE ---------------*/
       private class Node {
         private Object data;
         private Node previous;
      }     // end class node
   /*--------------  VARIABLES --------------*/
      private Node top;
  /*-- Push Method: pushes object onto LinkedStack --*/     
       public void push(Object data) {
         Node newTop = new Node();
         newTop.data = data;
         newTop.previous = top;
         top = newTop;
      }     // end function push
   /*--- Pop Method: pop obejct off of LinkedStack ---*/
       public Object pop()      {
         Object data = top.data;
         top = top.previous;
         return data;
      }     // end function pop
   } // end class linked stackEdited by: drmsndrgns on Mar 12, 2008 8:10 AM
Edited by: drmsndrgns on Mar 12, 2008 8:14 AM
Edited by: drmsndrgns on Mar 12, 2008 8:26 AM