Infix to postfix converter

[johnaw24]

I have written a program to perform an infix to postfix conversion. I'm getting an empty stack exception, I can't seem to figure out where it goes empty.
Thank you in advabce for your time.

package prog17_12;

import java.util.Scanner;
import java.util.Stack;

public class InfixToPostfixConverter 
{

    //take out the infix and change it into postfix
	public static StringBuffer convertToPostfix(StringBuffer infix)
	{
		Stack<Character> charStack = new Stack<Character>();
		StringBuffer temporary = new StringBuffer("");//to store the postfix expression

		//push a left paren onto the stack and add a right paren to infix
		charStack.push('(');
		infix.append(')');

		//convert the infix expression to postfix
		for (int infixCount = 0; !charStack.isEmpty(); ++infixCount) 
		{
			if (Character.isDigit(infix.charAt(infixCount)))
				temporary.append(infix.charAt(infixCount) + " ");
			else if (infix.charAt(infixCount) == '(')
				charStack.push('(');
			else if (isOperator(infix.charAt(infixCount))) 
			{
				while ( isOperator(stackTop(charStack)) && 
						precedence(stackTop(charStack), infix.charAt(infixCount)))
					temporary.append(charStack.pop() + " ");
		
				charStack.push(infix.charAt(infixCount));
			}//end else if
			
			else if (infix.charAt(infixCount) == ')')
			{
				while (isOperator(stackTop(charStack)) || stackTop(charStack) == '(')
				{
					if (isOperator(stackTop(charStack)))
						temporary.append(charStack.pop() + " ");
					else
						charStack.pop();
				}//end while
				
			} // end else if
			
		}//end for
		return temporary;
	}//end method convertToPostfix
	
	//checks to see character is an operator
	public static boolean isOperator(char c)
	{
		if (c == '+' || c == '-' || c == '*' || c == '/' || c == '^' || c == '%')
			return true;
		else
			return false;
	}//end method isOperator
	
	//determines if first operator(a) has equal or greater precedence over operator(b)
	public static boolean precedence(char a, char b)
	{
		String operatorString = "^*/%+-";
		if (operatorString.indexOf(a) >= operatorString.indexOf(a))
			return true;
		else
			return false;
	}//end method precedence
	
	//returns the top value of the stack without popping the stack
	public static Character stackTop(Stack<Character> stack)
	{
		return (stack.peek());
	}//end stackTop
	
	public static void main(String args[])
	{
		StringBuffer infix = new StringBuffer();
		Scanner input = new Scanner(System.in);

		//get mathematical expression from user
		System.out.println("Please input a mathematical expression");
		infix.append(input.next());
		
		System.out.println(convertToPostfix(infix).toString());
	}
	
}//InfixToPostfixConverter

here is the exception

Please input a mathematical expression
8+3-5
Exception in thread "main" java.util.EmptyStackException
	at java.util.Stack.peek(Unknown Source)
	at prog17_12.InfixToPostfixConverter.stackTop(InfixToPostfixConverter.java:73)
	at prog17_12.InfixToPostfixConverter.convertToPostfix(InfixToPostfixConverter.java:37)
	at prog17_12.InfixToPostfixConverter.main(InfixToPostfixConverter.java:85)