java基础源码 (2)--StringBuilder类

Serializable（接口）：
　　是一个IO的序列化接口，实现了这个接口，就代表这个类可以序列化或者反序列化，该接口没有方法或者字段，仅用于标识可串行话的语义。

Appendable（接口）：

/**
 * An object to which <tt>char</tt> sequences and values can be appended.  The
 * <tt>Appendable</tt> interface must be implemented by any class whose
 * instances are intended to receive formatted output from a {@link
 * java.util.Formatter}.
　　可附加char序列和值得对象，Appendable接口必须由其实例旨在从Formatter接收格式化输出的类型

 *
 * <p> The characters to be appended should be valid Unicode characters as
 * described in <a href="Character.html#unicode">Unicode Character
 * Representation</a>.  Note that supplementary characters may be composed of
 * multiple 16-bit <tt>char</tt> values.
　　要附加的字符应为Unicode Character Representation中描述的有效Unicode字符。请注意，
补充字符可以由最多16位char值组成
 *
 * <p> Appendables are not necessarily safe for multithreaded access.  Thread
 * safety is the responsibility of classes that extend and implement this
 * interface.
 *Appendable对于多线程访问，附加功能不一定是安全的。线程安全是扩展和实现这个接口的类的责任
 * <p> Since this interface may be implemented by existing classes
 * with different styles of error handling there is no guarantee that
 * errors will be propagated to the invoker.
由于此接口可能由具有不同样式的错误处理的现有类实现，因此不能保证将错误传播到调用者
 */
public interface Appendable {

    /**
     * Appends the specified character sequence to this <tt>Appendable</tt>.
     *将制定的字符序列追加到此Appendable
     * <p> Depending on which class implements the character sequence
     * <tt>csq</tt>, the entire sequence may not be appended.  For
     * instance, if <tt>csq</tt> is a {@link java.nio.CharBuffer} then
     * the subsequence to append is defined by the buffer's position and limit.
     *根据那个类实现字符序列csq,可能不会附加整个序列，假如，如果csq是CharBuff，则附加的子序列
　　　　由缓冲区的位置和限制来定义。
     * @param  csq
     *         The character sequence to append.  If <tt>csq</tt> is
　　　　　　　　　　要追加的字符序列，如果csq是null
     *         <tt>null</tt>, then the four characters <tt>"null"</tt> are
　　　　　　　　　　　　　　　　　　　则四个字符“null”附加到该附录
     *         appended to this Appendable.
     *
     * @return  A reference to this <tt>Appendable</tt>
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    Appendable append(CharSequence csq) throws IOException;

    /**
     * Appends a subsequence of the specified character sequence to this
     * <tt>Appendable</tt>.
     *将制定的字符序列的子序列附加到此Appendable
     * <p> An invocation of this method of the form <tt>out.append(csq, start,
     * end)</tt> when <tt>csq</tt> is not <tt>null</tt>, behaves in
     * exactly the same way as the invocation
     *形式的这种方法调用out.append(csq,start,end)csq不是null，行为以完全相同的方式调用
　　　　　out.append(csq.subSequence(start,end))
     * <pre>
     *     out.append(csq.subSequence(start, end)) </pre>
　　　　
     *
     * @param  csq
     *         The character sequence from which a subsequence will be
     *         appended.  If <tt>csq</tt> is <tt>null</tt>, then characters
     *         will be appended as if <tt>csq</tt> contained the four
     *         characters <tt>"null"</tt>.
　　　　　　　　追加子序列的字符序列，如果csq是null，则会附加字符，如果csq包含四个字符“null”
　　　　　　
     *
     * @param  start
     *         The index of the first character in the subsequence
　　　　　　　　　　子序列中第一个字符的索引
     *
     * @param  end
     *         The index of the character following the last character in the
     *         subsequence
　　　　　　　　　　子序列中最后一个字符后面的字符的索引
     *
     * @return  A reference to this <tt>Appendable</tt>
     *
     * @throws  IndexOutOfBoundsException
     *          If <tt>start</tt> or <tt>end</tt> are negative, <tt>start</tt>
     *          is greater than <tt>end</tt>, or <tt>end</tt> is greater than
     *          <tt>csq.length()</tt>
　　　　　　　　　　如果start或end为负数，则start大于end，或end大于csq.length()都会抛出异常
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    Appendable append(CharSequence csq, int start, int end) throws IOException;

    /**
     * Appends the specified character to this <tt>Appendable</tt>.
　　　　　将制定的字符附加到此Appendable
     *
     * @param  c
     *         The character to append
     *
     * @return  A reference to this <tt>Appendable</tt>
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    Appendable append(char c) throws IOException;
}

