Java-IO之CharArrayWriter（字符数组输出流）

CharArrayWriter用于写数据，数据单位是字符。
(1) 通过CharArrayWriter()创建的CharArrayWriter对应的字符数组大小是32。
(2) 通过CharArrayWriter(int size) 创建的CharArrayWriter对应的字符数组大小是size。
(3) write(int oneChar)的作用将int类型的oneChar换成char类型，然后写入到CharArrayWriter中。
(4) write(char[] buffer, int offset, int len) 是将字符数组buffer写入到输出流中，offset是从buffer中读取数据的起始偏移位置，len是读取的长度。
(5) write(String str, int offset, int count) 是将字符串str写入到输出流中，offset是从str中读取数据的起始位置，count是读取的长度。
(6) append(char c)的作用将char类型的c写入到CharArrayWriter中，然后返回CharArrayWriter对象。
注意：append(char c)与write(int c)都是将单个字符写入到CharArrayWriter中。它们的区别是，append(char c)会返回CharArrayWriter对象，但是write(int c)返回void。
(7) append(CharSequence csq, int start, int end)的作用将csq从start开始(包括)到end结束(不包括)的数据，写入到CharArrayWriter中。
注意：该函数返回CharArrayWriter对象！
(8) append(CharSequence csq)的作用将csq写入到CharArrayWriter中。
注意：该函数返回CharArrayWriter对象！
(9) writeTo(OutputStream out) 将该“字符数组输出流”的数据全部写入到“输出流out”中。

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();
}
}
}
运行结果：
caw=ABCdefgh0123456789ijkl
size=22
buf=ABCdefgh0123456789ijkl
caw2=ABCdefgh0123456789ijkl

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() { }

}