解读Integer类的parseInt和valueOf的区别

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

public static Integer valueOf(String s) throws NumberFormatException {
    return Integer.valueOf(parseInt(s, 10));
}

public class StringDemo {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String str = "123";
        long startTime = System.currentTimeMillis();
        for(int i = 0;i<100000000;i++){
            Integer.parseInt(str);
        }
        long endTime = System.currentTimeMillis();
        System.out.println(endTime-startTime);
    }
}

public class StringDemo {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String str = "123";
        long startTime = System.currentTimeMillis();
        for(int i = 0;i<100000000;i++){
            Integer.valueOf(str);
        }
        long endTime = System.currentTimeMillis();
        System.out.println(endTime-startTime);
    }
}

public class StringParseInt {
    public static void main(String[] args) {
        String str = "-1234";
        int i = Integer.parseInt(str);
        int y = Integer.valueOf(str);
 
        System.out.println(i + "\t" + y);
    }
}

    public static Integer valueOf(String s) throws NumberFormatException {
        return Integer.valueOf(parseInt(s, 10));
    }

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

    private static class IntegerCache {
        //下限固定-128
        static final int low = -128;
        //上限没有初始化
        static final int high;
        //核心缓存数组
        static final Integer cache[];
        //类加载初始化
        static {
            // high value may be configured by property
            //初始127上限
            int h = 127;
            //VM参数java.lang.Integer.IntegerCache.high可以配置Integer的最大上限
            String integerCacheHighPropValue =
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    //我没设置的值，如果不是int字符串型就会报错，然后被捕获
                    int i = parseInt(integerCacheHighPropValue);
                    //取大，对比127；也就是至少是127
                    i = Math.max(i, 127);
                    // Maximum array size is Integer.MAX_VALUE
                    // 上面官方注释很明显了，这里减128再减1是因为low是-128
                    h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;
 
            //创建缓存数组，如果设置java.lang.Integer.IntegerCache.high，不宜设置过大，过大很占连续空间
            cache = new Integer[(high - low) + 1];
            int j = low;
            //初始化缓存
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
 
            // range [-128, 127] must be interned (JLS7 5.1.7)
            //断言至少127，JLS7
            assert IntegerCache.high >= 127;
        }
 
        private IntegerCache() {}
    }

    public boolean equals(Object obj) {
        if (obj instanceof Integer) {
            return value == ((Integer)obj).intValue();
        }
        return false;
    }

    /**
     * 这段注释尤为重要，定义了符号位，定义了10进制数字
     * Parses the string argument as a signed decimal integer. The
     * characters in the string must all be decimal digits, except
     * that the first character may be an ASCII minus sign {@code '-'}
     * ({@code '\u005Cu002D'}) to indicate a negative value or an
     * ASCII plus sign {@code '+'} ({@code '\u005Cu002B'}) to
     * indicate a positive value. The resulting integer value is
     * returned, exactly as if the argument and the radix 10 were
     * given as arguments to the {@link #parseInt(java.lang.String,
     * int)} method.
     *
     * @param s    a {@code String} containing the {@code int}
     *             representation to be parsed
     * @return     the integer value represented by the argument in decimal.
     * @exception  NumberFormatException  if the string does not contain a
     *               parsable integer.
     */    
    public static int parseInt(String s) throws NumberFormatException {
        return parseInt(s,10);
    }

    public static long parseLong(String s) throws NumberFormatException {
        return parseLong(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.
         */
        //字符串不能为null，没有判断空字符串
        if (s == null) {
            throw new NumberFormatException("null");
        }
        //进制不能小于2，至少要2进制
        if (radix < Character.MIN_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " less than Character.MIN_RADIX");
        }
        //进制不能大于36
        if (radix > Character.MAX_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " greater than Character.MAX_RADIX");
        }
 
        int result = 0;
        //正负标记，默认正
        boolean negative = false;
        //字符串长度
        int i = 0, len = s.length();
        int limit = -Integer.MAX_VALUE;
        int multmin;
        int digit;
        //干活了
        if (len > 0) {
            //首位字符
            char firstChar = s.charAt(0);
            //这里玩了个计谋，0字符的ASCII是48，后面的数字包括ABCDEF的ASCII都比0大；
            //其中 + 43； - 45
            //只有带符号位的会判断，其他就默认正数
            if (firstChar < '0') { // Possible leading "+" or "-"
                //负数
                if (firstChar == '-') {
                    negative = true;
                    limit = Integer.MIN_VALUE;
                //非负即正，因为小于'0'
                } else if (firstChar != '+')
                    throw NumberFormatException.forInputString(s);
 
                if (len == 1) // Cannot have lone "+" or "-" 注释说明白了
                    throw NumberFormatException.forInputString(s);
                i++;
            }
            multmin = limit / radix;
            while (i < len) {
                // Accumulating negatively avoids surprises near MAX_VALUE
                // 拿到字符，转换为ASCII数字并按进制转为数字
                digit = Character.digit(s.charAt(i++),radix);
                //不能带符号位，前面已经验证了
                if (digit < 0) {
                    throw NumberFormatException.forInputString(s);
                }
                //最小限制，但是这里是一个负数
                if (result < multmin) {
                    throw NumberFormatException.forInputString(s);
                }
                //由于是从高位向低位，所以需要进制；字符每往后走，需要乘进制
                result *= radix;
                //同样最小验证，可能在某些地方有用，暂时没看出来
                if (result < limit + digit) {
                    throw NumberFormatException.forInputString(s);
                }
                //这里使用反向进位，为了照顾字符从前往后，也可以字符从后往前正向进位
                result -= digit;
            }
        } else {
            throw NumberFormatException.forInputString(s);
        }
        //负数正数校正，上面的算法是反向进位
        return negative ? result : -result;
    }

    public static int digit(char ch, int radix) {
        return digit((int)ch, radix);
    }