Java 7之传统I/O - 字符类 CharArrayReader和CharArrayWriter

CharArrayReader和CharArrayWriter是字符数组流。它和ByteArrayInputStream、ByteArrayOutputStream类似，只不过ByteArrayXXputStream是字节数组流，而CharArrayXX是字符数组流。

CharArrayWriter是用于读取字符数组，它继承于Writer类。以字符为单位进行数组的操作。下面来看一下主要方法实现的源代码：

1、创建及初始化

看一下CharArrayWritrer中定义的重要变量及构造函数：

    protected char buf[];  // 字符数组,用于数组的存取 
    protected int count;   // 数组中数据元素个数
    public CharArrayWriter() {
        this(32);
    }
    public CharArrayWriter(int initialSize) {
        if (initialSize < 0) {
            throw new IllegalArgumentException("Negative initial size: " + initialSize);
        }
        buf = new char[initialSize];
    }

默认buf数组的大小为32，也可以自己进行指定。

CharArrayReader中定义的重要变量及构造函数：

    protected char buf[];         // 字符数组,用于数组的存取
    protected int pos;            // 当前读取的位置
    protected int markedPos = 0;  // 设置的标识位
    
    protected int count;          // 数组中数据元素个数

    public CharArrayReader(char buf[]) {
        this.buf = buf;
        this.pos = 0;
        this.count = buf.length;
    }

    public CharArrayReader(char buf[], int offset, int length) {
        if ((offset < 0) || (offset > buf.length) || (length < 0) || ((offset + length) < 0)) {
            throw new IllegalArgumentException();
        }
        this.buf = buf;
        this.pos = offset;
        this.count = Math.min(offset + length, buf.length);
        this.markedPos = offset;
    }

2、CharArrayWritrer写入数据

    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;
        }
    }

    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;
        }
    }

    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;
        }
    }

实现了Write类中定义的write()方法和append()方法。其实现代码还是比较简单的，在前面类似的方法已经说过多遍了，这里不再赘述。

3、CharArrayReader读取数据

CharArrayReader 是用于读取字符数组，它继承于Reader。操作的数据是以字符为单位。下面来看一下主要方法实现的源代码：

    public int read() throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (pos >= count)
                return -1;
            else
                return buf[pos++];
        }
    }

    public int read(char b[], int off, int len) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if ((off < 0) || (off > b.length) || (len < 0) ||
                ((off + len) > b.length) || ((off + len) < 0)) {
                throw new IndexOutOfBoundsException();
            } else if (len == 0) {
                return 0;
            }

            if (pos >= count) {
                return -1;
            }
            if (pos + len > count) {
                len = count - pos;
            }
            if (len <= 0) {
                return 0;
            }
            System.arraycopy(buf, pos, b, off, len);
            pos += len;
            return len;
        }
    }

与操作字符串的类StringReader、StringWriter类似，其实字符数组与字符串操作有相通性，字符串在底层实际上就是由字符数组来实现的。如果理解了StringReader和StringWriter类，这个类会非常容易理解。另外提醒一点，这两个类也是线程安全的。

下面来编写一个简单的测试程序：

       char[] x={'a','d','p'};
       CharArrayWriter cw=new CharArrayWriter();
       cw.write(x,0,2);
       cw.append("x");
       System.out.println(cw.toString()); // adx
       
       CharArrayReader cr=new CharArrayReader(cw.toCharArray());
       System.out.println((char)cr.read());// a