java digit(Char,Int)把一个字符转换为一个数字

进制radix主要用来排错，因为转换的数为能大于radix。

    public static int digit(int codePoint, int radix) {
        // BEGIN android-changed
        if (radix < MIN_RADIX || radix > MAX_RADIX) {
            return -1;
        }
        if (codePoint < 128) {
            // Optimized for ASCII
            int result = -1;
            if ('0' <= codePoint && codePoint <= '9') {
                result = codePoint - '0';
            } else if ('a' <= codePoint && codePoint <= 'z') {
                result = 10 + (codePoint - 'a');
            } else if ('A' <= codePoint && codePoint <= 'Z') {
                result = 10 + (codePoint - 'A');
            }
            return result < radix ? result : -1;
        }
        return UCharacter.digit(codePoint, radix);
        // END android-changed
    }