java.lang.Integer源码浅析

1. Integer定义,final不可修改的类
   

   public final class Integer extends Number implements Comparable<Integer>

2. 常量定义
   

       /**
        * A constant holding the minimum value an {@code int} can
        * have, -2<sup>31</sup>.
        */
       @Native public static final int   MIN_VALUE = 0x80000000;
   
       /**
        * A constant holding the maximum value an {@code int} can
        * have, 2<sup>31</sup>-1.
        */
       @Native public static final int   MAX_VALUE = 0x7fffffff;
   
       /**
        * The {@code Class} instance representing the primitive type
        * {@code int}.
        *
        * @since   JDK1.1
        */
       @SuppressWarnings("unchecked")
       public static final Class<Integer>  TYPE = (Class<Integer>) Class.getPrimitiveClass("int");
       
       /**
        * All possible chars for representing a number as a String
        */
       final static char[] digits = {
           '0' , '1' , '2' , '3' , '4' , '5' ,
           '6' , '7' , '8' , '9' , 'a' , 'b' ,
           'c' , 'd' , 'e' , 'f' , 'g' , 'h' ,
           'i' , 'j' , 'k' , 'l' , 'm' , 'n' ,
           'o' , 'p' , 'q' , 'r' , 's' , 't' ,
           'u' , 'v' , 'w' , 'x' , 'y' , 'z'
       };
   
       final static char [] DigitTens = {
           '0', '0', '0', '0', '0', '0', '0', '0', '0', '0',
           '1', '1', '1', '1', '1', '1', '1', '1', '1', '1',
           '2', '2', '2', '2', '2', '2', '2', '2', '2', '2',
           '3', '3', '3', '3', '3', '3', '3', '3', '3', '3',
           '4', '4', '4', '4', '4', '4', '4', '4', '4', '4',
           '5', '5', '5', '5', '5', '5', '5', '5', '5', '5',
           '6', '6', '6', '6', '6', '6', '6', '6', '6', '6',
           '7', '7', '7', '7', '7', '7', '7', '7', '7', '7',
           '8', '8', '8', '8', '8', '8', '8', '8', '8', '8',
           '9', '9', '9', '9', '9', '9', '9', '9', '9', '9',
           } ;
   
       final static char [] DigitOnes = {
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
           } ;
       

   int表示的最大值MAX_VALUE，最小值负值MIN_VALUE，TYPE，digits的有效性数组定义。

3. Integer的toString方法
   

       // 静态toString方法，radix为基数
       public static String toString(int i, int radix) {
           // 最小为2，最大36，范围外默认10
           if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)
               radix = 10;
   
           /* Use the faster version */
           if (radix == 10) {
               // toString 默认为10
               return toString(i);
           }
           
           // buf数组为int 目标char。长度33，因为int为8字节32位,多出来的是符号位
           char buf[] = new char[33];
           boolean negative = (i < 0);
           int charPos = 32;
   
           if (!negative) {
               i = -i;
           }
   
           while (i <= -radix) {
               buf[charPos--] = digits[-(i % radix)];
               i = i / radix;
           }
           // 将最后一个余数赋值
           buf[charPos] = digits[-i];
   
           if (negative) {
               // 负数则需要把符号赋值最前面
               buf[--charPos] = '-';
           }
            
           // 使用char 加偏移进行String构建
           return new String(buf, charPos, (33 - charPos));
       }
   
       // 当int 无符号时，则可以表示long值，所以需要转long进行toStirng
       public static String toUnsignedString(int i, int radix) {
           return Long.toUnsignedString(toUnsignedLong(i), radix);
       }
   
       // 十六进制 toString, toUnsignedString0 的第二个参数是偏移量，4则为16进制
       public static String toHexString(int i) {
           return toUnsignedString0(i, 4);
       }
   
       // Octal 无符号的8进制
       public static String toOctalString(int i) {
           return toUnsignedString0(i, 3);
       }
   
       // Binary无符号的2进制
       public static String toBinaryString(int i) {
           return toUnsignedString0(i, 1);
       }
   
       // 私有无符号toString方法
       private static String toUnsignedString0(int val, int shift) {
           // assert shift > 0 && shift <=5 : "Illegal shift value";
          // 计算数值的字节数 
          int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val);
           // 定义char 的长度
           int chars = Math.max(((mag + (shift - 1)) / shift), 1);
           char[] buf = new char[chars];
   
           formatUnsignedInt(val, shift, buf, 0, chars);
   
           // Use special constructor which takes over "buf".
           return new String(buf, true);
       }
   
       /**
        * Format a long (treated as unsigned) into a character buffer.
        * @param val the unsigned int to format
        * @param shift the log2 of the base to format in (4 for hex, 3 for octal, 1 for binary)
        * @param buf the character buffer to write to
        * @param offset the offset in the destination buffer to start at
        * @param len the number of characters to write
        * @return the lowest character  location used
        */
        static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) {
           int charPos = len;
           int radix = 1 << shift;
           int mask = radix - 1;
           do {
               buf[offset + --charPos] = Integer.digits[val & mask];
               val >>>= shift;
           } while (val != 0 && charPos > 0);
   
           return charPos;
       }
   
       /**
        * Returns a {@code String} object representing the
        * specified integer. The argument is converted to signed decimal
        * representation and returned as a string, exactly as if the
        * argument and radix 10 were given as arguments to the {@link
        * #toString(int, int)} method.
        *
        * @param   i   an integer to be converted.
        * @return  a string representation of the argument in base&nbsp;10.
        */
       // 使用10进制进行toString，有-符号。
       public static String toString(int i) {
           if (i == Integer.MIN_VALUE)
               return "-2147483648";
           int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i);
           char[] buf = new char[size];
           getChars(i, size, buf);
           return new String(buf, true);
       }
   
       public static String toUnsignedString(int i) {
           // 当int 无符号时，则可以表示long值，所以需要转long进行toStirng
           return Long.toString(toUnsignedLong(i));
       }
   
