java的toString()及包装类的实现--Integer重点学习

1. toString()来源

2. toString()目的

3. toString()实现（JDK8）

1. toString()来源

　　源于java.lang.Object类，源码如下：

    /**
     * Returns a string representation of the object. In general, the
     * {@code toString} method returns a string that
     * "textually represents" this object. The result should
     * be a concise but informative representation that is easy for a
     * person to read.
     * It is recommended that all subclasses override this method.
     * <p>
     * The {@code toString} method for class {@code Object}
     * returns a string consisting of the name of the class of which the
     * object is an instance, the at-sign character `{@code @}', and
     * the unsigned hexadecimal representation of the hash code of the
     * object. In other words, this method returns a string equal to the
     * value of:
     * <blockquote>
     * <pre>
     * getClass().getName() + '@' + Integer.toHexString(hashCode())
     * </pre></blockquote>
     *
     * @return  a string representation of the object.
     */
    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

2. toString()目的

　　返回对象的字符串描述，是该对象的文本表示。该表示必须描述简洁且包含了该对象的信息，便于阅读。

　　推荐所有的子类重写该方法。

3. toString()实现（JDK8）

（1）Object类中的实现

　　在Object类中，该方法返回的是“该实例对象的类名@该对象的hashcode的十六进制表示”。对于我们而言，能阅读到的仅仅是该实例的类名。所以推荐对Object的子类重写该方法。

（2）包装类中的实现

　① Boolean

    /**
     * Returns a {@code String} object representing this Boolean's
     * value.  If this object represents the value {@code true},
     * a string equal to {@code "true"} is returned. Otherwise, a
     * string equal to {@code "false"} is returned.
     *
     * @return  a string representation of this object.
     */
    public String toString() {
        return value ? "true" : "false";
}

② Byte（借助于Integer）

    /**
     * Returns a new {@code String} object representing the
     * specified {@code byte}. The radix is assumed to be 10.
     *
     * @param b the {@code byte} to be converted
     * @return the string representation of the specified {@code byte}
     * @see java.lang.Integer#toString(int)
     */
    public static String toString(byte b) {
        return Integer.toString((int)b, 10);
}
    /**
     * Returns a {@code String} object representing this
     * {@code Byte}'s value.  The value is converted to signed
     * decimal representation and returned as a string, exactly as if
     * the {@code byte} value were given as an argument to the
     * {@link java.lang.Byte#toString(byte)} method.
     *
     * @return  a string representation of the value of this object in
     *          base 10.
     */
    public String toString() {
        return Integer.toString((int)value);
    }

③ Short（借助于Integer）

    /**
     * Returns a new {@code String} object representing the
     * specified {@code short}. The radix is assumed to be 10.
     *
     * @param s the {@code short} to be converted
     * @return the string representation of the specified {@code short}
     * @see java.lang.Integer#toString(int)
     */
    public static String toString(short s) {
        return Integer.toString((int)s, 10);
}
    /**
     * Returns a {@code String} object representing this
     * {@code Short}'s value.  The value is converted to signed
     * decimal representation and returned as a string, exactly as if
     * the {@code short} value were given as an argument to the
     * {@link java.lang.Short#toString(short)} method.
     *
     * @return  a string representation of the value of this object in
     *          base 10.
     */
    public String toString() {
        return Integer.toString((int)value);
    }

④ Character

    /**
     * Returns a {@code String} object representing this
     * {@code Character}'s value.  The result is a string of
     * length 1 whose sole component is the primitive
     * {@code char} value represented by this
     * {@code Character} object.
     *
     * @return  a string representation of this object.
     */
    public String toString() {
        char buf[] = {value}; // 转换为数组，可能和code point有关
        return String.valueOf(buf);
}
https://blog.csdn.net/qlql489/article/details/82780716

⑤ Integer

    /**
     * All possible chars for representing a number as a String
      所有可表示的进制下的所有字符
     */
    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.
     * 返回第一个参数的字符串表示，进制为第二个参数

     * <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.
     * 如果进制参数不再Character的可表示的进制范围内，设为十进制

