java.lang.Integer

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The Integer class wraps a value of the primitive type int in an object. An object of type Integer contains a single field whose type is int.

In addition, this class provides several methods for converting an int to a String and a String to an int, as well as other constants and methods useful when dealing with an int.

Implementation note: The implementations of the “bit twiddling” methods (such as highestOneBit and numberOfTrailingZeros) are based on material from Henry S. Warren, Jr.’s Hacker’s Delight, (Addison Wesley, 2002).

parseInt()

API 使用规范 - - - Java™ SE 8 API

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public static int parseInt(String s) throws NumberFormatException {
    return parseInt(s,10);
}

// s 为目标字符串，radix 为目标字符串的进制，默认为 10 进制
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");
    }

    // 进制范围为 2 - 36
    // Character.MIN_RADIX = 2
    // Character.MAX_RADIX = 36
    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;  // 转换成 10 进制的结果，默认当成负数处理
    boolean negative = false;  // 代表正负号
    int i = 0, len = s.length();
    int limit = -Integer.MAX_VALUE;  // 最大值限制，因为当成负数处理，所以为负数
    int multmin;  // 用来防止溢出
    int digit;  // 字符串里的字符，转换成 10 进制之后的值

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

            if (len == 1) // Cannot have lone "+" or "-"
                throw NumberFormatException.forInputString(s);
            i++;
        }
        multmin = limit / radix;  // 在 result *= radix 之前判断，防止溢出
        while (i < len) {
            // Accumulating negatively avoids surprises near MAX_VALUE
            digit = Character.digit(s.charAt(i++),radix);  // 无法转换时会返回 -1
            if (digit < 0) {
                throw NumberFormatException.forInputString(s);
            }
            if (result < multmin) {
                throw NumberFormatException.forInputString(s);
            }
            result *= radix;
            if (result < limit + digit) {  // 防止 result - digit 溢出
                throw NumberFormatException.forInputString(s);
            }
            result -= digit;  // 因为是当成负数处理，所以是 result - digit
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    return negative ? result : -result;
}