public String calculateResult(String expression) {
		String result = parseExpression(expression);
		Double lastOperand = 0.0;
		
		// If no exception pop the top operand
		if (result.equals("")) {
			lastOperand = operands.top();
			operands.pop();
		}
		
		// If no operands are left set result to the last operand
		if (operands.isEmpty() && result.equals(""))
			result = lastOperand.toString();
		
		// If operands remain, but not previous exceptions an operation needs to
		// be performed on the remaining operands
		if (!operands.isEmpty() && result.equals(""))
			result = "Invalid expression. There are two operands left, but no " +
					"operator.";
		
		// The result is out of range, throw an exception
		if (lastOperand < -500000000 || lastOperand > 1000000000)
			result = "The answer is out of the range -500,000,000 to 1,000,000,000";
		
		return result;
	}