CharSequence（接口）：
　　这个接口String类也是有实现的，该接口是char值得可读序列。

AbstractStringBuilder（类）：

/**
 * A mutable sequence of characters.
　　是一个可变的字符序列列表
 * <p>
 * Implements a modifiable string. At any point in time it contains some
 * particular sequence of characters, but the length and content of the
 * sequence can be changed through certain method calls.
实现可以修改的字符串，它都包含一些特定的字符列表，长度和内容的序列可以调用某些方法来改变
 *
 * <p>Unless otherwise noted, passing a {@code null} argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be
 * thrown.
　　构造器的参数如果传null，此类讲抛出NullPointerException异常
*/
abstract class AbstractStringBuilder implements Appendable, CharSequence {
    /**
     * The value is used for character storage.
　　　　类的成员变量：存储字符
     */
    char[] value;

    /**
     * The count is the number of characters used.
　类的成员变量：有多少个有效数据
     */
    int count;

    /**
     * This no-arg constructor is necessary for serialization of subclasses.
　　　　无参构造器     

*/
    AbstractStringBuilder() {
    }

    /**
     * Creates an AbstractStringBuilder of the specified capacity.
　　　　创建制定容量的     

*/
    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

    /**
     * Returns the length (character count).
　　　　返回字符长度
     *
     * @return  the length of the sequence of characters currently
     *          represented by this object
　　　　返回字符序列的长度
     */
    @Override
    public int length() {
        return count;
    }

    /**
     * Returns the current capacity. The capacity is the amount of storage
     * available for newly inserted characters, beyond which an allocation
     * will occur.
　　　　返回存储序列数组的长度，
     *
     * @return  the current capacity
     */
    public int capacity() {
        return value.length;
    }

    /**
     * Ensures that the capacity is at least equal to the specified minimum.
     * If the current capacity is less than the argument, then a new internal
     * array is allocated with greater capacity. The new capacity is the
     * larger of:
　　　　确保容量至少等于指定的最小值，如果当前容量小于参数，那么新的内部数据分配更大的容量
     * <ul>
     * <li>The {@code minimumCapacity} argument.
     * <li>Twice the old capacity, plus {@code 2}.
     * </ul>
     * If the {@code minimumCapacity} argument is nonpositive, this
     * method takes no action and simply returns.
     * Note that subsequent operations on this object can reduce the
     * actual capacity below that requested here.
　　　　如果参数为非正数，则不采取任何动作，只返回。
     *
     * @param   minimumCapacity   the minimum desired capacity.
     */
    public void ensureCapacity(int minimumCapacity) {
        if (minimumCapacity > 0)
            ensureCapacityInternal(minimumCapacity);
    }

    /**
     * For positive values of {@code minimumCapacity}, this method
     * behaves like {@code ensureCapacity}, however it is never
     * synchronized.
     * If {@code minimumCapacity} is non positive due to numeric
     * overflow, this method throws {@code OutOfMemoryError}.
　　　　参数minimumCapacity大于value.length则扩容
     */
    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }

    /**
     * The maximum size of array to allocate (unless necessary).
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
　　　　要分配的最大数据大小（除非必须要）
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * Returns a capacity at least as large as the given minimum capacity.
     * Returns the current capacity increased by the same amount + 2 if
     * that suffices.
　　返回至少给定最小容量一样大的容量，返回当前容量增加相同数量+2
     * Will not return a capacity greater than {@code MAX_ARRAY_SIZE}
     * unless the given minimum capacity is greater than that.
     *不会返回大于MAX_ARRAY_SIZE的，除非给定的最小容量大于此值
     * @param  minCapacity the desired minimum capacity
　　　　　　　　　　minCapacity所需的最小容量
     * @throws OutOfMemoryError if minCapacity is less than zero or
     *         greater than Integer.MAX_VALUE
　　　　　　　　如果minCapacity小于零或者大于interger.MAX_VALUE就会抛出异常
     */
    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }

    private int hugeCapacity(int minCapacity) {
        if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
            throw new OutOfMemoryError();
        }
        return (minCapacity > MAX_ARRAY_SIZE)
            ? minCapacity : MAX_ARRAY_SIZE;
    }

    /**
     * Attempts to reduce storage used for the character sequence.
     * If the buffer is larger than necessary to hold its current sequence of
　　　　如果缓冲区大于保持其当前序列所需的的缓冲区字符
     * characters, then it may be resized to become more space efficient.
　　　　　　　　　　　　然后可以根据调整数组的大小提高空间效率
     * Calling this method may, but is not required to, affect the value
     * returned by a subsequent call to the {@link #capacity()} method.
     */
    public void trimToSize() {
        if (count < value.length) {
            value = Arrays.copyOf(value, count);
        }
    }