       /**
        * Places characters representing the integer i into the
        * character array buf. The characters are placed into
        * the buffer backwards starting with the least significant
        * digit at the specified index (exclusive), and working
        * backwards from there.
        *
        * Will fail if i == Integer.MIN_VALUE
        */
       static void getChars(int i, int index, char[] buf) {
           int q, r;
           int charPos = index;
           char sign = 0;
   
           if (i < 0) {
               sign = '-';
               i = -i;
           }
           // 两位两位进行转char操作
           // Generate two digits per iteration
           while (i >= 65536) {
               q = i / 100;
           // really: r = i - (q * 100);
               // * 100 拆解成 二进制运算 
               r = i - ((q << 6) + (q << 5) + (q << 2));
               i = q;
               // 赋值 个位数
               buf [--charPos] = DigitOnes[r];
               // 赋值十位数
               buf [--charPos] = DigitTens[r];
           }
   
           // 处理一个字节的小数值
           // Fall thru to fast mode for smaller numbers
           // assert(i <= 65536, i);
           for (;;) {
               q = (i * 52429) >>> (16+3);
               r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
               buf [--charPos] = digits [r];
               i = q;
               if (i == 0) break;
           }
           if (sign != 0) {
               buf [--charPos] = sign;
           }
       }
       
       // 使用数值对应索引来计算一个数需要多少长度char来表示
       final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,
                                         99999999, 999999999, Integer.MAX_VALUE };
   
       // Requires positive x
       static int stringSize(int x) {
           for (int i=0; ; i++)
               if (x <= sizeTable[i])
                   return i+1;
       }
   
       

4. parseInt方法
   

       /**
        * Parses the string argument as a signed integer in the radix
        * specified by the second argument. The characters in the string
        * must all be digits of the specified radix (as determined by
        * whether {@link java.lang.Character#digit(char, int)} returns a
        * nonnegative value), 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.
        *
        * <p>An exception of type {@code NumberFormatException} is
        * thrown if any of the following situations occurs:
        * <ul>
        * <li>The first argument is {@code null} or is a string of
        * length zero.
        *
        * <li>The radix is either smaller than
        * {@link java.lang.Character#MIN_RADIX} or
        * larger than {@link java.lang.Character#MAX_RADIX}.
        *
        * <li>Any character of the string is not a digit of the specified
        * radix, except that the first character may be a minus sign
        * {@code '-'} ({@code '\u005Cu002D'}) or plus sign
        * {@code '+'} ({@code '\u005Cu002B'}) provided that the
        * string is longer than length 1.
        *
        * <li>The value represented by the string is not a value of type
        * {@code int}.
        * </ul>
        *
        * <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>
        *
        * @param      s   the {@code String} containing the integer
        *                  representation to be parsed
        * @param      radix   the radix to be used while parsing {@code s}.
        * @return     the integer represented by the string argument in the
        *             specified radix.
        * @exception  NumberFormatException if the {@code String}
        *             does not contain a parsable {@code int}.
        */
       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);
                   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;
       }
   
       /**
        * 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);
       }
   
       /**
        * Parses the string argument as an unsigned integer in the radix
        * specified by the second argument.  An unsigned integer maps the
        * values usually associated with negative numbers to positive
        * numbers larger than {@code MAX_VALUE}.
        *
        * The characters in the string must all be digits of the
        * specified radix (as determined by whether {@link
        * java.lang.Character#digit(char, int)} returns a nonnegative
        * value), except that the first character may be an ASCII plus
        * sign {@code '+'} ({@code '\u005Cu002B'}). The resulting
        * integer value is returned.
        *
        * <p>An exception of type {@code NumberFormatException} is
        * thrown if any of the following situations occurs:
        * <ul>
        * <li>The first argument is {@code null} or is a string of
        * length zero.
        *
        * <li>The radix is either smaller than
        * {@link java.lang.Character#MIN_RADIX} or
        * larger than {@link java.lang.Character#MAX_RADIX}.
        *
        * <li>Any character of the string is not a digit of the specified
        * radix, except that the first character may be a plus sign
        * {@code '+'} ({@code '\u005Cu002B'}) provided that the
        * string is longer than length 1.
        *
        * <li>The value represented by the string is larger than the
        * largest unsigned {@code int}, 2<sup>32</sup>-1.
        *
        * </ul>
        *
        *
        * @param      s   the {@code String} containing the unsigned integer
        *                  representation to be parsed
        * @param      radix   the radix to be used while parsing {@code s}.
        * @return     the integer represented by the string argument in the
        *             specified radix.
        * @throws     NumberFormatException if the {@code String}
        *             does not contain a parsable {@code int}.
        * @since 1.8
        */
       public static int parseUnsignedInt(String s, int radix)
                   throws NumberFormatException {
           if (s == null)  {
               throw new NumberFormatException("null");
           }
   
