Integer的源码分析

Integer的源码

 

/*

 * Copyright (c) 1994, 2010, Oracle and/or its affiliates. All rights reserved.

 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

 */

 

package java.lang;

 

import java.util.Properties;

 

/**

 * The {@code Integer} class wraps a value of the primitive type

 * {@code int} in an object. An object of type {@code Integer}

 * contains a single field whose type is {@code int}.

 *

 * <p>In addition, this class provides several methods for converting

 * an {@code int} to a {@code String} and a {@code String} to an

 * {@code int}, as well as other constants and methods useful when

 * dealing with an {@code int}.

 *

 * <p>Implementation note: The implementations of the "bit twiddling"

 * methods (such as {@link #highestOneBit(int) highestOneBit} and

 * {@link #numberOfTrailingZeros(int) numberOfTrailingZeros}) are

 * based on material from Henry S. Warren, Jr.'s <i>Hacker's

 * Delight</i>, (Addison Wesley, 2002).

 *

 * @author  Lee Boynton

 * @author  Arthur van Hoff

 * @author  Josh Bloch

 * @author  Joseph D. Darcy

 * @since JDK1.0

 */

public final class Integer extends Number implements Comparable<Integer> {

    /**

     * A constant holding the minimum value an {@code int} can

     * have, -2³¹.

     */

    public static final int   MIN_VALUE = 0x80000000;

 

    /**

     * A constant holding the maximum value an {@code int} can

     * have, 2³¹-1.

     */

    public static final int   MAX_VALUE = 0x7fffffff;

 

    /**

     * The {@code Class} instance representing the primitive type

     * {@code int}.

     *

     * @since   JDK1.1

     */

    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.

     *

     * <p>If the radix is smaller than {@code Character.MIN_RADIX}

     * or larger than {@code Character.MAX_RADIX}, then the radix

     * {@code 10} is used instead.

     *

     * <p>If the first argument is negative, the first element of the

     * result is the ASCII minus character {@code '-'}

     * (<code>'&#92;u002D'</code>). If the first argument is not

     * negative, no sign character appears in the result.

     *

     * <p>The remaining characters of the result represent the magnitude

     * of the first argument. If the magnitude is zero, it is

     * represented by a single zero character {@code '0'}

     * (<code>'&#92;u0030'</code>); otherwise, the first character of

     * the representation of the magnitude will not be the zero

     * character.  The following ASCII characters are used as digits:

     *

     * <blockquote>

     *   {@code 0123456789abcdefghijklmnopqrstuvwxyz}

     * </blockquote>

     *

     * These are <code>'&#92;u0030'</code> through

     * <code>'&#92;u0039'</code> and <code>'&#92;u0061'</code> through

     * <code>'&#92;u007A'</code>. If {@code radix} is

     * <var>N</var>, then the first <var>N</var> of these characters

     * are used as radix-<var>N</var> digits in the order shown. Thus,

     * the digits for hexadecimal (radix 16) are

     * {@code 0123456789abcdef}. If uppercase letters are

     * desired, the {@link java.lang.String#toUpperCase()} method may

     * be called on the result:

     *

     * <blockquote>

     *  {@code Integer.toString(n, 16).toUpperCase()}

     * </blockquote>

     *

     * @param   i       an integer to be converted to a string.

     * @param   radix   the radix to use in the string representation.

     * @return  a string representation of the argument in the specified radix.

     * @see     java.lang.Character#MAX_RADIX

     * @see     java.lang.Character#MIN_RADIX

     */

    public static String toString(int i, int radix) {

 

        if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)

            radix = 10;

 

        /* Use the faster version */

        if (radix == 10) {

            return toString(i);

        }

 

        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;

        }

        buf[charPos] = digits[-i];

 

        if (negative) {

            buf[--charPos] = '-';

        }

 

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

    }

 

    /**

     * Returns a string representation of the integer argument as an

     * unsigned integer in base&nbsp;16.