    /**
     * Sets the length of the character sequence.
　　　　设置字符序列的长度
     * The sequence is changed to a new character sequence
　　　　序列更改为新的字符序列
     * whose length is specified by the argument. For every nonnegative
　　　　其长度有参数指定，对于每个非负
     * index <i>k</i> less than {@code newLength}, the character at
　　　　index k 小于 newLength ,                     字符位于
     * index <i>k</i> in the new character sequence is the same as the
　　　　新字符序列中的 index k
     * character at index <i>k</i> in the old sequence if <i>k</i> is less
　　　　如果 k  较小，则旧序列中索引 k 处的字符
     * than the length of the old character sequence; otherwise, it is the
　　　　比旧字符序列的长度，否则，他就是 null 序列
     * null character {@code '\u005Cu0000'}.
     *
     * In other words, if the {@code newLength} argument is less than
　　　　换句话说，如果参数newLength小于当前长度
     * the current length, the length is changed to the specified length.
　　　　长度则更改为newlength
     * <p>
     * If the {@code newLength} argument is greater than or equal
　　　　如果参数newlength参数大于或等于
     * to the current length, sufficient null characters
　　　　当前长度，足够的空字符
     * ({@code '\u005Cu0000'}) are appended so that
　　　　附加
     * length becomes the {@code newLength} argument.
　　　　　　length成为newLength参数
     * <p>
     * The {@code newLength} argument must be greater than or equal
　　　　newLength参数必须大于或等于
     * to {@code 0}.
     *
     * @param      newLength   the new length
     * @throws     IndexOutOfBoundsException  if the
     *               {@code newLength} argument is negative.
     */
    public void setLength(int newLength) {
        if (newLength < 0)
            throw new StringIndexOutOfBoundsException(newLength);
        ensureCapacityInternal(newLength);

        if (count < newLength) {
            Arrays.fill(value, count, newLength, '\0');
        }

        count = newLength;
    }

    /**
     * Returns the {@code char} value in this sequence at the specified index.
　　　　返回此序列中指定下标的char值
     * The first {@code char} value is at index {@code 0}, the next at index
　　　　第一个索引值是从0开始，然后1，以此类推
     * {@code 1}, and so on, as in array indexing.
　　　　就像在数组索引中一样
     * <p>
     * The index argument must be greater than or equal to
     * {@code 0}, and less than the length of this sequence.
     *
     * <p>If the {@code char} value specified by the index is a
     * <a href="Character.html#unicode">surrogate</a>, the surrogate
     * value is returned.
     *
     * @param      index   the index of the desired {@code char} value.
     * @return     the {@code char} value at the specified index.
     * @throws     IndexOutOfBoundsException  if {@code index} is
     *             negative or greater than or equal to {@code length()}.
     */
    @Override
    public char charAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        return value[index];
    }

    /**
     * Returns the character (Unicode code point) at the specified
　　　　返回指定的字符
     * index. The index refers to {@code char} values
　　　　指数。索引引用char值
     * (Unicode code units) and ranges from {@code 0} to
     * {@link #length()}{@code  - 1}.
     *
     * <p> If the {@code char} value specified at the given index
     * is in the high-surrogate range, the following index is less
     * than the length of this sequence, and the
     * {@code char} value at the following index is in the
     * low-surrogate range, then the supplementary code point
     * corresponding to this surrogate pair is returned. Otherwise,
     * the {@code char} value at the given index is returned.
     *
     * @param      index the index to the {@code char} values
     * @return     the code point value of the character at the
     *             {@code index}
     * @exception  IndexOutOfBoundsException  if the {@code index}
     *             argument is negative or not less than the length of this
     *             sequence.
     */
    public int codePointAt(int index) {
        if ((index < 0) || (index >= count)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return Character.codePointAtImpl(value, index, count);
    }

    /**
     * Returns the character (Unicode code point) before the specified
     * index. The index refers to {@code char} values
     * (Unicode code units) and ranges from {@code 1} to {@link
     * #length()}.
     *
     * <p> If the {@code char} value at {@code (index - 1)}
     * is in the low-surrogate range, {@code (index - 2)} is not
     * negative, and the {@code char} value at {@code (index -
     * 2)} is in the high-surrogate range, then the
     * supplementary code point value of the surrogate pair is
     * returned. If the {@code char} value at {@code index -
     * 1} is an unpaired low-surrogate or a high-surrogate, the
     * surrogate value is returned.
     *
     * @param     index the index following the code point that should be returned
     * @return    the Unicode code point value before the given index.
     * @exception IndexOutOfBoundsException if the {@code index}
     *            argument is less than 1 or greater than the length
     *            of this sequence.
     */
    public int codePointBefore(int index) {
        int i = index - 1;
        if ((i < 0) || (i >= count)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return Character.codePointBeforeImpl(value, index, 0);
    }

