示例代码：

public class CharArrayWriterTest {

    private static final int LEN = 5;
    // 对应英文字母“abcdefghijklmnopqrstuvwxyz”
    private static final char[] ArrayLetters = new char[] {'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'};

    public static void main(String[] args) {
        tesCharArrayWriter() ;
    }
    private static void tesCharArrayWriter() {
        try {
            // 创建CharArrayWriter字符流
            CharArrayWriter caw = new CharArrayWriter();
            // 写入“A”个字符
            caw.write('A');
            // 写入字符串“BC”个字符
            caw.write("BC");
            //System.out.printf("caw=%s\n", caw);
            // 将ArrayLetters数组中从“3”开始的后5个字符(defgh)写入到caw中。
            caw.write(ArrayLetters, 3, 5);
            //System.out.printf("caw=%s\n", caw);

            // (01) 写入字符0
            // (02) 然后接着写入“123456789”
            // (03) 再接着写入ArrayLetters中第8-12个字符(ijkl)
            caw.append('0').append("123456789").append(String.valueOf(ArrayLetters), 8, 12);
            System.out.printf("caw=%s\n", caw);

            // 计算长度
            int size = caw.size();
            System.out.printf("size=%s\n", size);

            // 转换成byte[]数组
            char[] buf = caw.toCharArray();
            System.out.printf("buf=%s\n", String.valueOf(buf));

            // 将caw写入到另一个输出流中
            CharArrayWriter caw2 = new CharArrayWriter();
            caw.writeTo(caw2);
            System.out.printf("caw2=%s\n", caw2);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

基于JDK8的CharArrayWriter的源码分析：

public class CharArrayWriter extends Writer {
    /**
     * The buffer where data is stored.
     */
    protected char buf[];//字符数组

    /**
     * The number of chars in the buffer.
     */
    protected int count;//数组中字符的数量

    /**
     * Creates a new CharArrayWriter.
     */
    public CharArrayWriter() {//默认字符数组初始大小是32
        this(32);
    }

    /**
     * Creates a new CharArrayWriter with the specified initial size.
     *
     * @param initialSize  an int specifying the initial buffer size.
     * @exception IllegalArgumentException if initialSize is negative
     */
    //构造initialSize大小的字符数组
    public CharArrayWriter(int initialSize) {
        if (initialSize < 0) {
            throw new IllegalArgumentException("Negative initial size: "+ initialSize);
        }
        buf = new char[initialSize];
    }

    /**
     * Writes a character to the buffer.
     */
    //向字符数组里写一个字符
    public void write(int c) {
        synchronized (lock) {
            int newcount = count + 1;
            if (newcount > buf.length) {
                buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
            }
            buf[count] = (char)c;
            count = newcount;
        }
    }

    /**
     * Writes characters to the buffer.
     * @param c the data to be written
     * @param off       the start offset in the data
     * @param len       the number of chars that are written
     */
    //向字符数组中写C，其实off，长度len
    public void write(char c[], int off, int len) {
        if ((off < 0) || (off > c.length) || (len < 0) ||
                ((off + len) > c.length) || ((off + len) < 0)) {
                    throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return;
        }
        synchronized (lock) {
            int newcount = count + len;
            if (newcount > buf.length) {
                buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
            }
            System.arraycopy(c, off, buf, count, len);
            count = newcount;
        }
    }

    /**
     * Write a portion of a string to the buffer.
     * @param  str  String to be written from
     * @param  off  Offset from which to start reading characters
     * @param  len  Number of characters to be written
     */
    //向字符数组中写字符串
    public void write(String str, int off, int len) {
        synchronized (lock) {
            int newcount = count + len;
            if (newcount > buf.length) {
                buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
            }
            str.getChars(off, off + len, buf, count);
            count = newcount;
        }
    }

    /**
     * Writes the contents of the buffer to another character stream.
     *
     * @param out       the output stream to write to
     * @throws IOException If an I/O error occurs.
     */
    //将字符数组中的数据写到另外一个流中
    public void writeTo(Writer out) throws IOException {
        synchronized (lock) {
            out.write(buf, 0, count);
        }
    }

    /**
     * Appends the specified character sequence to this writer.
     *
     * <p> An invocation of this method of the form <tt>out.append(csq)</tt>
     * behaves in exactly the same way as the invocation
     *
     * <pre>
     *     out.write(csq.toString()) </pre>
     *
     * <p> Depending on the specification of <tt>toString</tt> for the
     * character sequence <tt>csq</tt>, the entire sequence may not be
     * appended. For instance, invoking the <tt>toString</tt> method of a
     * character buffer will return a subsequence whose content depends upon
     * the buffer's position and limit.
     *
     * @param  csq
     *         The character sequence to append.  If <tt>csq</tt> is
     *         <tt>null</tt>, then the four characters <tt>"null"</tt> are
     *         appended to this writer.
     *
     * @return  This writer
     *
     * @since  1.5
     */
    //追加
    public CharArrayWriter append(CharSequence csq) {
        String s = (csq == null ? "null" : csq.toString());
        write(s, 0, s.length());
        return this;
    }

    /**
     * Appends a subsequence of the specified character sequence to this writer.
     *
     * <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
     *
     * <pre>
     *     out.write(csq.subSequence(start, end).toString()) </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>.
     *
     * @param  start
     *         The index of the first character in the subsequence
     *
     * @param  end
     *         The index of the character following the last character in the
     *         subsequence
     *
     * @return  This writer
     *
     * @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>
     *
     * @since  1.5
     */
     //追加部分到字符数组
    public CharArrayWriter append(CharSequence csq, int start, int end) {
        String s = (csq == null ? "null" : csq).subSequence(start, end).toString();
        write(s, 0, s.length());
        return this;
    }

    /**
     * Appends the specified character to this writer.
     *
     * <p> An invocation of this method of the form <tt>out.append(c)</tt>
     * behaves in exactly the same way as the invocation
     *
     * <pre>
     *     out.write(c) </pre>
     *
     * @param  c
     *         The 16-bit character to append
     *
     * @return  This writer
     *
     * @since 1.5
     */
    public CharArrayWriter append(char c) {
        write(c);
        return this;
    }

    /**
     * Resets the buffer so that you can use it again without
     * throwing away the already allocated buffer.
     */
    //重置
    public void reset() {
        count = 0;
    }

    /**
     * Returns a copy of the input data.
     *
     * @return an array of chars copied from the input data.
     */
    public char toCharArray()[] {
        synchronized (lock) {
            return Arrays.copyOf(buf, count);
        }
    }

    /**
     * Returns the current size of the buffer.
     *
     * @return an int representing the current size of the buffer.
     */
    //字符数组当前大小
    public int size() {
        return count;
    }

    /**
     * Converts input data to a string.
     * @return the string.
     */
    //
    public String toString() {
        synchronized (lock) {
            return new String(buf, 0, count);
        }
    }

    /**
     * Flush the stream.
     */
    public void flush() { }

    /**
     * Close the stream.  This method does not release the buffer, since its
     * contents might still be required. Note: Invoking this method in this class
     * will have no effect.
     */
    public void close() { }

}