Postfix evaluation using stacks error (java)

Question

So I am making a program that evaluates a postfix expression from a file. However I keep getting the error:

 0
Exception in thread "main" java.lang.NumberFormatException: For input string: "+"
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Integer.parseInt(Integer.java:484)
    at java.lang.Integer.valueOf(Integer.java:582)
    at postfixEvaluater.postfixEvaluation(postfixEvaluater.java:89)
    at postfixEvaluater.readFromTheFile(postfixEvaluater.java:49)
    at postfixEvaluater.main(postfixEvaluater.java:21)

So I am guessing that it has something to do with my operators based on this.

the contents of the input .txt file are as follows:

 511+93/2-932/-149657%/*-+14+96*5/7%-149-+6*57%/ 

and the expected output is:

 5
 2
 1
 8
-4
 2
-4

Here is all my code so far:

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;

public class postfixEvaluater {
   public static void main(String[] commandlineArguments) {
      //Error Checking
      if(commandlineArguments.length == 0){
         System.out.println("Please enter the file name.");
      }
      else{
         postfixEvaluater.readFromTheFile(commandlineArguments[0]);
      }

   }        


   public static void readFromTheFile(String fileName) {
         //connect to file 
      File file = new File(fileName);
      Scanner scanFile = null;
      try {
         scanFile = new Scanner(file);
      } 
      catch (FileNotFoundException exception) {

         System.out.print("File not found for " + fileName);

         System.exit(1);
      }        

      while (scanFile.hasNextLine()) {

         String line = scanFile.nextLine();

         Integer postfixLine = postfixEvaluater.postfixEvaluation(line);
         System.out.println(postfixLine);



      }
   }


   public static Integer postfixEvaluation(String input){




      StackInterface<Integer> stack = new LinkedStack<Integer>();

      Integer result = new Integer(0);



      for(int i=0;i<input.length();i++){

         String characterNum = input.substring(i, i+1);

         Integer integer1 = new Integer(0);
         Integer integer2 = new Integer(0);

        Integer character = (Integer.valueOf(characterNum));



         if (character.equals("0")) {
            stack.push(character);

         }
         else if (character.equals("1")) {
            stack.push(character);

         }
         else if (character.equals("2")) {
            stack.push(character);
         }
         else if (character.equals("3")) {
            stack.push(character);
         }
         else if (character.equals("4")) {
            stack.push(character);
         }
         else if (character.equals("5")) {
            stack.push(character);
         }
         else if (character.equals("6")) {
            stack.push(character);
         }
         else if (character.equals("7")) {
            stack.push(character);
         }
         else if (character.equals("8")) {
            stack.push(character);
         }
         else if (character.equals("9")) {
            stack.push(character);
         }
         else if (character.equals("*")) {
            integer2 = stack.pop();
            integer1 = stack.pop();


            result = integer1 * integer2;


            stack.push(result);



         }
        else if (character.equals("/")) {
            integer2 = stack.pop();
            integer1 = stack.pop();

             result = integer1 / integer2;


            stack.push(result);

         }
        else if (character.equals("%")) {
            integer2 = stack.pop();
            integer1 = stack.pop();


            result = integer1 % integer2;


            stack.push(result);
          }
         else if (character.equals("+")) {
            integer2 = stack.pop();
            integer1 = stack.pop();


            result = integer1 + integer2;


            stack.push(result);

         }
         else if (character.equals("-")) {
            integer2 = stack.pop();
            integer1 = stack.pop();

             result = integer1 - integer2;


            stack.push(result);

         }


      }
      return result;   
   }
}

Here is the part of my code where I feel the problem resides(btw I am pushing and popping nothing but integers):

public static Integer postfixEvaluation(String input){




      StackInterface<Integer> stack = new LinkedStack<Integer>();

      Integer result = new Integer(0);



      for(int i=0;i<input.length();i++){

         String characterNum = input.substring(i, i+1);

         Integer integer1 = new Integer(0);
         Integer integer2 = new Integer(0);

        Integer character = (Integer.valueOf(characterNum));



         if (character.equals("0")) {
            stack.push(character);

         }
         else if (character.equals("1")) {
            stack.push(character);

         }
         else if (character.equals("2")) {
            stack.push(character);
         }
         else if (character.equals("3")) {
            stack.push(character);
         }
         else if (character.equals("4")) {
            stack.push(character);
         }
         else if (character.equals("5")) {
            stack.push(character);
         }
         else if (character.equals("6")) {
            stack.push(character);
         }
         else if (character.equals("7")) {
            stack.push(character);
         }
         else if (character.equals("8")) {
            stack.push(character);
         }
         else if (character.equals("9")) {
            stack.push(character);
         }
         else if (character.equals("*")) {
            integer2 = stack.pop();
            integer1 = stack.pop();


            result = integer1 * integer2;


            stack.push(result);



         }
        else if (character.equals("/")) {
            integer2 = stack.pop();
            integer1 = stack.pop();

             result = integer1 / integer2;


            stack.push(result);

         }
        else if (character.equals("%")) {
            integer2 = stack.pop();
            integer1 = stack.pop();


            result = integer1 % integer2;


            stack.push(result);
          }
         else if (character.equals("+")) {
            integer2 = stack.pop();
            integer1 = stack.pop();


            result = integer1 + integer2;


            stack.push(result);

         }
         else if (character.equals("-")) {
            integer2 = stack.pop();
            integer1 = stack.pop();

             result = integer1 - integer2;


            stack.push(result);

         }


      }
      return result;   
   }

Answer 1

Your problem is the following code line:

Integer character = (Integer.valueOf(characterNum));

Since you are trying to convert any character to a number and your String also contains operators(which can't be converted to numbers), you get an exception.

You should probably think about it that way:

• is my character a digit?

  Yes => convert it to number and push it on the stack

  No => it's an operand, therefore pop the top 2 elements from the stack, perform the operation and push the result.