    /**
     * Returns the number of Unicode code points in the specified text
     * range of this sequence. The text range begins at the specified
     * {@code beginIndex} and extends to the {@code char} at
     * index {@code endIndex - 1}. Thus the length (in
     * {@code char}s) of the text range is
     * {@code endIndex-beginIndex}. Unpaired surrogates within
     * this sequence count as one code point each.
     *
     * @param beginIndex the index to the first {@code char} of
     * the text range.
     * @param endIndex the index after the last {@code char} of
     * the text range.
     * @return the number of Unicode code points in the specified text
     * range
     * @exception IndexOutOfBoundsException if the
     * {@code beginIndex} is negative, or {@code endIndex}
     * is larger than the length of this sequence, or
     * {@code beginIndex} is larger than {@code endIndex}.
     */
    public int codePointCount(int beginIndex, int endIndex) {
        if (beginIndex < 0 || endIndex > count || beginIndex > endIndex) {
            throw new IndexOutOfBoundsException();
        }
        return Character.codePointCountImpl(value, beginIndex, endIndex-beginIndex);
    }

    /**
     * Returns the index within this sequence that is offset from the
     * given {@code index} by {@code codePointOffset} code
     * points. Unpaired surrogates within the text range given by
     * {@code index} and {@code codePointOffset} count as
     * one code point each.
     *
     * @param index the index to be offset
     * @param codePointOffset the offset in code points
     * @return the index within this sequence
     * @exception IndexOutOfBoundsException if {@code index}
     *   is negative or larger then the length of this sequence,
     *   or if {@code codePointOffset} is positive and the subsequence
     *   starting with {@code index} has fewer than
     *   {@code codePointOffset} code points,
     *   or if {@code codePointOffset} is negative and the subsequence
     *   before {@code index} has fewer than the absolute value of
     *   {@code codePointOffset} code points.
     */
    public int offsetByCodePoints(int index, int codePointOffset) {
        if (index < 0 || index > count) {
            throw new IndexOutOfBoundsException();
        }
        return Character.offsetByCodePointsImpl(value, 0, count,
                                                index, codePointOffset);
    }

    /**
     * Characters are copied from this sequence into the
　　　　字符从此序列复制到
     * destination character array {@code dst}. The first character to
　　　　目标字符数组 dst。第一个字符
     * be copied is at index {@code srcBegin}; the last character to
　　　　被复制在索引，最后一个字符
     * be copied is at index {@code srcEnd-1}. The total number of
　　　　被复制在索引 srcEnd-1，总数
     * characters to be copied is {@code srcEnd-srcBegin}. The
　　　　要复制的字符是srcEnd-srcBegin.该字符要被复制到dst的子数组中
     * characters are copied into the subarray of {@code dst} starting
     * at index {@code dstBegin} and ending at index:
     * <pre>{@code
     * dstbegin + (srcEnd-srcBegin) - 1
     * }</pre>
     *
     * @param      srcBegin   start copying at this offset.
     * @param      srcEnd     stop copying at this offset.
     * @param      dst        the array to copy the data into.
     * @param      dstBegin   offset into {@code dst}.
     * @throws     IndexOutOfBoundsException  if any of the following is true:
     *             <ul>
     *             <li>{@code srcBegin} is negative
     *             <li>{@code dstBegin} is negative
     *             <li>the {@code srcBegin} argument is greater than
     *             the {@code srcEnd} argument.
     *             <li>{@code srcEnd} is greater than
     *             {@code this.length()}.
     *             <li>{@code dstBegin+srcEnd-srcBegin} is greater than
     *             {@code dst.length}
     *             </ul>
     */
    public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
    {
        if (srcBegin < 0)
            throw new StringIndexOutOfBoundsException(srcBegin);
        if ((srcEnd < 0) || (srcEnd > count))
            throw new StringIndexOutOfBoundsException(srcEnd);
        if (srcBegin > srcEnd)
            throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");
        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
    }

