# Java Integer源码解读

### 1、引言

```public class IntegerDemo {
public static void main(String[] args){
Integer i=10;
Integer j=10;
System.out.println(i==j);
System.out.println(i.equals(j));
Integer x=128;
Integer y=128;
System.out.println(x==y);
System.out.println(x.equals(y));
}
}```

### 2、源码解读

```package java.lang;

import java.lang.annotation.Native;

/**
*  从类定义中我们可以知道以下几点：
*  1.Integer类是final型的，不能被继承
*  2.Integer类实现了Comparable接口，可以用compareTo进行比较并且Integer对象只能和Integer类型的对象进行比较
*  3.Integer类继承了Number，所以该类可以调用longValue、floatValue、doubleValue等系列方法返回对应的类型的值
*/
public final class Integer extends Number implements Comparable<Integer> {
/**
* A constant holding the minimum value an {@code int} can
* have, -2<sup>31</sup>.  最小值 －（2的31次方）
*/
@Native
public static final int   MIN_VALUE = 0x80000000;

/**
* A constant holding the maximum value an {@code int} can
* have, 2<sup>31</sup>-1.   最大值（2的31次方）－1
*/
@Native
public static final int   MAX_VALUE = 0x7fffffff;

/**
* The {@code Class} instance representing the primitive type
* {@code int}.
* 表示基本类型 int 的 Class 实例
* @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'
};

/**
* Returns a string representation of the first argument in the
* radix specified by the second argument.
*
*/
public static String toString(int i, int radix) {

/* Use the faster version */
}

char buf[] = new char[33];
boolean negative = (i < 0);
int charPos = 32;

if (!negative) {
i = -i;
}

}
buf[charPos] = digits[-i];

if (negative) {
buf[--charPos] = '-';
}

return new String(buf, charPos, (33 - charPos));
}

/**
* Returns a string representation of the first argument as an
* unsigned integer value in the radix specified by the second
* argument.
*/
public static String toUnsignedString(int i, int radix) {
}

/**
* Returns a string representation of the integer argument as an
* unsigned integer in base&nbsp;16.
*
*/
public static String toHexString(int i) {
}

/**
* Returns a string representation of the integer argument as an
* unsigned integer in base&nbsp;8.
*
*/
public static String toOctalString(int i) {
}

/**
* Returns a string representation of the integer argument as an
* unsigned integer in base&nbsp;2.
*
*/
public static String toBinaryString(int i) {
}

/**
* Convert the integer to an unsigned number.
*/
private static String toUnsignedString0(int val, int shift) {
// assert shift > 0 && shift <=5 : "Illegal shift value";
int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val);
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.
*/
static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) {
int charPos = len;
int radix = 1 << shift;
do {
buf[offset + --charPos] = Integer.digits[val & mask];
val >>>= shift;
} while (val != 0 && charPos > 0);

return charPos;
}

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',
} ;

/**
* 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.
*/
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);
}

/**
* Returns a string representation of the argument as an unsigned
* decimal value.
*/
public static String toUnsignedString(int i) {
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;
}

// Generate two digits per iteration
while (i >= 65536) {
q = i / 100;
// really: r = i - (q * 100);
r = i - ((q << 6) + (q << 5) + (q << 2));
i = q;
buf [--charPos] = DigitOnes[r];
buf [--charPos] = DigitTens[r];
}

// 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;
}
}

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;
}

/**
* 所有将String转成Integer的方法都是基于parseInt方法实现的
* 第二个参数指定的基数(如果没指定，则按照十进制处理）
*
*
* <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{

if (s == null) {
throw new NumberFormatException("null");
}

}

}

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++;
}
while (i < len) {
// Accumulating negatively avoids surprises near MAX_VALUE
if (digit < 0) {
throw NumberFormatException.forInputString(s);
}
if (result < multmin) {
throw NumberFormatException.forInputString(s);
}
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
* 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
*
* <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
* @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
} else {
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);
}

/**
* 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:
*
*/
public static Integer valueOf(String s, int radix) throws NumberFormatException {
}

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

/**
* Cache to support the object identity semantics of autoboxing for values between
* -128 and 127 (inclusive) as required by JLS.
*
* The cache is initialized on first usage.  The size of the cache
* may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
* During VM initialization, java.lang.Integer.IntegerCache.high property
* may be set and saved in the private system properties in the
* sun.misc.VM class.
*/

private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];

/**
*静态代码块，Integer类被加载的时候就执行
*当Integer被加载时，就新建了-128到127的所有数字并存放在Integer数组cache中
*/
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.
*  当调用valueOf方法时，如果参数的值在-127到128之间，则直接从缓存中返回一个已经存在的对象。
*  如果参数的值不在这个范围内，则new一个Integer对象返回。
*  当把一个int变量转成Integer的时候（或者新建一个Integer的时候），建议使用valueOf方法来代替构造函数。
*  或者直接使用Integer i = 100;编译器会转成Integer s = Integer.valueOf(100)
*/
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}.
* value属性就是Integer对象中真正保存int值的
*/
private final int value;

/**
* Constructs a newly allocated {@code Integer} object that
* represents the specified {@code int} value.
* 当我们使用new Integer(10)创建一个Integer对象的时候，就会用以下形式给value赋值
*/
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.
* 从构造方法中我们可以知道，初始化一个Integer对象的时候只能创建一个十进制的整数
*/
public Integer(String s) throws NumberFormatException {
this.value = parseInt(s, 10);
}

/**
* 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;
}

/**
* 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&nbsp;10.
*/
public String toString() {
}

/**
* 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;
}

/**
* 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>If there is no property with the specified name, if the
* specified name is empty or {@code null}, or if the property
* does not have the correct numeric format, then {@code null} is
* returned.
*
* <p>In other words, this method returns an {@code Integer}
* object equal to the value of:
*
* <blockquote>
*  {@code getInteger(nm, null)}
* </blockquote>
*
* @param   nm   property name.
* @return  the {@code Integer} value of the property.
* @throws  SecurityException for the same reasons as
* @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) {
return getInteger(nm, null);
}

/**
* 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
* @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
* 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
* <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
* <li>Otherwise, the property value is parsed as a decimal integer
* exactly as by the method {@link #valueOf(java.lang.String, int)}
* </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
* @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;
}

/**
* 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
* 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 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;
}
else if (nm.startsWith("#", index)) {
index ++;
}
else if (nm.startsWith("0", index) && nm.length() > 1 + index) {
index ++;
}

if (nm.startsWith("-", index) || nm.startsWith("+", index))
throw new NumberFormatException("Sign character in wrong position");

try {
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);
}
return result;
}

/**
* 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);
}

/**
* 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));
}

// Bit twiddling

/**
* 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));
}

/**
* 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);
}

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

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