private int calculate(String first, String second, String operator) {
		assert (!isNullOrEmpty(first) && !isNullOrEmpty(second) && !isNullOrEmpty(operator)) : "Please check input!";
		int a = Integer.parseInt(first);
		int b = Integer.parseInt(second);
		int result = 0;
		switch (operator){
		case "+":
			result = a + b;
			break;
		case "-":
			result = a - b;
			break;
		case "*":
			result = a * b;
			break;
		case "/":
			result = a / b;
			break;
		}
		return result;
	}