           int len = s.length();
           if (len > 0) {
               char firstChar = s.charAt(0);
               if (firstChar == '-') {
                   throw new
                       NumberFormatException(String.format("Illegal leading minus sign " +
                                                          "on unsigned string %s.", s));
               } else {
                   if (len <= 5 || // Integer.MAX_VALUE in Character.MAX_RADIX is 6 digits
                       (radix == 10 && len <= 9) ) { // Integer.MAX_VALUE in base 10 is 10 digits
                       return parseInt(s, radix);
                   } else {
                       long ell = Long.parseLong(s, radix);
                       if ((ell & 0xffff_ffff_0000_0000L) == 0) {
                           return (int) ell;
                       } else {
                           throw new
                               NumberFormatException(String.format("String value %s exceeds " +
                                                                   "range of unsigned int.", s));
                       }
                   }
               }
           } else {
               throw NumberFormatException.forInputString(s);
           }
       }
   
       /**
        * Parses the string argument as an unsigned decimal integer. The
        * characters in the string must all be decimal digits, except
        * that the first character may be an an ASCII plus sign {@code
        * '+'} ({@code '\u005Cu002B'}). The resulting integer value
        * is returned, exactly as if the argument and the radix 10 were
        * given as arguments to the {@link
        * #parseUnsignedInt(java.lang.String, int)} method.
        *
        * @param s   a {@code String} containing the unsigned {@code int}
        *            representation to be parsed
        * @return    the unsigned integer value represented by the argument in decimal.
        * @throws    NumberFormatException  if the string does not contain a
        *            parsable unsigned integer.
        * @since 1.8
        */
       public static int parseUnsignedInt(String s) throws NumberFormatException {
           return parseUnsignedInt(s, 10);
       }

   若String为null，会抛出null 异常。要注意的是返回值为int，不是Integer对象。

5. valueOf方法
   

   /**
        * Returns an {@code Integer} object holding the value
        * extracted from the specified {@code String} when parsed
        * with the radix given by the second argument. The first argument
        * is interpreted as representing a signed integer in the radix
        * specified by the second argument, exactly as if the arguments
        * were given to the {@link #parseInt(java.lang.String, int)}
        * method. The result is an {@code Integer} object that
        * represents the integer value specified by the string.
        *
        * <p>In other words, this method returns an {@code Integer}
        * object equal to the value of:
        *
        * <blockquote>
        *  {@code new Integer(Integer.parseInt(s, radix))}
        * </blockquote>
        *
        * @param      s   the string to be parsed.
        * @param      radix the radix to be used in interpreting {@code s}
        * @return     an {@code Integer} object holding the value
        *             represented by the string argument in the specified
        *             radix.
        * @exception NumberFormatException if the {@code String}
        *            does not contain a parsable {@code int}.
        */
       public static Integer valueOf(String s, int radix) throws NumberFormatException {
           return Integer.valueOf(parseInt(s,radix));
       }
   
       /**
        * Returns an {@code Integer} object holding the
        * value of the specified {@code String}. The argument is
        * interpreted as representing a signed decimal integer, exactly
        * as if the argument were given to the {@link
        * #parseInt(java.lang.String)} method. The result is an
        * {@code Integer} object that represents the integer value
        * specified by the string.
        *
        * <p>In other words, this method returns an {@code Integer}
        * object equal to the value of:
        *
        * <blockquote>
        *  {@code new Integer(Integer.parseInt(s))}
        * </blockquote>
        *
        * @param      s   the string to be parsed.
        * @return     an {@code Integer} object holding the value
        *             represented by the string argument.
        * @exception  NumberFormatException  if the string cannot be parsed
        *             as an integer.
        */
       public static Integer valueOf(String s) throws NumberFormatException {
           return Integer.valueOf(parseInt(s, 10));
       }

   valueOf默认是10进制有符号转换，原理也是parseInt，但是与parseInt不同的是 返回值为Integer

6. Integer 的重要定义,IntegerCache 
   

   private static class IntegerCache {
           static final int low = -128;
           static final int high;
           static final Integer cache[];
   
           static {
               // high value may be configured by property
               int h = 127;
               String integerCacheHighPropValue =
                   sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
               if (integerCacheHighPropValue != null) {
                   try {
                       int i = parseInt(integerCacheHighPropValue);
                       i = Math.max(i, 127);
                       // Maximum array size is Integer.MAX_VALUE
                       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;
   
               cache = new Integer[(high - low) + 1];
               int j = low;
               for(int k = 0; k < cache.length; k++)
                   cache[k] = new Integer(j++);
   
               // range [-128, 127] must be interned (JLS7 5.1.7)
               assert IntegerCache.high >= 127;
           }
   
           private IntegerCache() {}
       }
   
       /**
        * Returns an {@code Integer} instance representing the specified
        * {@code int} value.  If a new {@code Integer} instance is not
        * required, this method should generally be used in preference to
        * the constructor {@link #Integer(int)}, as this method is likely
        * to yield significantly better space and time performance by
        * caching frequently requested values.
        *
        * This method will always cache values in the range -128 to 127,
        * inclusive, and may cache other values outside of this range.
        *
        * @param  i an {@code int} value.
        * @return an {@code Integer} instance representing {@code i}.
        * @since  1.5
        */
       public static Integer valueOf(int i) {
           if (i >= IntegerCache.low && i <= IntegerCache.high)
               return IntegerCache.cache[i + (-IntegerCache.low)];
           return new Integer(i);
       }
   