     * <p>If the first argument is negative, the first element of the
     * result is the ASCII minus character {@code '-'}
     * ({@code '\u005Cu002D'}). If the first argument is not
     * negative, no sign character appears in the result.
     * 如果第一个参数是负数，则返回的字符串第一个字符是‘-’，否则为无符号

     * <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 '\u005Cu0030'}); otherwise, the first character of
     * the representation of the magnitude will not be the zero
     * character.  The following ASCII characters are used as digits:
     * 结果的剩余字符串表示了第一个参数的大小，如果参数为0，则返回‘0’，否则字符串的第一个字符不为0。使用ASCII字符：0-9a-z

     * <blockquote>
     *   {@code 0123456789abcdefghijklmnopqrstuvwxyz}
     * </blockquote>
     *
     * These are {@code '\u005Cu0030'} through
     * {@code '\u005Cu0039'} and {@code '\u005Cu0061'} through
     * {@code '\u005Cu007A'}. 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) {
    // 进制范围
        if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)
            radix = 10;

        /* Use the faster version */
        if (radix == 10) {
            return toString(i);
        }

        // 除十进制外的其他进制
        // 返回的字符串，最长为33个字符（32个二进制+1个符号）
        char buf[] = new char[33];
        // 记录正负
        boolean negative = (i < 0);
        int charPos = 32;
        // 保证i<=0
        if (!negative) {
            i = -i;
        }
        // 从后往前转字符 123->3,2,1
        while (i <= -radix) {
            buf[charPos--] = digits[-(i % radix)];
            i = i / radix;
        }
        // 最高位
        buf[charPos] = digits[-i];
        // 符号位
        if (negative) {
            buf[--charPos] = '-';
        }
        // 返回不为0的字符部分
        return new String(buf, charPos, (33 - 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.
     */
    // 十进制
public static String toString(int i) {
    // 无法得到最小值的绝对值（超范围）
        if (i == Integer.MIN_VALUE)
            return "-2147483648";
        // 需要的字符个数
        int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i);
        char[] buf = new char[size];
        getChars(i, size, buf);
        return new String(buf, true);
}
// Requires positive x
// 该十进制表示需要的位数（不包括符号）
    static int stringSize(int x) {
        for (int i=0; ; i++)
            if (x <= sizeTable[i])
                return i+1;
    }
   final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999,
9999999,99999999, 999999999, Integer.MAX_VALUE };
    /**
     * 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  最小值的绝对值超范围
     */
    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
        // 一次迭代产生两位字符，从低到高
        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);
        // 以65536为分界线，下面使用移位和乘法的形式提高速度，
        // 所以（i*52429）不能超过Integer.MAX_VALUE，2^15 < 52429 < 2^16,所以i<2^16，所以这里i的值不能超过65536
        for (;;) {
            // 2^19 = 524288,52429>>>19=0.1xxx
            // 移位最快，乘 > 除
            q = (i * 52429) >>> (16+3);  // q = i/10;
            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 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',
        } ;
    // 10x10
    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 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);
    }
两个toString,一个是在多进制下转字符串，有多少位就要循环多少次。
toString(value)的方法默认为十进制，毕竟使用的多，而上面的方法太慢了，toString(value)明显减少了迭代次数。

⑥ Long（同Integer）

   /**
     * 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 sign {@code '-'}
     * ({@code '\u005Cu002d'}). 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 '\u005Cu0030'}); 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 '\u005Cu0030'} through
     * {@code '\u005Cu0039'} and {@code '\u005Cu0061'} through
     * {@code '\u005Cu007a'}. 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 Long.toString(n, 16).toUpperCase()}
     * </blockquote>
     *
     * @param   i       a {@code long} 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(long i, int radix) {
        if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)
            radix = 10;
        if (radix == 10)
            return toString(i);
        char[] buf = new char[65];
        int charPos = 64;
        boolean negative = (i < 0);

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

        while (i <= -radix) {
            buf[charPos--] = Integer.digits[(int)(-(i % radix))];
            i = i / radix;
        }
        buf[charPos] = Integer.digits[(int)(-i)];

