PostFixCalculator while/try error

Question

import java.util.Scanner;

public class PostFixCalculator
{
      public static void main(String [] args)
      {
            Scanner kbd = new Scanner(System.in);
            int result;
            String expression;

            System.out.println("Student name, CS-304, Fall 2014, Asst 2c.");

            System.out.println("To quit this program, just hit 'return'.\n");

            System.out.print("Enter a postfix expression: ");
            expression = kbd.nextLine();

         while (!expression.equals(""))
         {
               try
               {
               }
            catch(RuntimeException e)
            {
            }

            System.out.print("\nEnter a postfix expression: ");
            expression = kbd.nextLine();

         } // end while (!expression.equals(""))

         System.out.println("\nBye!");

      } // end public static void main(String [] args)


   public static int postFixEvaluate(String input)
   {
      Scanner tokenizer; 
      int result, operand1, operand2, value;
      String operator;
      LinkedStack s = new LinkedStack();

      tokenizer = new Scanner(input);

      while (tokenizer.hasNext())
      {
         if (tokenizer.hasNextInt())
         {
            value = tokenizer.nextInt();
            s.push(value);
         }

         else // we have an operator
         {
            operator = tokenizer.next();

            if (s.isEmpty())
                throw new RuntimeException ("Not Enough Operands");
                operand2 = s.pop();

            if (s.isEmpty())
                throw new RuntimeException ("Not Enough Operands");
                operand1 = s.pop();

            if (operator.equals("+"))
               result = operand1 + operand2;
            else if (operator.equals("-"))
               result = operand1 - operand2;
            else if (operator.equals("*"))
               result = operand1 * operand2;
            else if (operator.equals("/"))
               result = operand1 / operand2;
            else
                throw new RuntimeException ("Not Enough Operands");

            s.push(result);
         } // end else // we have an operator

      } // end while (tokenizer.hasNext())

      if (s.isEmpty())
          throw new RuntimeException ("Not Enough Operands");

         result = s.pop();

      if (!s.isEmpty())
          throw new RuntimeException ("Not Enough Operands");

      return result;

   } // end public static int postFixEvaluate(String input)

} // end public class PostFixCalculator

I have this code, a PostFixCalculator but whatever I put in for the while and try, I kept getting an error. The program compiles and runs the way it is posted, but it does not run correctly. I've hit a wall..

Answer 1

Your while loop is incorrct. Try revisiting it and make correction

Answer 2

Just try to do e.printStackTrace() in catch block, and you will see that error is NullPointerException. If you are doing empty catch block you will lose all this information.

try this one:

package test;

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class PostFixCalculator {
  public static void main(String[] args) {
    Scanner kbd = new Scanner(System.in);
    String expression;

    System.out.println("Student name, CS-304, Fall 2014, Asst 2c.");

    System.out.println("To quit this program, just hit 'return'.\n");

    System.out.print("Enter a postfix expression: ");
    expression = kbd.nextLine();

    while (!expression.equals("")) {
        try {
            System.out.println(postFixEvaluate(expression));
        } catch (RuntimeException e) {
            System.out.println(e.getLocalizedMessage());
        }

        System.out.print("\nEnter a postfix expression: ");
        expression = kbd.nextLine();

    } // end while (!expression.equals(""))

    System.out.println("\nBye!");
    kbd.close();

  } 

  public static String postFixEvaluate(String input)
  {

  List<String> operatorsAsStrings = getListOfValuesAsString(input.split("\\d+"));
  List<String> digitsAsStrings = getListOfValuesAsString(input.split("\\W+"));


  if(operatorsAsStrings.size() >= digitsAsStrings.size()){
      throw new RuntimeException("Incorrect format of expression.");
  }

  if(digitsAsStrings.size()<2){
      throw new RuntimeException("Not Enough Operands.");
  }

  BigInteger result = new BigInteger("0");
  boolean firstTime = true;

  for (int i=0; i<=digitsAsStrings.size()-1; i++) {

      if(firstTime){
          result = result.add(new BigInteger(digitsAsStrings.get(i)));
          firstTime = false;

      }else{

          String operator = operatorsAsStrings.get(i-1);
          if (operator.equals("+"))
              result = result.add(new BigInteger(digitsAsStrings.get(i)));
          else if (operator.equals("-"))
               result = result.subtract(new BigInteger(digitsAsStrings.get(i)));
          else if (operator.equals("*"))
               result = result.multiply(new BigInteger(digitsAsStrings.get(i)));
          else if (operator.equals("/"))
               result = result.divide(new BigInteger(digitsAsStrings.get(i)));

      }

  }
  return result.toString(); 
 } 

 private static List<String> getListOfValuesAsString(String[] split) {
    List<String> resultList = new ArrayList<>();

    for(String string: split){
        if(string != null && !string.isEmpty()){
            resultList.add(string);
        }
    }

    return resultList;
 }
}