    /**
     * The character at the specified index is set to {@code ch}. This
　　　　指定索引处的字符设置为ch,这个
     * sequence is altered to represent a new character sequence that is
　　　　序列被改变以表示新的字符序列
     * identical to the old character sequence, except that it contains the
　　　　与就字符序列相同，但它包含位置 index 的字符
     * character {@code ch} at position {@code index}.
     * <p>
     * The index argument must be greater than or equal to
　　　　　　index参数必须大于0或等于小于此序列的长度
     * {@code 0}, and less than the length of this sequence.
     *
     * @param      index   the index of the character to modify.
     * @param      ch      the new character.
     * @throws     IndexOutOfBoundsException  if {@code index} is
     *             negative or greater than or equal to {@code length()}.
     */
    public void setCharAt(int index, char ch) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        value[index] = ch;
    }

    /**
     * Appends the string representation of the {@code Object} argument.
　　　　附加 Object 参数的字符串表示形式
     * <p>
     * The overall effect is exactly as if the argument were converted
　　　　整体效果就像参数被转换一样
     * to a string by the method {@link String#valueOf(Object)},
　　　　　通过String.valueOf(Object)到一个字符串
     * and the characters of that string were then
　　　　然后那个字符串的字符
     * {@link #append(String) appended} to this character sequence.
　　　　　　appended 附加到此字符序列
     *
     * @param   obj   an {@code Object}.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

    /**
     * Appends the specified string to this character sequence.
　　　　将指定的字符串追加到此字符序列。
     * <p>
     * The characters of the {@code String} argument are appended, in
     * order, increasing the length of this sequence by the length of the
     * argument. If {@code str} is {@code null}, then the four
     * characters {@code "null"} are appended.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at
     * index <i>k</i> in the new character sequence is equal to the character
     * at index <i>k</i> in the old character sequence, if <i>k</i> is less
     * than <i>n</i>; otherwise, it is equal to the character at index
     * <i>k-n</i> in the argument {@code str}.
     *
     * @param   str   a string.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

    // Documentation in subclasses because of synchro difference
    public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return appendNull();
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }

    /**
     * @since 1.8
     */
    AbstractStringBuilder append(AbstractStringBuilder asb) {
        if (asb == null)
            return appendNull();
        int len = asb.length();
        ensureCapacityInternal(count + len);
        asb.getChars(0, len, value, count);
        count += len;
        return this;
    }

    // Documentation in subclasses because of synchro difference
    @Override
    public AbstractStringBuilder append(CharSequence s) {
        if (s == null)
            return appendNull();
        if (s instanceof String)
            return this.append((String)s);
        if (s instanceof AbstractStringBuilder)
            return this.append((AbstractStringBuilder)s);

        return this.append(s, 0, s.length());
    }

    private AbstractStringBuilder appendNull() {
        int c = count;
        ensureCapacityInternal(c + 4);
        final char[] value = this.value;
        value[c++] = 'n';
        value[c++] = 'u';
        value[c++] = 'l';
        value[c++] = 'l';
        count = c;
        return this;
    }

    /**
     * Appends a subsequence of the specified {@code CharSequence} to this
     * sequence.
　　　　将指定的charSequence的子序列附加到此序列
     * <p>
     * Characters of the argument {@code s}, starting at
　　　　参数的字符s,从
     * index {@code start}, are appended, in order, to the contents of
　　　　　index开始，按顺序附加到内容中
     * this sequence up to the (exclusive) index {@code end}. The length
　　　　　　此序列最多为end，
     * of this sequence is increased by the value of {@code end - start}.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at
     * index <i>k</i> in this character sequence becomes equal to the
     * character at index <i>k</i> in this sequence, if <i>k</i> is less than
     * <i>n</i>; otherwise, it is equal to the character at index
     * <i>k+start-n</i> in the argument {@code s}.
     * <p>
     * If {@code s} is {@code null}, then this method appends
     * characters as if the s parameter was a sequence containing the four
     * characters {@code "null"}.
     *
     * @param   s the sequence to append.
     * @param   start   the starting index of the subsequence to be appended.
     * @param   end     the end index of the subsequence to be appended.
     * @return  a reference to this object.
     * @throws     IndexOutOfBoundsException if
     *             {@code start} is negative, or
     *             {@code start} is greater than {@code end} or
     *             {@code end} is greater than {@code s.length()}
     */
    @Override
    public AbstractStringBuilder append(CharSequence s, int start, int end) {
        if (s == null)
            s = "null";
        if ((start < 0) || (start > end) || (end > s.length()))
            throw new IndexOutOfBoundsException(
                "start " + start + ", end " + end + ", s.length() "
                + s.length());
        int len = end - start;
        ensureCapacityInternal(count + len);
        for (int i = start, j = count; i < end; i++, j++)
            value[j] = s.charAt(i);
        count += len;
        return this;
    }