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

        return new String(buf, charPos, (65 - charPos));
    }
    /**
     * Returns a {@code String} object representing the specified
     * {@code long}.  The argument is converted to signed decimal
     * representation and returned as a string, exactly as if the
     * argument and the radix 10 were given as arguments to the {@link
     * #toString(long, int)} method.
     *
     * @param   i   a {@code long} to be converted.
     * @return  a string representation of the argument in base&nbsp;10.
     */
    public static String toString(long i) {
        if (i == Long.MIN_VALUE)
            return "-9223372036854775808";
        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 == Long.MIN_VALUE
     */
    static void getChars(long i, int index, char[] buf) {
        long q;
        int r;
        int charPos = index;
        char sign = 0;

        if (i < 0) {
            sign = '-';
            i = -i;
        }

        // Get 2 digits/iteration using longs until quotient fits into an int
        while (i > Integer.MAX_VALUE) {
            q = i / 100;
            // really: r = i - (q * 100);
            r = (int)(i - ((q << 6) + (q << 5) + (q << 2)));
            i = q;
            buf[--charPos] = Integer.DigitOnes[r];
            buf[--charPos] = Integer.DigitTens[r];
        }

        // Get 2 digits/iteration using ints
        int q2;
        int i2 = (int)i;
        while (i2 >= 65536) {
            q2 = i2 / 100;
            // really: r = i2 - (q * 100);
            r = i2 - ((q2 << 6) + (q2 << 5) + (q2 << 2));
            i2 = q2;
            buf[--charPos] = Integer.DigitOnes[r];
            buf[--charPos] = Integer.DigitTens[r];
        }

        // Fall thru to fast mode for smaller numbers
        // assert(i2 <= 65536, i2);
        for (;;) {
            q2 = (i2 * 52429) >>> (16+3);
            r = i2 - ((q2 << 3) + (q2 << 1));  // r = i2-(q2*10) ...
            buf[--charPos] = Integer.digits[r];
            i2 = q2;
            if (i2 == 0) break;
        }
        if (sign != 0) {
            buf[--charPos] = sign;
        }
    }

    // Requires positive x
    static int stringSize(long x) {
        long p = 10;
        for (int i=1; i<19; i++) {
            if (x < p)
                return i;
            p = 10*p;
        }
        return 19;
}
    /**
     * Returns a {@code String} object representing this
     * {@code Long}'s value.  The value is converted to signed
     * decimal representation and returned as a string, exactly as if
     * the {@code long} value were given as an argument to the
     * {@link java.lang.Long#toString(long)} method.
     *
     * @return  a string representation of the value of this object in
     *          base&nbsp;10.
     */
    public String toString() {
        return toString(value);
    }

⑦ Float

    /**
     * Returns a string representation of the {@code float}
     * argument. All characters mentioned below are ASCII characters.
     * <ul>
     * <li>If the argument is NaN, the result is the string
     * "{@code NaN}".
     * <li>Otherwise, the result is a string that represents the sign and
     *     magnitude (absolute value) of the argument. If the sign is
     *     negative, the first character of the result is
     *     '{@code -}' ({@code '\u005Cu002D'}); if the sign is
     *     positive, no sign character appears in the result. As for
     *     the magnitude <i>m</i>:
     * <ul>
     * <li>If <i>m</i> is infinity, it is represented by the characters
     *     {@code "Infinity"}; thus, positive infinity produces
     *     the result {@code "Infinity"} and negative infinity
     *     produces the result {@code "-Infinity"}.
     * <li>If <i>m</i> is zero, it is represented by the characters
     *     {@code "0.0"}; thus, negative zero produces the result
     *     {@code "-0.0"} and positive zero produces the result
     *     {@code "0.0"}.
     * <li> If <i>m</i> is greater than or equal to 10<sup>-3</sup> but
     *      less than 10<sup>7</sup>, then it is represented as the
     *      integer part of <i>m</i>, in decimal form with no leading
     *      zeroes, followed by '{@code .}'
     *      ({@code '\u005Cu002E'}), followed by one or more
     *      decimal digits representing the fractional part of
     *      <i>m</i>.
     10^-3<= float < 10^7 : 整数.小数形式（1025.35）
     float < 10^-3 || float >= 10^7 ：科学计数法（1.024E7）

