Postfix Expression Evaluation Program

Write a program to evaluate a postfix expression using stack

-------------------------Program---------------------------------------

import java.util.Stack;



public class PostfixEvaluation {
 private static String postfixExpr;
    public PostfixEvaluation(String postfix) {
     this.postfixExpr= postfix;
   
    }
   

     /**
      * Evaluate postfix expression
      *
      * @param postfix The postfix expression
      */
     public  Double evaluate() {
      Stack s = new Stack();
      Double result = 0.0;
         String operand = "";


         for(int i = 0; i<postfixExpr.length();i++){
             if(Character.isDigit(postfixExpr.charAt(i)) == true || ( postfixExpr.charAt(i)=='-'  ) ){
              
                   if( postfixExpr.charAt(i)=='-' && i!=postfixExpr.length()-2   && i!=postfixExpr.length()-1)
                   {
                    if( Character.isDigit(postfixExpr.charAt(i+1)) &&  Character.isDigit(postfixExpr.charAt(i+1)) == true) {
                    operand="-";
                      i++;
                    }
                   }
                   if ( postfixExpr.charAt(i)!='-' )
                   {  
                 operand = operand + postfixExpr.charAt(i);
                 if(Character.isDigit(postfixExpr.charAt(i+1)) == false){
                     s.push(operand);
                     operand = "";

                     }
                   }
             }
            
              if(postfixExpr.charAt(i) == '+'){
              Double x = Double.parseDouble((String) s.pop()) + Double.parseDouble((String) s.pop());
                    result = x ;
                    s.push(String.valueOf(x));
             }
              if(postfixExpr.charAt(i) == '-'){
              Double p =Double.parseDouble((String) s.pop());
              Double q =Double.parseDouble((String) s.pop());
              Double x = p-q;    
                    result = x ;
                    s.push(String.valueOf(x));
                 }
              if(postfixExpr.charAt(i) == '*'){
              Double x = Double.parseDouble((String) s.pop()) * Double.parseDouble((String) s.pop());    
                    result = x ;
                    s.push(String.valueOf(x));
                 }
              if(postfixExpr.charAt(i) == '/'){
              Double p =Double.parseDouble((String) s.pop());
              Double q =Double.parseDouble((String) s.pop());
              if(q!=0){
              Double x = p/q; 
                    result =  x ;
                    s.push(String.valueOf(x));
                 }
              
             else
             {
              throw new ArithmeticException("Division by zero error");
             }

         }
         }
        return result;
      } 
   

     
//     public static void main(String[] args) {
//         String postfix = "3 2 /";
//
//         String output;
//         
//         PostfixEvaluation pfe = new PostfixEvaluation(postfix);
//         
//         Double value = pfe.evaluate();
//         System.out.println(value);
//         
//     }
 }