       /**
        * The value of the {@code Integer}.
        *
        * @serial
        */
       private final int value;
   
       /**
        * Constructs a newly allocated {@code Integer} object that
        * represents the specified {@code int} value.
        *
        * @param   value   the value to be represented by the
        *                  {@code Integer} object.
        */
       public Integer(int value) {
           this.value = value;
       }
   
       /**
        * Constructs a newly allocated {@code Integer} object that
        * represents the {@code int} value indicated by the
        * {@code String} parameter. The string is converted to an
        * {@code int} value in exactly the manner used by the
        * {@code parseInt} method for radix 10.
        *
        * @param      s   the {@code String} to be converted to an
        *                 {@code Integer}.
        * @exception  NumberFormatException  if the {@code String} does not
        *               contain a parsable integer.
        * @see        java.lang.Integer#parseInt(java.lang.String, int)
        */
       public Integer(String s) throws NumberFormatException {
           this.value = parseInt(s, 10);
       }

   IntegerCache 为Integer的缓冲池，当创建Integer对象时，若大小在-128~127之间，则先在缓冲池返回，若空则新建相应对象返回并进入缓冲池。
   从源码可以看出，最大值127是可以通过jvm启动设置进行修改。
   Integer i = 100; 直接赋值语句是自动装箱，原理就是 Integer.valueOf(int i);

7. Integer实现
   

   /**
        * The value of the {@code Integer}.
        *
        * @serial
        */
       private final int value;
   
       /**
        * Constructs a newly allocated {@code Integer} object that
        * represents the specified {@code int} value.
        *
        * @param   value   the value to be represented by the
        *                  {@code Integer} object.
        */
       public Integer(int value) {
           this.value = value;
       }
   
       /**
        * Constructs a newly allocated {@code Integer} object that
        * represents the {@code int} value indicated by the
        * {@code String} parameter. The string is converted to an
        * {@code int} value in exactly the manner used by the
        * {@code parseInt} method for radix 10.
        *
        * @param      s   the {@code String} to be converted to an
        *                 {@code Integer}.
        * @exception  NumberFormatException  if the {@code String} does not
        *               contain a parsable integer.
        * @see        java.lang.Integer#parseInt(java.lang.String, int)
        */
       public Integer(String s) throws NumberFormatException {
           this.value = parseInt(s, 10);
       }

   value为final int，Integer 对象一旦创建不可改变。

8. 转化int方法
   

   /**
        * Returns the value of this {@code Integer} as a {@code byte}
        * after a narrowing primitive conversion.
        * @jls 5.1.3 Narrowing Primitive Conversions
        */
       public byte byteValue() {
           return (byte)value;
       }
   
       /**
        * Returns the value of this {@code Integer} as a {@code short}
        * after a narrowing primitive conversion.
        * @jls 5.1.3 Narrowing Primitive Conversions
        */
       public short shortValue() {
           return (short)value;
       }
   
       /**
        * Returns the value of this {@code Integer} as an
        * {@code int}.
        */
       public int intValue() {
           return value;
       }
   
       /**
        * Returns the value of this {@code Integer} as a {@code long}
        * after a widening primitive conversion.
        * @jls 5.1.2 Widening Primitive Conversions
        * @see Integer#toUnsignedLong(int)
        */
       public long longValue() {
           return (long)value;
       }
   
       /**
        * Returns the value of this {@code Integer} as a {@code float}
        * after a widening primitive conversion.
        * @jls 5.1.2 Widening Primitive Conversions
        */
       public float floatValue() {
           return (float)value;
       }
   
       /**
        * Returns the value of this {@code Integer} as a {@code double}
        * after a widening primitive conversion.
        * @jls 5.1.2 Widening Primitive Conversions
        */
       public double doubleValue() {
           return (double)value;
       }

9. 重写equals , hashcode，toString
   

   /**
        * Returns a {@code String} object representing this
        * {@code Integer}'s value. The value is converted to signed
        * decimal representation and returned as a string, exactly as if
        * the integer value were given as an argument to the {@link
        * java.lang.Integer#toString(int)} method.
        *
        * @return  a string representation of the value of this object in
        *          base 10.
        */
       public String toString() {
           return toString(value);
       }
   
       /**
        * Returns a hash code for this {@code Integer}.
        *
        * @return  a hash code value for this object, equal to the
        *          primitive {@code int} value represented by this
        *          {@code Integer} object.
        */
       @Override
       public int hashCode() {
           return Integer.hashCode(value);
       }
   
       /**
        * Returns a hash code for a {@code int} value; compatible with
        * {@code Integer.hashCode()}.
        *
        * @param value the value to hash
        * @since 1.8
        *
        * @return a hash code value for a {@code int} value.
        */
       public static int hashCode(int value) {
           return value;
       }
   
       /**
        * Compares this object to the specified object.  The result is
        * {@code true} if and only if the argument is not
        * {@code null} and is an {@code Integer} object that
        * contains the same {@code int} value as this object.
        *
        * @param   obj   the object to compare with.
        * @return  {@code true} if the objects are the same;
        *          {@code false} otherwise.
        */
       public boolean equals(Object obj) {
           if (obj instanceof Integer) {
               return value == ((Integer)obj).intValue();
           }
           return false;
       }

