Java Integer.parseInt(String s, int radix) 实现A、B、C分别代表10,11,12转换

在实际业务过程中我们常遇到 Y 一码代表月份，A，B，C 分别代表10、11、12月

public static void main(String[] args) {
        //A B C 分别代表 10,11,12
        String M ="A";
        int    month = Integer.parseInt(M, 13);
        System.out.println("month = " + month);
    }

parseInt(String s, int radix) 源码如下：

/**
 * Parses the string argument as a signed integer in the radix
 * specified by the second argument.
 *
 * <p>Examples:
 * <blockquote><pre>
 * parseInt("0", 10) returns 0
 * parseInt("473", 10) returns 473
 * parseInt("+42", 10) returns 42
 * parseInt("-0", 10) returns 0
 * parseInt("-FF", 16) returns -255
 * parseInt("1100110", 2) returns 102
 * parseInt("2147483647", 10) returns 2147483647
 * parseInt("-2147483648", 10) returns -2147483648
 * parseInt("2147483648", 10) throws a NumberFormatException
 * parseInt("99", 8) throws a NumberFormatException
 * parseInt("Kona", 10) throws a NumberFormatException
 * parseInt("Kona", 27) returns 411787
 * </pre></blockquote>
 */

public static int parseInt(String s, int radix)
            throws NumberFormatException
{
    /*
     * WARNING: This method may be invoked early during VM initialization
     * before IntegerCache is initialized. Care must be taken to not use
     * the valueOf method.
     */

    if (s == null) {    // 如果接受的字符串为空, 就报空字符串的异常
        throw new NumberFormatException("null");
    }

    if (radix < Character.MIN_RADIX) {      // 判断基数是不是符合要求
        throw new NumberFormatException("radix " + radix +
                                        " less than Character.MIN_RADIX");
    }

    if (radix > Character.MAX_RADIX) {  // 判断基数是不是符合要求
        throw new NumberFormatException("radix " + radix +
                                        " greater than Character.MAX_RADIX");
    }

    int result = 0;
    boolean negative = false;       // 判断符号
    int i = 0, len = s.length();    // 设置初始位置和字符串的长度
    int limit = -Integer.MAX_VALUE; 
    int multmin;
    int digit;

    if (len > 0) {      // 字符串的长度必须大于零
        char firstChar = s.charAt(0);   // 获得字符串的第一个字符
        if (firstChar < '0') { // Possible leading "+" or "-"
            if (firstChar == '-') {
                negative = true;
                limit = Integer.MIN_VALUE;
            } else if (firstChar != '+') // 如果不为++的话就报错
                throw NumberFormatException.forInputString(s);
            // 字符串的长度为1但是又不是数字, 出错
            if (len == 1) // Cannot have lone "+" or "-" 
                throw NumberFormatException.forInputString(s);
            i++;
        }
        multmin = limit / radix;

        /*
         * 下面的过程其实很好理解, 以8进制的"534"为例
         * (-5*8-3)*8-4 = -348, 根据符号位判断返回的是348
         */

        while (i < len) {
            // Accumulating negatively avoids surprises near MAX_VALUE 

            digit = Character.digit(s.charAt(i++),radix);
            if (digit < 0) {
                throw NumberFormatException.forInputString(s);
            }
            if (result < multmin) {
                throw NumberFormatException.forInputString(s);
            }
            result *= radix;
            if (result < limit + digit) {
                throw NumberFormatException.forInputString(s);
            }
            result -= digit;
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    return negative ? result : -result;     // 根据符号位来判断返回哪一个
}