Code atoi

Java code:

class atoi
{
    public static int atoi(String str)
        throws NumberFormatException
    {
        if (str == null) throw new NumberFormatException("null");
        if (str == "-2147483648") return Integer.MIN_VALUE;
        int retVal = 0;
        int i = 0;
        char[] s = str.toCharArray();
        boolean isNegative = s[0] == '-';
        if (isNegative) i++;
        if ((isNegative && str.length() > 11) || (!isNegative && str.length() > 10))
                throw new NumberFormatException(str);

        while (i < s.length)
        {
            retVal *= 10;
            if (s[i] > '9' || s[i] < '0')
                throw new NumberFormatException(str);
            retVal += s[i++] - '0';
            if (retVal < 0)
                throw new NumberFormatException(str);
        }
        if (isNegative)
            retVal *= -1;

        return retVal;
    }
    public static void main(String[] args)
    {
        String s = null;
        System.out.println("atoi="+atoi(s));
        System.out.println("max="+Integer.MAX_VALUE);
        System.out.println("min="+Integer.MIN_VALUE);
        System.out.println("parseInt="+Integer.parseInt(s));
    }
}