PostFix Calculator in Java

I am fairly a new java programmer who needs to learn and brush up?
i am having issue on how to start my first java project that involves calculation and need guidance?

Postfix Calculator
I need to make a program that evaluate an arithmetic expression in postfix notation.
The arithmetic expression will be input as a String (by the user) and will contain only integer operands.
Postfix Notation Definition

I think the link below will help it contains suitable commenting too so that you can understand better.

link text

I read about postfix and understood bit.
I wrote this and trying to figure out how to add expression as user string?
and values will contain only integer operands? program won’t compile?

public class PostfixCalculator { int
length; Stack ri; String expression;
String postfix;

PostFixCalculator(String s){
ri = new Stack(); //create an empty stack
expression = s;
postfix = “”;
length = expression.length; }

boolean isBalance(){
boolean fail =false;
int index = 0; }

try{
    while (index < length)
    {
        char ch = expression.charAt(index);
        switch(ch)
        {
            case ')':stk.pop();
            break;
            case '(': stk.push(new Character(ch));
            break;
            default:
            break;
        }
            index++;
    }
}
catch(EmptyStackException e)
{
    fail = true;
}
    return ri.empty() && !fail;
 void postfixExpression(){
String token = "";
Scanner scan = new Scanner(exp);
ri.clear();

while(scan.hasNext())
{
    token = scan.next();
    char current = token.charAt(0);

    if (isNumber(token)){
        postfix = postfix + token + " ";
    }
    else if(isParentheses(current)){
        if(current == '(')
        {
            ri.push(current);
        }
        else
        {
            Character ch = (Character)ri.peek();
            char nextToken = ch.charValue();

            while(nextToken != '(')
            {
                postfix = postfix + ri.pop() + " ";
                ch = (Character)ri.peek();
                nextToken = ch.charValue();
            }
                ri.pop();
        }
    }
    else
    {
        if(ri.empty())
        {
            ri.push(current);
        }
        else
        {
            Character ch = (Character)ri.peek();
            char top = ch.charValue();
            if(hasHigherPrecedence(top, current))
            {
                ri.push(current);
            }
            else
            {
                ch = (Character)ri.pop();
                top = ch.charValue();
                ri.push(current);
                ri.push(top);
            }
        }
    }
} }
try
{
    Character ch = (Character)ri.peek();
    char nextToken = ch.charValue();

    while(isOperator(nextToken))
    {
        postfix = postfix + ri.pop() + " ";
        ch = (Character)ri.peek();
        nextToken = ch.charValue();
    }
}catch(EmptyStackException e)
{
}

boolean isNumber(String s){
try
{
int Num = Integer.parseInt(s);
}
catch(NumberFormatException e)
{
return false;
}
return true; }

System.out.print(“Please write an
arithmetic expression: ”); }