10. 获取系统properties Integer
    

    /**
         * Determines the integer value of the system property with the
         * specified name.
         *
         * <p>The first argument is treated as the name of a system
         * property.  System properties are accessible through the {@link
         * java.lang.System#getProperty(java.lang.String)} method. The
         * string value of this property is then interpreted as an integer
         * value using the grammar supported by {@link Integer#decode decode} and
         * an {@code Integer} object representing this value is returned.
         *
         * <p>The second argument is the default value. An {@code Integer} object
         * that represents the value of the second argument is returned if there
         * is no property of the specified name, if the property does not have
         * the correct numeric format, or if the specified name is empty or
         * {@code null}.
         *
         * <p>In other words, this method returns an {@code Integer} object
         * equal to the value of:
         *
         * <blockquote>
         *  {@code getInteger(nm, new Integer(val))}
         * </blockquote>
         *
         * but in practice it may be implemented in a manner such as:
         *
         * <blockquote><pre>
         * Integer result = getInteger(nm, null);
         * return (result == null) ? new Integer(val) : result;
         * </pre></blockquote>
         *
         * to avoid the unnecessary allocation of an {@code Integer}
         * object when the default value is not needed.
         *
         * @param   nm   property name.
         * @param   val   default value.
         * @return  the {@code Integer} value of the property.
         * @throws  SecurityException for the same reasons as
         *          {@link System#getProperty(String) System.getProperty}
         * @see     java.lang.System#getProperty(java.lang.String)
         * @see     java.lang.System#getProperty(java.lang.String, java.lang.String)
         */
        public static Integer getInteger(String nm, int val) {
            Integer result = getInteger(nm, null);
            return (result == null) ? Integer.valueOf(val) : result;
        }
    
        /**
         * Returns the integer value of the system property with the
         * specified name.  The first argument is treated as the name of a
         * system property.  System properties are accessible through the
         * {@link java.lang.System#getProperty(java.lang.String)} method.
         * The string value of this property is then interpreted as an
         * integer value, as per the {@link Integer#decode decode} method,
         * and an {@code Integer} object representing this value is
         * returned; in summary:
         *
         * <ul><li>If the property value begins with the two ASCII characters
         *         {@code 0x} or the ASCII character {@code #}, not
         *      followed by a minus sign, then the rest of it is parsed as a
         *      hexadecimal integer exactly as by the method
         *      {@link #valueOf(java.lang.String, int)} with radix 16.
         * <li>If the property value begins with the ASCII character
         *     {@code 0} followed by another character, it is parsed as an
         *     octal integer exactly as by the method
         *     {@link #valueOf(java.lang.String, int)} with radix 8.
         * <li>Otherwise, the property value is parsed as a decimal integer
         * exactly as by the method {@link #valueOf(java.lang.String, int)}
         * with radix 10.
         * </ul>
         *
         * <p>The second argument is the default value. The default value is
         * returned if there is no property of the specified name, if the
         * property does not have the correct numeric format, or if the
         * specified name is empty or {@code null}.
         *
         * @param   nm   property name.
         * @param   val   default value.
         * @return  the {@code Integer} value of the property.
         * @throws  SecurityException for the same reasons as
         *          {@link System#getProperty(String) System.getProperty}
         * @see     System#getProperty(java.lang.String)
         * @see     System#getProperty(java.lang.String, java.lang.String)
         */
        public static Integer getInteger(String nm, Integer val) {
            String v = null;
            try {
                v = System.getProperty(nm);
            } catch (IllegalArgumentException | NullPointerException e) {
            }
            if (v != null) {
                try {
                    return Integer.decode(v);
                } catch (NumberFormatException e) {
                }
            }
            return val;
        }

11.  decode，解析String的数值（二进制|八进制|十进制|十六进制）
    

    /**
         * Decodes a {@code String} into an {@code Integer}.
         * Accepts decimal, hexadecimal, and octal numbers given
         * by the following grammar:
         *
         * <blockquote>
         * <dl>
         * <dt><i>DecodableString:</i>
         * <dd><i>Sign<sub>opt</sub> DecimalNumeral</i>
         * <dd><i>Sign<sub>opt</sub></i> {@code 0x} <i>HexDigits</i>
         * <dd><i>Sign<sub>opt</sub></i> {@code 0X} <i>HexDigits</i>
         * <dd><i>Sign<sub>opt</sub></i> {@code #} <i>HexDigits</i>
         * <dd><i>Sign<sub>opt</sub></i> {@code 0} <i>OctalDigits</i>
         *
         * <dt><i>Sign:</i>
         * <dd>{@code -}
         * <dd>{@code +}
         * </dl>
         * </blockquote>
         *
         * <i>DecimalNumeral</i>, <i>HexDigits</i>, and <i>OctalDigits</i>
         * are as defined in section 3.10.1 of
         * <cite>The Java&trade; Language Specification</cite>,
         * except that underscores are not accepted between digits.
         *
         * <p>The sequence of characters following an optional
         * sign and/or radix specifier ("{@code 0x}", "{@code 0X}",
         * "{@code #}", or leading zero) is parsed as by the {@code
         * Integer.parseInt} method with the indicated radix (10, 16, or
         * 8).  This sequence of characters must represent a positive
         * value or a {@link NumberFormatException} will be thrown.  The
         * result is negated if first character of the specified {@code
         * String} is the minus sign.  No whitespace characters are
         * permitted in the {@code String}.
         *
         * @param     nm the {@code String} to decode.
         * @return    an {@code Integer} object holding the {@code int}
         *             value represented by {@code nm}
         * @exception NumberFormatException  if the {@code String} does not
         *            contain a parsable integer.
         * @see java.lang.Integer#parseInt(java.lang.String, int)
         */
        public static Integer decode(String nm) throws NumberFormatException {
            int radix = 10;
            int index = 0;
            boolean negative = false;
            Integer result;
    
