Converting Infix to Postfix Expressions

[Christopher Beech]

Having some problems converting infix to postfix. for example, if I enter 1+2*3, it gives me 1+12+*23*3 instead of 123*+

  import java.util.*;
 
/* Author: Chris Beech
 *
 * Purpose: The puropse is to implement a node in a linked list of Student
 * Records.
 *
 * Created:  April 1, 2006
 * Modified: April 17, 2006
 */
 
public class InfixCalculator
{
  public static void main(String[] args)
  {
    
    Stack <Character> s = new Stack <Character>();
    Stack <Double> t = new Stack <Double>();
    String postfixExpr = "";
    String inFix = "";
    String another = "y";
    char ch = ' ';
    double result = 0;
    
    Scanner console = new Scanner (System.in);
    
    while (another.equalsIgnoreCase("y"))
    {
      System.out.println ("Please enter an infix expression: ");
      inFix = console.nextLine();
      System.out.println(inFix);
      
      for (int index = 0; index < inFix.length(); index++)
      {
        ch = inFix.charAt(index);
        
        if (Integer.valueOf(ch)  > -10 || Integer.valueOf(ch)  < 10)
        {
          postfixExpr = postfixExpr + ch;
          if (ch == '(')
          {
            s.push(ch);
          }
          else if (ch == ')')
          {
            while (s.peek() != '(')
            {
              postfixExpr = postfixExpr + s.pop();
            }
            s.pop();
          }
          else
          {
            while (!s.isEmpty() && s.peek() != '(' && precedence(ch,s.peek()) != false)
            {
              postfixExpr = postfixExpr + s.pop();
            }
            s.push(ch);
          }
        }
      }
      while (!s.isEmpty())
      {
        postfixExpr = postfixExpr + s.pop();
      }
      System.out.println(postfixExpr);
      
      for (int index = 0; index < postfixExpr.length(); index++)
      {
        double operand2 = 0;
        double operand1 = 0;
        
        ch = postfixExpr.charAt(index);
        
        if (Double.valueOf(ch)  > -10 || Double.valueOf(ch) < 10)
        {
          t.push((double)ch - '0');
        }
        else if(ch == t.pop())
        {
          operand2 = t.pop();
          operand1 = t.pop();
          
          if (ch == '+')
          {
            result = (operand1 - '0') + (operand2 - '0');
            t.push((double)result - '0');
          }
          else if (ch == '/')
          {
            result =(operand1 - '0')/(operand2 - '0');
            t.push((double)result - '0');
          }
          else if (ch == '-')
          {
            result =(operand1 - '0')-(operand2 - '0');
            t.push((double)result - '0');
          }
          else if (ch == '*')
          {
            result = (operand1 - '0')*(operand2 - '0');
            t.push((double)result - '0');
          }
        }
        result = t.pop();
      }
      System.out.println ("The result is:" + result);
      System.out.print ("Do another expression (y/n)?");
      another = console.nextLine();
    }
  }
  public static boolean precedence(char ch, char peek)
  {
    if (ch == '*' && peek == '*')
    {
      return true;
    }
    else if (ch == '*' && peek == '/')
    {
      return true;
    }
    else if (ch == '*' && peek == '+')
    {
      return false;
    }
    else if (ch == '*' && peek == '-')
    {
      return false;
    }
    else if (ch == '/' && peek == '*')
    {
      return true;
    }
    else if (ch == '/' && peek == '/')
    {
      return true;
    }
    else if (ch == '/' && peek == '+')
    {
      return false;
    }
    else if (ch == '/' && peek == '-')
    {
      return false;
    }
    else if (ch == '+' && peek == '*')
    {
      return true;
    }
    else if (ch == '+' && peek == '/')
    {
      return true;
    }
    else if (ch == '+' && peek == '+')
    {
      return true;
    }
    else if (ch == '+' && peek == '-')
    {
      return true;
    }
    else if (ch == '-' && peek == '*')
    {
      return true;
    }
    else if (ch == '-' && peek == '/')
    {
      return true;
    }
    else if (ch == '-' && peek == '+')
    {
      return true;
    }
    else
    {
      return true;
    } 
  }
}