     *

     * <p>The unsigned integer value is the argument plus 2³²

     * if the argument is negative; otherwise, it is equal to the

     * argument.  This value is converted to a string of ASCII digits

     * in hexadecimal (base&nbsp;16) with no extra leading

     * {@code 0}s. If the unsigned magnitude is zero, it is

     * represented by a single zero character {@code '0'}

     * (<code>'&#92;u0030'</code>); otherwise, the first character of

     * the representation of the unsigned magnitude will not be the

     * zero character. The following characters are used as

     * hexadecimal digits:

     *

     * <blockquote>

     *  {@code 0123456789abcdef}

     * </blockquote>

     *

     * These are the characters <code>'&#92;u0030'</code> through

     * <code>'&#92;u0039'</code> and <code>'&#92;u0061'</code> through

     * <code>'&#92;u0066'</code>. If uppercase letters are

     * desired, the {@link java.lang.String#toUpperCase()} method may

     * be called on the result:

     *

     * <blockquote>

     *  {@code Integer.toHexString(n).toUpperCase()}

     * </blockquote>

     *

     * @param   i   an integer to be converted to a string.

     * @return  the string representation of the unsigned integer value

     *          represented by the argument in hexadecimal (base&nbsp;16).

     * @since   JDK1.0.2

     */

    public static String toHexString(int i) {

        return toUnsignedString(i, 4);

    }

 

    /**

     * Returns a string representation of the integer argument as an

     * unsigned integer in base&nbsp;8.

     *

     * <p>The unsigned integer value is the argument plus 2³²

     * if the argument is negative; otherwise, it is equal to the

     * argument.  This value is converted to a string of ASCII digits

     * in octal (base&nbsp;8) with no extra leading {@code 0}s.

     *

     * <p>If the unsigned magnitude is zero, it is represented by a

     * single zero character {@code '0'}

     * (<code>'&#92;u0030'</code>); otherwise, the first character of

     * the representation of the unsigned magnitude will not be the

     * zero character. The following characters are used as octal

     * digits:

     *

     * <blockquote>

     * {@code 01234567}

     * </blockquote>

     *

     * These are the characters <code>'&#92;u0030'</code> through

     * <code>'&#92;u0037'</code>.

     *

     * @param   i   an integer to be converted to a string.

     * @return  the string representation of the unsigned integer value

     *          represented by the argument in octal (base&nbsp;8).

     * @since   JDK1.0.2

     */

    public static String toOctalString(int i) {

        return toUnsignedString(i, 3);

    }

 

    /**

     * Returns a string representation of the integer argument as an

     * unsigned integer in base&nbsp;2.

     *

     * <p>The unsigned integer value is the argument plus 2³²

     * if the argument is negative; otherwise it is equal to the

     * argument.  This value is converted to a string of ASCII digits

     * in binary (base&nbsp;2) with no extra leading {@code 0}s.

     * If the unsigned magnitude is zero, it is represented by a

     * single zero character {@code '0'}

     * (<code>'&#92;u0030'</code>); otherwise, the first character of

     * the representation of the unsigned magnitude will not be the

     * zero character. The characters {@code '0'}

     * (<code>'&#92;u0030'</code>) and {@code '1'}

     * (<code>'&#92;u0031'</code>) are used as binary digits.

     *

     * @param   i   an integer to be converted to a string.

     * @return  the string representation of the unsigned integer value

     *          represented by the argument in binary (base&nbsp;2).

     * @since   JDK1.0.2

     */

    public static String toBinaryString(int i) {

        return toUnsignedString(i, 1);

    }

 

    /**

     * Convert the integer to an unsigned number.

     */

    private static String toUnsignedString(int i, int shift) {

        char[] buf = new char[32];

        int charPos = 32;

        int radix = 1 << shift;

        int mask = radix - 1;

        do {

            buf[--charPos] = digits[i & mask];

            i >>>= shift;

        } while (i != 0);

 

        return new String(buf, charPos, (32 - 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',

        } ;

 

        // I use the "invariant division by multiplication" trick to

        // accelerate Integer.toString.  In particular we want to

        // avoid division by 10.

        //

        // The "trick" has roughly the same performance characteristics

        // as the "classic" Integer.toString code on a non-JIT VM.

        // The trick avoids .rem and .div calls but has a longer code

        // path and is thus dominated by dispatch overhead.  In the

        // JIT case the dispatch overhead doesn't exist and the

        // "trick" is considerably faster than the classic code.

        //

        // TODO-FIXME: convert (x * 52429) into the equiv shift-add

        // sequence.