            if (nm.length() == 0)
                throw new NumberFormatException("Zero length string");
            char firstChar = nm.charAt(0);
            // Handle sign, if present
            if (firstChar == '-') {
                negative = true;
                index++;
            } else if (firstChar == '+')
                index++;
    
            // Handle radix specifier, if present
            if (nm.startsWith("0x", index) || nm.startsWith("0X", index)) {
                index += 2;
                radix = 16;
            }
            else if (nm.startsWith("#", index)) {
                index ++;
                radix = 16;
            }
            else if (nm.startsWith("0", index) && nm.length() > 1 + index) {
                index ++;
                radix = 8;
            }
    
            if (nm.startsWith("-", index) || nm.startsWith("+", index))
                throw new NumberFormatException("Sign character in wrong position");
    
            try {
                result = Integer.valueOf(nm.substring(index), radix);
                result = negative ? Integer.valueOf(-result.intValue()) : result;
            } catch (NumberFormatException e) {
                // If number is Integer.MIN_VALUE, we'll end up here. The next line
                // handles this case, and causes any genuine format error to be
                // rethrown.
                String constant = negative ? ("-" + nm.substring(index))
                                           : nm.substring(index);
                result = Integer.valueOf(constant, radix);
            }
            return result;
        }

12. compareTo 方法,比较一定用 value，或者int值来进行大小比较
    

    /**
         * Compares two {@code Integer} objects numerically.
         *
         * @param   anotherInteger   the {@code Integer} to be compared.
         * @return  the value {@code 0} if this {@code Integer} is
         *          equal to the argument {@code Integer}; a value less than
         *          {@code 0} if this {@code Integer} is numerically less
         *          than the argument {@code Integer}; and a value greater
         *          than {@code 0} if this {@code Integer} is numerically
         *           greater than the argument {@code Integer} (signed
         *           comparison).
         * @since   1.2
         */
        public int compareTo(Integer anotherInteger) {
            return compare(this.value, anotherInteger.value);
        }
    
        /**
         * Compares two {@code int} values numerically.
         * The value returned is identical to what would be returned by:
         * <pre>
         *    Integer.valueOf(x).compareTo(Integer.valueOf(y))
         * </pre>
         *
         * @param  x the first {@code int} to compare
         * @param  y the second {@code int} to compare
         * @return the value {@code 0} if {@code x == y};
         *         a value less than {@code 0} if {@code x < y}; and
         *         a value greater than {@code 0} if {@code x > y}
         * @since 1.7
         */
        public static int compare(int x, int y) {
            return (x < y) ? -1 : ((x == y) ? 0 : 1);
        }
    
        /**
         * Compares two {@code int} values numerically treating the values
         * as unsigned.
         *
         * @param  x the first {@code int} to compare
         * @param  y the second {@code int} to compare
         * @return the value {@code 0} if {@code x == y}; a value less
         *         than {@code 0} if {@code x < y} as unsigned values; and
         *         a value greater than {@code 0} if {@code x > y} as
         *         unsigned values
         * @since 1.8
         */
        public static int compareUnsigned(int x, int y) {
            return compare(x + MIN_VALUE, y + MIN_VALUE);
        }

13. Integer内置运算方法
    

        /**
         * Converts the argument to a {@code long} by an unsigned
         * conversion.  In an unsigned conversion to a {@code long}, the
         * high-order 32 bits of the {@code long} are zero and the
         * low-order 32 bits are equal to the bits of the integer
         * argument.
         *
         * Consequently, zero and positive {@code int} values are mapped
         * to a numerically equal {@code long} value and negative {@code
         * int} values are mapped to a {@code long} value equal to the
         * input plus 2<sup>32</sup>.
         *
         * @param  x the value to convert to an unsigned {@code long}
         * @return the argument converted to {@code long} by an unsigned
         *         conversion
         * @since 1.8
         */
        public static long toUnsignedLong(int x) {
            return ((long) x) & 0xffffffffL;
        }
    
        /**
         * Returns the unsigned quotient of dividing the first argument by
         * the second where each argument and the result is interpreted as
         * an unsigned value.
         *
         * <p>Note that in two's complement arithmetic, the three other
         * basic arithmetic operations of add, subtract, and multiply are
         * bit-wise identical if the two operands are regarded as both
         * being signed or both being unsigned.  Therefore separate {@code
         * addUnsigned}, etc. methods are not provided.
         *
         * @param dividend the value to be divided
         * @param divisor the value doing the dividing
         * @return the unsigned quotient of the first argument divided by
         * the second argument
         * @see #remainderUnsigned
         * @since 1.8
         */
        public static int divideUnsigned(int dividend, int divisor) {
            // In lieu of tricky code, for now just use long arithmetic.
            return (int)(toUnsignedLong(dividend) / toUnsignedLong(divisor));
        }
    
