public static int parseInt(String s) throws NumberFormatException {

    return parseInt(s,10);

}

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) {

        throw new NumberFormatException("null");

    }

    if (radix < Character.MIN_RADIX) {

        throw new NumberFormatException("radix " + radix +" less than Character.MIN_RADIX");

    }

    if (radix > Character.MAX_RADIX) {

            throw new NumberFormatException("radix " + radix +" greater than Character.MAX_RADIX");

    }

    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 NumberFormatException.forInputString(s);

            if (len == 1) // Cannot have lone "+" or "-"

                 throw NumberFormatException.forInputString(s);

            i++;

        }

        multmin = limit / radix;

        while (i < len) {

            // Accumulating negatively avoids surprises near MAX_VALUE

            digit = Character.digit(s.charAt(i++),radix); //传入一个字符参数和基数，返回一个int值

            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;
}

public static Integer valueOf(String s) throws NumberFormatException {

    return Integer.valueOf(parseInt(s, 10));
} 

public static Integer valueOf(int i) {

    assert IntegerCache.high >= 127;

    if (i >= IntegerCache.low && i <= IntegerCache.high)

        return IntegerCache.cache[i + (-IntegerCache.low)];

    return new Integer(i);
}