        //

        // RE:  Division by Invariant Integers using Multiplication

        //      T Gralund, P Montgomery

        //      ACM PLDI 1994

        //

 

    /**

     * 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);

    }

 

    /**

     * 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;

    }

 

    /**

     * 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>'&#92;u002D'</code>) to

     * indicate a negative value or an ASCII plus sign {@code '+'}

     * (<code>'&#92;u002B'</code>) 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>'&#92;u002D'</code>) or plus sign

     * {@code '+'} (<code>'&#92;u002B'</code>) 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>'&#92;u002D'</code>) to indicate a negative value or an

     * ASCII plus sign {@code '+'} (<code>'&#92;u002B'</code>) 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);

    }

 

    /**

     * 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));

    }

 

    /**

     * 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 -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[];

 

        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) {

                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);

            }

            high = h;

 

            cache = new Integer[(high - low) + 1];

            int j = low;

            for(int k = 0; k < cache.length; k++)

                cache[k] = new Integer(j++);

        }

 

        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) {

        assert IntegerCache.high >= 127;

        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);

    }

 

    /**

     * Returns the value of this {@code Integer} as a

     * {@code byte}.

     */

    public byte byteValue() {

        return (byte)value;

    }

 

    /**

     * Returns the value of this {@code Integer} as a

     * {@code short}.

     */

    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}.

     */

    public long longValue() {

        return (long)value;

    }

 

    /**

     * Returns the value of this {@code Integer} as a

     * {@code float}.

     */

    public float floatValue() {

        return (float)value;

    }

 

    /**

     * Returns the value of this {@code Integer} as a

     * {@code double}.

     */

    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() {

        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.

     */

    public int hashCode() {

        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 and an {@code Integer} object representing this value is

     * returned. Details of possible numeric formats can be found with

     * the definition of {@code getProperty}.

     *

     * <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.

     * @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 and an {@code Integer} object representing this value is

     * returned. Details of possible numeric formats can be found with

     * the definition of {@code getProperty}.

     *

     * <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.

     * @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 {@code Integer.decode} method,

     * and an {@code Integer} object representing this value is

     * returned.

     *

     * <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.

     * @see     java.lang.System#getProperty(java.lang.String)

     * @see java.lang.System#getProperty(java.lang.String, java.lang.String)

     * @see java.lang.Integer#decode

     */

    public static Integer getInteger(String nm, Integer val) {

        String v = null;

        try {

            v = System.getProperty(nm);

        } catch (IllegalArgumentException e) {

        } catch (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_opt DecimalNumeral</i>

     * <dd><i>Sign_opt</i> {@code 0x} <i>HexDigits</i>

     * <dd><i>Sign_opt</i> {@code 0X} <i>HexDigits</i>

     * <dd><i>Sign_opt</i> {@code #} <i>HexDigits</i>

     * <dd><i>Sign_opt</i> {@code 0} <i>OctalDigits</i>

     * <p>

     * <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;

    }

 

    /**

     * 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);

    }

 

 

    // Bit twiddling

 

    /**

     * The number of bits used to represent an {@code int} value in two's

     * complement binary form.

     *

     * @since 1.5

     */

    public static final int SIZE = 32;

 

    /**

     * 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.

     *

     * @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); //向右移动一位，然后与自己或运算，相当于使最高位的1的右边一位也置成1

        i |= (i >>  2); //把最高位1的右边三位，置成1

        i |= (i >>  4); //把最高位1的右边七位，置成1

        i |= (i >>  8); //把最高位1的右边十五位，置成1

        i |= (i >> 16); //把最高位1的右边三十一位，置成1，因为整数32位，已经足够了

        return i - (i >>> 1);   //无符号右移动一位，然后把最高位腾为0，想剪得到最高位

    }

 

    /**

     * 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.

     *

     * @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(log₂(x)) = {@code 31 - numberOfLeadingZeros(x)}

     * <li>ceil(log₂(x)) = {@code 32 - numberOfLeadingZeros(x - 1)}

     * </ul>

     *

     * @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.

     *

     * @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>.

     *

     * @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)}.

     *

     * @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)}.

     *

     * @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.

     *

     * @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.)

     *

     * @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.

     *

     * @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));

    }

 

    /** use serialVersionUID from JDK 1.0.2 for interoperability */

    private static final long serialVersionUID = 1360826667806852920L;

}