    /**
     * Appends the string representation of the {@code char} array
　　　　往后面追加一个char类型的数组
     * argument to this sequence.
     * <p>
     * The characters of the array argument are appended, in order, to
     * the contents of this sequence. The length of this sequence
     * increases by the length of the argument.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(char[])},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   str   the characters to be appended.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(char[] str) {
        int len = str.length;
        ensureCapacityInternal(count + len);
        System.arraycopy(str, 0, value, count, len);
        count += len;
        return this;
    }

    /**
     * Appends the string representation of a subarray of the
　　　　追加子数组的字符串表示
     * {@code char} array argument to this sequence.
     * <p>
     * Characters of the {@code char} array {@code str}, starting at
     * index {@code offset}, are appended, in order, to the contents
     * of this sequence. The length of this sequence increases
     * by the value of {@code len}.
     * <p>
     * The overall effect is exactly as if the arguments were converted
     * to a string by the method {@link String#valueOf(char[],int,int)},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   str      the characters to be appended.
     * @param   offset   the index of the first {@code char} to append.
     * @param   len      the number of {@code char}s to append.
     * @return  a reference to this object.
     * @throws IndexOutOfBoundsException
     *         if {@code offset < 0} or {@code len < 0}
     *         or {@code offset+len > str.length}
     */
    public AbstractStringBuilder append(char str[], int offset, int len) {
        if (len > 0)                // let arraycopy report AIOOBE for len < 0
            ensureCapacityInternal(count + len);
        System.arraycopy(str, offset, value, count, len);
        count += len;
        return this;
    }

    /**
     * Appends the string representation of the {@code boolean}
　　　　追加 布尔类型的字符串表示序列的参数
     * argument to the sequence.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(boolean)},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   b   a {@code boolean}.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(boolean b) {
        if (b) {
            ensureCapacityInternal(count + 4);
            value[count++] = 't';
            value[count++] = 'r';
            value[count++] = 'u';
            value[count++] = 'e';
        } else {
            ensureCapacityInternal(count + 5);
            value[count++] = 'f';
            value[count++] = 'a';
            value[count++] = 'l';
            value[count++] = 's';
            value[count++] = 'e';
        }
        return this;
    }

    /**
     * Appends the string representation of the {@code char}
     * argument to this sequence.
     * <p>
     * The argument is appended to the contents of this sequence.
     * The length of this sequence increases by {@code 1}.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(char)},
     * and the character in that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   c   a {@code char}.
     * @return  a reference to this object.
     */
    @Override
    public AbstractStringBuilder append(char c) {
        ensureCapacityInternal(count + 1);
        value[count++] = c;
        return this;
    }

    /**
     * Appends the string representation of the {@code int}
     * argument to this sequence.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(int)},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   i   an {@code int}.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(int i) {
        if (i == Integer.MIN_VALUE) {
            append("-2147483648");
            return this;
        }
        int appendedLength = (i < 0) ? Integer.stringSize(-i) + 1
                                     : Integer.stringSize(i);
        int spaceNeeded = count + appendedLength;
        ensureCapacityInternal(spaceNeeded);
        Integer.getChars(i, spaceNeeded, value);
        count = spaceNeeded;
        return this;
    }

    /**
     * Appends the string representation of the {@code long}
     * argument to this sequence.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(long)},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   l   a {@code long}.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(long l) {
        if (l == Long.MIN_VALUE) {
            append("-9223372036854775808");
            return this;
        }
        int appendedLength = (l < 0) ? Long.stringSize(-l) + 1
                                     : Long.stringSize(l);
        int spaceNeeded = count + appendedLength;
        ensureCapacityInternal(spaceNeeded);
        Long.getChars(l, spaceNeeded, value);
        count = spaceNeeded;
        return this;
    }

    /**
     * Appends the string representation of the {@code float}
     * argument to this sequence.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(float)},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   f   a {@code float}.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(float f) {
        FloatingDecimal.appendTo(f,this);
        return this;
    }

    /**
     * Appends the string representation of the {@code double}
     * argument to this sequence.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(double)},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   d   a {@code double}.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(double d) {
        FloatingDecimal.appendTo(d,this);
        return this;
    }