     * <li> If <i>m</i> is less than 10<sup>-3</sup> or greater than or
     *      equal to 10<sup>7</sup>, then it is represented in
     *      so-called "computerized scientific notation." Let <i>n</i>
     *      be the unique integer such that 10<sup><i>n</i> </sup>&le;
     *      <i>m</i> {@literal <} 10<sup><i>n</i>+1</sup>; then let <i>a</i>
     *      be the mathematically exact quotient of <i>m</i> and
     *      10<sup><i>n</i></sup> so that 1 &le; <i>a</i> {@literal <} 10.
     *      The magnitude is then represented as the integer part of
     *      <i>a</i>, as a single decimal digit, followed by
     *      '{@code .}' ({@code '\u005Cu002E'}), followed by
     *      decimal digits representing the fractional part of
     *      <i>a</i>, followed by the letter '{@code E}'
     *      ({@code '\u005Cu0045'}), followed by a representation
     *      of <i>n</i> as a decimal integer, as produced by the
     *      method {@link java.lang.Integer#toString(int)}.
     *
     * </ul>
     * </ul>
     * How many digits must be printed for the fractional part of
     * <i>m</i> or <i>a</i>? There must be at least one digit
     * to represent the fractional part, and beyond that as many, but
     * only as many, more digits as are needed to uniquely distinguish
     * the argument value from adjacent values of type
     * {@code float}. That is, suppose that <i>x</i> is the
     * exact mathematical value represented by the decimal
     * representation produced by this method for a finite nonzero
     * argument <i>f</i>. Then <i>f</i> must be the {@code float}
     * value nearest to <i>x</i>; or, if two {@code float} values are
     * equally close to <i>x</i>, then <i>f</i> must be one of
     * them and the least significant bit of the significand of
     * <i>f</i> must be {@code 0}.
     *
     * <p>To create localized string representations of a floating-point
     * value, use subclasses of {@link java.text.NumberFormat}.
     *
     * @param   f   the float to be converted.
     * @return a string representation of the argument.
     */
    public static String toString(float f) {
        return FloatingDecimal.toJavaFormatString(f);
    }
    /**
     * Returns a string representation of this {@code Float} object.
     * The primitive {@code float} value represented by this object
     * is converted to a {@code String} exactly as if by the method
     * {@code toString} of one argument.
     *
     * @return  a {@code String} representation of this object.
     * @see java.lang.Float#toString(float)
     */
    public String toString() {
        return Float.toString(value);
}

⑧ Double

  /**
    * <li>If <i>m</i> is greater than or equal to 10<sup>-3</sup> but less
     * than 10<sup></sup>, then it is represented as the integer part of
     * <i>m</i>, in decimal form with no leading zeroes, followed by
     * '{@code .}' ({@code '\u005Cu002E'}), followed by one or
     * more decimal digits representing the fractional part of <i>m</i>.
     *
     * <li>If <i>m</i> is less than 10<sup>-3</sup> or greater than or
     * equal to 10<sup></sup>, then it is represented in so-called
     * "computerized scientific notation." Let <i>n</i> be the unique
     * integer such that 10<sup><i>n</i></sup> &le; <i>m</i> {@literal <}
     * 10<sup><i>n</i>+1</sup>; then let <i>a</i> be the
     * mathematically exact quotient of <i>m</i> and
     * 10<sup><i>n</i></sup> so that 1 &le; <i>a</i> {@literal <} 10. The
     * magnitude is then represented as the integer part of <i>a</i>,
     * as a single decimal digit, followed by '{@code .}'
     * ({@code '\u005Cu002E'}), followed by decimal digits
     * representing the fractional part of <i>a</i>, followed by the
     * letter '{@code E}' ({@code '\u005Cu0045'}), followed
     * by a representation of <i>n</i> as a decimal integer, as
     * produced by the method {@link Integer#toString(int)}.
*/
public static String toString(double d) {
        return FloatingDecimal.toJavaFormatString(d);
}

⑨ String

/**
 * This object (which is already a string!) is itself returned.
 *
 * @return  the string itself.
 */
public String toString() {
    return this;
}