Evaluate Postfix Using Stack

Question

I am trying to create a program which evaluates postfix expression.For example "3500 43 12 * 47 2 / + -" .Here is my code

 public static int EvaluatePostfixExpression(String postfixExpr){
     Stack s = new Stack();
     int result = 0;
     String operand = null;


     for(int i = 0; i<postfixExpr.length();i++){
         if(Character.isDigit(postfixExpr.charAt(i)) == true){

             operand = operand + postfixExpr.charAt(i);
             if(Character.isDigit(postfixExpr.charAt(i+1)) == false){
                 s.push(operand);
                 operand = null;

                 }

         }
         if(postfixExpr.charAt(i) == '+'){
            result = result + Integer.parseInt((String) s.pop()) + Integer.parseInt((String) s.pop()) ;
         }
         if(postfixExpr.charAt(i) == '-'){
                result = result + Integer.parseInt((String) s.pop()) - Integer.parseInt((String) s.pop()) ;
             }
         if(postfixExpr.charAt(i) == '*'){
                result = result + Integer.parseInt((String) s.pop()) * Integer.parseInt((String) s.pop()) ;
             }
         if(postfixExpr.charAt(i) == '/'){
                result = result + Integer.parseInt((String) s.pop()) / Integer.parseInt((String) s.pop()) ;
             }

     }

    return result;
  } //end-EvaluatePostfixExpression

When I try to run it, there occurs an error.

Exception in thread "main" java.lang.NumberFormatException: For input string: "null12"
    at java.lang.NumberFormatException.forInputString(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)

I can't find a solution.It would be great if anyone could help.

Edit: I handled the errors and my code now works;

 public static int EvaluatePostfixExpression(String postfixExpr){
     Stack s = new Stack();
     int result = 0;
     String operand = "";


     for(int i = 0; i<postfixExpr.length();i++){
         if(Character.isDigit(postfixExpr.charAt(i)) == true){

             operand = operand + postfixExpr.charAt(i);
             if(Character.isDigit(postfixExpr.charAt(i+1)) == false){
                 s.push(operand);
                 operand = "";

                 }

         }
         if(postfixExpr.charAt(i) == '+'){
                int x = Integer.parseInt((String) s.pop()) + Integer.parseInt((String) s.pop());
                result = result + x ;
                s.push(String.valueOf(x));
         }
         if(postfixExpr.charAt(i) == '-'){
                int x = Integer.parseInt((String) s.pop()) - Integer.parseInt((String) s.pop());    
                result = result + x ;
                s.push(String.valueOf(x));
             }
         if(postfixExpr.charAt(i) == '*'){
                int x = Integer.parseInt("" + s.pop()) * Integer.parseInt("" + s.pop());    
                result = result + x ;
                s.push(String.valueOf(x));
             }
         if(postfixExpr.charAt(i) == '/'){
                int x = Integer.parseInt((String) s.pop()) / Integer.parseInt((String) s.pop());    
                result = result + x ;
                s.push(String.valueOf(x));
             }

     }

    return result;
  } 

but now the result is wrong.It should be 2961 but I am getting -1952.

Answer 1

Try this one:

 String operand = "";

Change it also when

if(Character.isDigit(postfixExpr.charAt(i+1)) == false){
    s.push(operand);
    operand = "";
}