    /**
     * Removes the characters in a substring of this sequence.
　　　　删除此序列中的指定的子字符串的字符
     * The substring begins at the specified {@code start} and extends to
　　　　　子字符串从指定的start开始并延伸到
     * the character at index {@code end - 1} or to the end of the
　　　　end-1或结尾处的字符序列
     * sequence if no such character exists. If
　　　　如果
     * {@code start} is equal to {@code end}, no changes are made.
　　　　　　start等于end，则不做任何更改
     *
     * @param      start  The beginning index, inclusive.
     * @param      end    The ending index, exclusive.
     * @return     This object.
     * @throws     StringIndexOutOfBoundsException  if {@code start}
     *             is negative, greater than {@code length()}, or
     *             greater than {@code end}.
     */
    public AbstractStringBuilder delete(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            end = count;
        if (start > end)
            throw new StringIndexOutOfBoundsException();
        int len = end - start;
        if (len > 0) {
            System.arraycopy(value, start+len, value, start, count-end);
            count -= len;
        }
        return this;
    }
}

StringBuilder类：

　　

/**
 * A mutable sequence of characters.  This class provides an API compatible
 * with {@code StringBuffer}, but with no guarantee of synchronization.
 * This class is designed for use as a drop-in replacement for
 * {@code StringBuffer} in places where the string buffer was being
 * used by a single thread (as is generally the case).   Where possible,
 * it is recommended that this class be used in preference to
 * {@code StringBuffer} as it will be faster under most implementations.
　　一个可变的字符序列，此类提供与StringBuffer的API，但不保证同步，此类设计用作简易替换为StringBuffer
　　在，正在使用由单个线程字符串缓冲区的地方（如通常是这种情况），在可能的情况下，简易使用这个类别优于
　　StringBuffer，因为它大大多数实现中将更快
 *
 * <p>The principal operations on a {@code StringBuilder} are the
 * {@code append} and {@code insert} methods, which are
 * overloaded so as to accept data of any type. Each effectively
 * converts a given datum to a string and then appends or inserts the
 * characters of that string to the string builder. The
 * {@code append} method always adds these characters at the end
 * of the builder; the {@code insert} method adds the characters at
 * a specified point.
　　StringBuilder的主要StringBuilder是append和insert方法，它们是重载的，以便接受
　　任何类型的数据，每个都有效地将给定的数据转换为字符串，然后将该字符串附加或插入字符
　　串构造器，append方法始终在构造器的末尾添加这些字符，insert方法将insert添加到
　　指定点
 * <p>
 * For example, if {@code z} refers to a string builder object
 * whose current contents are "{@code start}", then
 * the method call {@code z.append("le")} would cause the string
 * builder to contain "{@code startle}", whereas
 * {@code z.insert(4, "le")} would alter the string builder to
 * contain "{@code starlet}".
　　例如，如果z引用当前内容“start”的字符串构造器对象，那么方法调用z.append("le")将
　　导致字符串构造器包含“startle”,而z.insert(4,"le")会将字符串构造器更改为包含
　　“starlet”
 * <p>
 * In general, if sb refers to an instance of a {@code StringBuilder},
 * then {@code sb.append(x)} has the same effect as
 * {@code sb.insert(sb.length(), x)}.
　　一般情况下，如果某人是指的一个实例StringBuilder，则sb.append(x)具有相同的效果
　　sb.insert(sb.length(),x).
 * <p>
 * Every string builder has a capacity. As long as the length of the
 * character sequence contained in the string builder does not exceed
 * the capacity, it is not necessary to allocate a new internal
 * buffer. If the internal buffer overflows, it is automatically made larger.
 *每个字符串构造器都有一个容器，只要字符串构造器中包含的字符序列的长度不超过容量，则不需要分配
　　新的内部缓冲区，如果内部缓冲区溢出，则会自动变大
 * <p>Instances of {@code StringBuilder} are not safe for
 * use by multiple threads. If such synchronization is required then it is
 * recommended that {@link java.lang.StringBuffer} be used.
 *StringBuilder不能安全使用多线程，如果需要同步，那么简易使用StringBuffer
 * <p>Unless otherwise noted, passing a {@code null} argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be
 * thrown.
 *
 * @author      Michael McCloskey
 * @see         java.lang.StringBuffer
 * @see         java.lang.String
 * @since       1.5
 */
public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    /** use serialVersionUID for interoperability */
    static final long serialVersionUID = 4383685877147921099L;

    /**
     * Constructs a string builder with no characters in it and an
　　　　构造一个没有字符的字符串构造器和一个初始化容量为16个字符。
     * initial capacity of 16 characters.
     */
    public StringBuilder() {
        super(16);
    }

    /**
     * Constructs a string builder with no characters in it and an
　　　　构造一个字符串生成器，其中没有字符和capacity参数指定的初始容量
     * initial capacity specified by the {@code capacity} argument.
     *
     * @param      capacity  the initial capacity.
     * @throws     NegativeArraySizeException  if the {@code capacity}
     *               argument is less than {@code 0}.
     */
    public StringBuilder(int capacity) {
        super(capacity);
    }

