Question

Im writing a method to evaluate a postfix expression. Using my own stack class. Here is my code but I keep getting a classcastexception where it says java.lang.Character cannot be cast to java.lang,Integer. Im not sure how to fix this.

public class Evaluator {

public static void evaluatePost(String postFix)
   {
       LinkedStack stack2 = new LinkedStack();
       int val1;
       int val2;
       int result;
       for(int i = 0; i < postFix.length(); i++)
       {
           char m = postFix.charAt(i);
           if(Character.isDigit(m))
           {
               stack2.push(m);
           }
           else
           {
               val1 = (int) stack2.pop();
               val2 =(int) stack2.pop();

               if (m == '-')
               {
                   result =val2 - val1;
               }
               else if (m == '=')
               {
                   result = val2 + val1;
               }
               else if ( m == '%')
               {
                   result = val2 % val1;
               }
               else if (m == '/')
               {
                   result = val2 / val1;
               }
               else if (m == '*')
               {
                   result = val2 * val1;
               }
               else
               {
                   result = Math.pow(val2, val1);
               }
               stack2.push(result);
              
           }
          
       }

}

Stack class

public class LinkedStack
{
   private class Node
   {
       private Object data;
       private Node next;
       public Node(Object e, Node n)
       {
           this.data = e;
           this.next = n;
       }
      
   }//end of node class
  
   private Node top;
   private int size;
  
   public LinkedStack() //constructor
   {
       this.top = null;
       this.size = 0;
   }
  
   public int size()
   {
       return this.size;
   }
  
   public boolean isEmpty()
   {
       return (this.top == null || this.size == 0);
   }
  
   public void push(Object elem)
   {
       Node nn = new Node(elem, this.top);
       this.top = nn;
       this.size++;
   }
  
   public Object pop() throws EmptyStackException
   {
       if (isEmpty())
       {
           throw new EmptyStackException("Stack is empty");
       }
       Object temp = this.top.data;
       this.top = this.top.next;
       this.size--;
       return temp;
   }
  
  
   public Object top() throws EmptyStackException
   {
       if (isEmpty())
       {
           throw new EmptyStackException("Stack is empty");
       }
       return this.top.data;
   }
  
   class EmptyStackException extends RuntimeException
   {
       public EmptyStackException(String err)
       {
           super(err);
       }

   }
}

Answer 1

/*
As pop() method have a retrun type of 'Object', and we can't convert 'Object' type to
'int' type directly. That's why java.lang.ClassCastException is being thrown.
So one possible solution is:
convert the 'Object' to string with the help of
'toString()' method of Object class and then parse this string
into Integer using 'Integer.ParseInt()' method of Integer class
*/

import java.util.*;

public class Evaluator
{
public static void main(String[] args) {
    Evaluate e = new Evaluate();
    e.evaluatePost("231*+9-");
}
}

class Evaluate {

public static void evaluatePost(String postFix)
     {
         LinkedStack stack2 = new LinkedStack();
         int val1;
         int val2;
         int result;
         for(int i = 0; i < postFix.length(); i++)
         {
             char m = postFix.charAt(i);
             if(Character.isDigit(m))
             {
                 stack2.push(m);
             }
             else
             {
                 val1 = Integer.parseInt(stack2.pop().toString()); ///// modified code
                 val2 = Integer.parseInt(stack2.pop().toString()); ///// modified code

                 if (m == '-')
                 {
                     result =val2 - val1;
                 }
                 else if (m == '=')
                 {
                     result = val2 + val1;
                 }
                 else if ( m == '%')
                 {
                     result = val2 % val1;
                 }
                 else if (m == '/')
                 {
                     result = val2 / val1;
                 }
                 else if (m == '*')
                 {
                     result = val2 * val1;
                 }
                 else
                 {
                     result = (int)Math.pow(val2, val1);
                 }
                 stack2.push(result);
              
             }
          
         }

         System.out.println(stack2.pop());
}
}

//Stack class

class LinkedStack
{
   private class Node
   {
       private Object data;
       private Node next;
       public Node(Object e, Node n)
       {
           this.data = e;
           this.next = n;
       }
    
   }//end of node class

   private Node top;
   private int size;

   public LinkedStack() //constructor
   {
       this.top = null;
       this.size = 0;
   }

   public int size()
   {
       return this.size;
   }

   public boolean isEmpty()
   {
       return (this.top == null || this.size == 0);
   }

   public void push(Object elem)
   {
       Node nn = new Node(elem, this.top);
       this.top = nn;
       this.size++;
   }

   public Object pop() throws EmptyStackException
   {
       if (isEmpty())
       {
           throw new EmptyStackException("Stack is empty");
       }
       Object temp = this.top.data;
       this.top = this.top.next;
       this.size--;
       return temp;
   }


   public Object top() throws EmptyStackException
   {
       if (isEmpty())
       {
           throw new EmptyStackException("Stack is empty");
       }
       return this.top.data;
   }

   class EmptyStackException extends RuntimeException
   {
       public EmptyStackException(String err)
       {
           super(err);
       }

   }
}