Java implementation of Postfix Evaluation

import java.util.Scanner;

class PostFixEval

{

    boolean isDigit(char ch)

    {

        if(ch>='0'&& ch<='9')

            return true;

        else

            return false;

    }

    void Eval(String expr)

    {

        int position;

        char c;

        double num1,num2,value;

        Stack_Num s=new Stack_Num();

        s.top=-1;

        for(position=0;position<expr.length(); position++)

        {

            c=expr.charAt(position);

            if(isDigit(c))

                s.Push((double)(c -'0'));

            else

            {

                num2=s.Pop();

                num1=s.Pop();

                value=Calc(c,num1,num2);

                s.Push(value);

            }

        }

        value=s.Pop();

        if(!s.isEmpty())

            System.out.println("Expression is wrong!!!!!!!!!!!");

        else

            System.out.println("Result of postfix expression is "+value);

    }

    double Calc(char ch,double num1,double num2)

    {

        switch(ch)

        {

            case '+':return(num1+num2);

            case '-':return(num1-num2);

            case '*':return(num1*num2);

            case '/':return(num1/num2);

            case '^':return(Math.pow(num1,num2));

            default:System.out.println("\nillegal operator found");

        }

        return(0);

    }

    public static void main(String args[])

    {

        String expr=new String();

        Scanner in = new Scanner(System.in);

        PostFixEval eqn=new PostFixEval();

        System.out.println("Use digits(0-9)and operators(+,-,*,/,$)");

        System.out.println("Enter an expression in postfix form :");

        expr=in.nextLine();

        System.out.println("Original postfix expression is " + expr);

        eqn.Eval(expr);

    }

}