    /**
     * Constructs a string builder initialized to the contents of the
　　　　初始化一个字符串长度为16的
     * specified string. The initial capacity of the string builder is
     * {@code 16} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

    /**
     * Constructs a string builder that contains the same characters
　　　　构造一个包含与指定的相同字符的字符串构造器CharSequence
     * as the specified {@code CharSequence}. The initial capacity of
     * the string builder is {@code 16} plus the length of the
     * {@code CharSequence} argument.
     *
     * @param      seq   the sequence to copy.
     */
    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }

    @Override
    public StringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

    /**
     * Appends the specified {@code StringBuffer} to this sequence.
　　　　将指定StringBuffer这个序列
     * <p>
     * The characters of the {@code StringBuffer} argument are appended,
     * in order, to this sequence, increasing the
     * length of this sequence by the length of the argument.
     * If {@code sb} is {@code null}, then the four characters
     * {@code "null"} are appended to this sequence.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at index
     * <i>k</i> in the new character sequence is equal to the character at
     * index <i>k</i> in the old character sequence, if <i>k</i> is less than
     * <i>n</i>; otherwise, it is equal to the character at index <i>k-n</i>
     * in the argument {@code sb}.
     *
     * @param   sb   the {@code StringBuffer} to append.
     * @return  a reference to this object.
     */
    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }

    @Override
    public StringBuilder append(CharSequence s) {
        super.append(s);
        return this;
    }

    /**
     * @throws     IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder append(CharSequence s, int start, int end) {
        super.append(s, start, end);
        return this;
    }

    @Override
    public StringBuilder append(char[] str) {
        super.append(str);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder append(char[] str, int offset, int len) {
        super.append(str, offset, len);
        return this;
    }

    @Override
    public StringBuilder append(boolean b) {
        super.append(b);
        return this;
    }

    @Override
    public StringBuilder append(char c) {
        super.append(c);
        return this;
    }

    @Override
    public StringBuilder append(int i) {
        super.append(i);
        return this;
    }

    @Override
    public StringBuilder append(long lng) {
        super.append(lng);
        return this;
    }

    @Override
    public StringBuilder append(float f) {
        super.append(f);
        return this;
    }

    @Override
    public StringBuilder append(double d) {
        super.append(d);
        return this;
    }

    /**
     * @since 1.5
     */
    @Override
    public StringBuilder appendCodePoint(int codePoint) {
        super.appendCodePoint(codePoint);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder delete(int start, int end) {
        super.delete(start, end);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder deleteCharAt(int index) {
        super.deleteCharAt(index);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder replace(int start, int end, String str) {
        super.replace(start, end, str);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int index, char[] str, int offset,
                                int len)
    {
        super.insert(index, str, offset, len);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, Object obj) {
            super.insert(offset, obj);
            return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, String str) {
        super.insert(offset, str);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, char[] str) {
        super.insert(offset, str);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int dstOffset, CharSequence s) {
            super.insert(dstOffset, s);
            return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int dstOffset, CharSequence s,
                                int start, int end)
    {
        super.insert(dstOffset, s, start, end);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, boolean b) {
        super.insert(offset, b);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, char c) {
        super.insert(offset, c);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, int i) {
        super.insert(offset, i);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, long l) {
        super.insert(offset, l);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, float f) {
        super.insert(offset, f);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, double d) {
        super.insert(offset, d);
        return this;
    }

    @Override
    public int indexOf(String str) {
        return super.indexOf(str);
    }

    @Override
    public int indexOf(String str, int fromIndex) {
        return super.indexOf(str, fromIndex);
    }

    @Override
    public int lastIndexOf(String str) {
        return super.lastIndexOf(str);
    }

    @Override
    public int lastIndexOf(String str, int fromIndex) {
        return super.lastIndexOf(str, fromIndex);
    }

    @Override
    public StringBuilder reverse() {
        super.reverse();
        return this;
    }

    @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }

    /**
     * Save the state of the {@code StringBuilder} instance to a stream
     * (that is, serialize it).
     *将StringBuilder实例的状态保存到流中，即序列化
     * @serialData the number of characters currently stored in the string
     *             builder ({@code int}), followed by the characters in the
     *             string builder ({@code char[]}).   The length of the
     *             {@code char} array may be greater than the number of
     *             characters currently stored in the string builder, in which
     *             case extra characters are ignored.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeInt(count);
        s.writeObject(value);
    }

    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        count = s.readInt();
        value = (char[]) s.readObject();
    }

}