二进制转10进制

public class BB {
    public static void main(String[] args) throws Exception {
        System.out.println(parseInt("11100000000000000000000000000000",2));
        System.out.println(parseInt("11100000000000000000000000000001",2));
        System.out.println(parseInt("11100000000000000000000000000011",2));
        System.out.println(parseInt("01111111111111111111111111111111",2));
    }
    
    public static int parseInt(String s, int radix) throws Exception {

        int result = 0;
        boolean negative = false;
        int i = 0, len = s.length();
        int limit = -Integer.MAX_VALUE;
        int multmin;
        int digit;

        if (len > 0) {
            char firstChar = s.charAt(0);
            if (firstChar < '0') { // Possible leading "+" or "-"
                if (firstChar == '-') {
                    negative = true;
                    limit = Integer.MIN_VALUE;
                } else if (firstChar != '+')
                    throw new Exception(s);

                if (len == 1) // Cannot have lone "+" or "-"
                    throw new Exception(s);
                i++;
            }
            multmin = limit / 1;
            while (i < len) {
                digit = Character.digit(s.charAt(i++), radix);
                if (digit < 0) {
                    throw new Exception(s);
                }
                if (result < multmin) {
                    throw new Exception(s);
                }
                result *= radix;
                result -= digit;
            }
        } else {
            throw new Exception(s);
        }
        return negative ? result : -result;
    }
}