        /**
         * Returns the unsigned remainder from dividing the first argument
         * by the second where each argument and the result is interpreted
         * as an unsigned value.
         *
         * @param dividend the value to be divided
         * @param divisor the value doing the dividing
         * @return the unsigned remainder of the first argument divided by
         * the second argument
         * @see #divideUnsigned
         * @since 1.8
         */
        public static int remainderUnsigned(int dividend, int divisor) {
            // In lieu of tricky code, for now just use long arithmetic.
            return (int)(toUnsignedLong(dividend) % toUnsignedLong(divisor));
        }
    
        /**
          * Adds two integers together as per the + operator.
         *
         * @param a the first operand
         * @param b the second operand
         * @return the sum of {@code a} and {@code b}
         * @see java.util.function.BinaryOperator
         * @since 1.8
         */
        public static int sum(int a, int b) {
            return a + b;
        }
    
        /**
         * Returns the greater of two {@code int} values
         * as if by calling {@link Math#max(int, int) Math.max}.
         *
         * @param a the first operand
         * @param b the second operand
         * @return the greater of {@code a} and {@code b}
         * @see java.util.function.BinaryOperator
         * @since 1.8
         */
        public static int max(int a, int b) {
            return Math.max(a, b);
        }
    
        /**
         * Returns the smaller of two {@code int} values
         * as if by calling {@link Math#min(int, int) Math.min}.
         *
         * @param a the first operand
         * @param b the second operand
         * @return the smaller of {@code a} and {@code b}
         * @see java.util.function.BinaryOperator
         * @since 1.8
         */
        public static int min(int a, int b) {
            return Math.min(a, b);
        }

14. Integer  位相关方法
    

    /**
         * The number of bits used to represent an {@code int} value in two's
         * complement binary form.
         *
         * @since 1.5
         */
        @Native public static final int SIZE = 32;
    
        /**
         * The number of bytes used to represent a {@code int} value in two's
         * complement binary form.
         *
         * @since 1.8
         */
        public static final int BYTES = SIZE / Byte.SIZE;
    
        /**
         * Returns an {@code int} value with at most a single one-bit, in the
         * position of the highest-order ("leftmost") one-bit in the specified
         * {@code int} value.  Returns zero if the specified value has no
         * one-bits in its two's complement binary representation, that is, if it
         * is equal to zero.
         *
         * @param i the value whose highest one bit is to be computed
         * @return an {@code int} value with a single one-bit, in the position
         *     of the highest-order one-bit in the specified value, or zero if
         *     the specified value is itself equal to zero.
         * @since 1.5
         */
        public static int highestOneBit(int i) {
            // HD, Figure 3-1
            i |= (i >>  1);
            i |= (i >>  2);
            i |= (i >>  4);
            i |= (i >>  8);
            i |= (i >> 16);
            return i - (i >>> 1);
        }
    
        /**
         * Returns an {@code int} value with at most a single one-bit, in the
         * position of the lowest-order ("rightmost") one-bit in the specified
         * {@code int} value.  Returns zero if the specified value has no
         * one-bits in its two's complement binary representation, that is, if it
         * is equal to zero.
         *
         * @param i the value whose lowest one bit is to be computed
         * @return an {@code int} value with a single one-bit, in the position
         *     of the lowest-order one-bit in the specified value, or zero if
         *     the specified value is itself equal to zero.
         * @since 1.5
         */
        public static int lowestOneBit(int i) {
            // HD, Section 2-1
            return i & -i;
        }
    
        /**
         * Returns the number of zero bits preceding the highest-order
         * ("leftmost") one-bit in the two's complement binary representation
         * of the specified {@code int} value.  Returns 32 if the
         * specified value has no one-bits in its two's complement representation,
         * in other words if it is equal to zero.
         *
         * <p>Note that this method is closely related to the logarithm base 2.
         * For all positive {@code int} values x:
         * <ul>
         * <li>floor(log<sub>2</sub>(x)) = {@code 31 - numberOfLeadingZeros(x)}
         * <li>ceil(log<sub>2</sub>(x)) = {@code 32 - numberOfLeadingZeros(x - 1)}
         * </ul>
         *
         * @param i the value whose number of leading zeros is to be computed
         * @return the number of zero bits preceding the highest-order
         *     ("leftmost") one-bit in the two's complement binary representation
         *     of the specified {@code int} value, or 32 if the value
         *     is equal to zero.
         * @since 1.5
         */
        public static int numberOfLeadingZeros(int i) {
            // HD, Figure 5-6
            if (i == 0)
                return 32;
            int n = 1;
            if (i >>> 16 == 0) { n += 16; i <<= 16; }
            if (i >>> 24 == 0) { n +=  8; i <<=  8; }
            if (i >>> 28 == 0) { n +=  4; i <<=  4; }
            if (i >>> 30 == 0) { n +=  2; i <<=  2; }
            n -= i >>> 31;
            return n;
        }
    
        /**
         * Returns the number of zero bits following the lowest-order ("rightmost")
         * one-bit in the two's complement binary representation of the specified
         * {@code int} value.  Returns 32 if the specified value has no
         * one-bits in its two's complement representation, in other words if it is
         * equal to zero.
         *
         * @param i the value whose number of trailing zeros is to be computed
         * @return the number of zero bits following the lowest-order ("rightmost")
         *     one-bit in the two's complement binary representation of the
         *     specified {@code int} value, or 32 if the value is equal
         *     to zero.
         * @since 1.5
         */
        public static int numberOfTrailingZeros(int i) {
            // HD, Figure 5-14
            int y;
            if (i == 0) return 32;
            int n = 31;
            y = i <<16; if (y != 0) { n = n -16; i = y; }
            y = i << 8; if (y != 0) { n = n - 8; i = y; }
            y = i << 4; if (y != 0) { n = n - 4; i = y; }
            y = i << 2; if (y != 0) { n = n - 2; i = y; }
            return n - ((i << 1) >>> 31);
        }
    
        /**
         * Returns the number of one-bits in the two's complement binary
         * representation of the specified {@code int} value.  This function is
         * sometimes referred to as the <i>population count</i>.
         *
         * @param i the value whose bits are to be counted
         * @return the number of one-bits in the two's complement binary
         *     representation of the specified {@code int} value.
         * @since 1.5
         */
        public static int bitCount(int i) {
            // HD, Figure 5-2
            i = i - ((i >>> 1) & 0x55555555);
            i = (i & 0x33333333) + ((i >>> 2) & 0x33333333);
            i = (i + (i >>> 4)) & 0x0f0f0f0f;
            i = i + (i >>> 8);
            i = i + (i >>> 16);
            return i & 0x3f;
        }
    
        /**
         * Returns the value obtained by rotating the two's complement binary
         * representation of the specified {@code int} value left by the
         * specified number of bits.  (Bits shifted out of the left hand, or
         * high-order, side reenter on the right, or low-order.)
         *
         * <p>Note that left rotation with a negative distance is equivalent to
         * right rotation: {@code rotateLeft(val, -distance) == rotateRight(val,
         * distance)}.  Note also that rotation by any multiple of 32 is a
         * no-op, so all but the last five bits of the rotation distance can be
         * ignored, even if the distance is negative: {@code rotateLeft(val,
         * distance) == rotateLeft(val, distance & 0x1F)}.
         *
         * @param i the value whose bits are to be rotated left
         * @param distance the number of bit positions to rotate left
         * @return the value obtained by rotating the two's complement binary
         *     representation of the specified {@code int} value left by the
         *     specified number of bits.
         * @since 1.5
         */
        public static int rotateLeft(int i, int distance) {
            return (i << distance) | (i >>> -distance);
        }
    
        /**
         * Returns the value obtained by rotating the two's complement binary
         * representation of the specified {@code int} value right by the
         * specified number of bits.  (Bits shifted out of the right hand, or
         * low-order, side reenter on the left, or high-order.)
         *
         * <p>Note that right rotation with a negative distance is equivalent to
         * left rotation: {@code rotateRight(val, -distance) == rotateLeft(val,
         * distance)}.  Note also that rotation by any multiple of 32 is a
         * no-op, so all but the last five bits of the rotation distance can be
         * ignored, even if the distance is negative: {@code rotateRight(val,
         * distance) == rotateRight(val, distance & 0x1F)}.
         *
         * @param i the value whose bits are to be rotated right
         * @param distance the number of bit positions to rotate right
         * @return the value obtained by rotating the two's complement binary
         *     representation of the specified {@code int} value right by the
         *     specified number of bits.
         * @since 1.5
         */
        public static int rotateRight(int i, int distance) {
            return (i >>> distance) | (i << -distance);
        }
    
        /**
         * Returns the value obtained by reversing the order of the bits in the
         * two's complement binary representation of the specified {@code int}
         * value.
         *
         * @param i the value to be reversed
         * @return the value obtained by reversing order of the bits in the
         *     specified {@code int} value.
         * @since 1.5
         */
        public static int reverse(int i) {
            // HD, Figure 7-1
            i = (i & 0x55555555) << 1 | (i >>> 1) & 0x55555555;
            i = (i & 0x33333333) << 2 | (i >>> 2) & 0x33333333;
            i = (i & 0x0f0f0f0f) << 4 | (i >>> 4) & 0x0f0f0f0f;
            i = (i << 24) | ((i & 0xff00) << 8) |
                ((i >>> 8) & 0xff00) | (i >>> 24);
            return i;
        }
    
        /**
         * Returns the signum function of the specified {@code int} value.  (The
         * return value is -1 if the specified value is negative; 0 if the
         * specified value is zero; and 1 if the specified value is positive.)
         *
         * @param i the value whose signum is to be computed
         * @return the signum function of the specified {@code int} value.
         * @since 1.5
         */
        public static int signum(int i) {
            // HD, Section 2-7
            return (i >> 31) | (-i >>> 31);
        }
    
        /**
         * Returns the value obtained by reversing the order of the bytes in the
         * two's complement representation of the specified {@code int} value.
         *
         * @param i the value whose bytes are to be reversed
         * @return the value obtained by reversing the bytes in the specified
         *     {@code int} value.
         * @since 1.5
         */
        public static int reverseBytes(int i) {
            return ((i >>> 24)           ) |
                   ((i >>   8) &   0xFF00) |
                   ((i <<   8) & 0xFF0000) |
                   ((i << 24));
        }

15. 序列化
    

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
        @Native private static final long serialVersionUID = 1360826667806852920L;