下面是java se 7 API 對於java.io.BufferedWriter 繼承關系的描述。

BufferedWriter可以將文本寫入字符流。它會將字符緩存，目的是提高寫入字符的效率。

buffer的大小必須明確，否則將會使用默認的大小。默認的大小對於大多數情況是足夠大的。

BufferedWriter提供了一個newLine()的方法，目的是用來換行。畢竟不是所有的平臺都使用‘\n‘的換行方式。

一個Writer對象會將輸出立即寫入當前的字符流或者字節流。

通常來說，如果這個寫入不是非常緊急，建議使用BufferedWriter對象包裹一個費時的Writer對象（比如，FileWriters，OutputStreamWriters）

具體例子如下：

PrintWriter out = new PrintWriter(new BufferWriter(new FileWriter("foo.out")));

上面這個例子，會將PrintWriter對象的輸出緩存到文件。如果沒有緩存，每次調用print()方法都會講要輸出的字符立即轉換為字節，這樣的話寫入的效率非常不高。

————————————————————————————————————————————————————————————————————

API中已經說明了BufferedWriter可以提高寫入字符流的效率。對於耗時的寫入操作，使用BufferWriter對象是非常明智的選擇。

下面是BufferedWriter的源碼。

package java.io;

public class BufferedWriter extends Writer {

    private Writer out;

    private char cb[];
    private int nChars, nextChar;

    private static int defaultCharBufferSize = 8192;

    private String lineSeparator;

    public BufferedWriter(Writer out) {
        
 
this(out, defaultCharBufferSize);
    }

    public BufferedWriter(Writer out, int sz) {
        super(out);
        if (sz <= 0)
            throw new IllegalArgumentException("Buffer size <= 0");
        this.out = out;
        cb = new char[sz];
        nChars = sz;
        nextChar = 0;

        lineSeparator = java.security.AccessController.doPrivileged(
            new sun.security.action.GetPropertyAction("line.separator"));
    }

    private void ensureOpen() throws IOException {
        if (out == null)
            throw new IOException("Stream closed");
    }

    void flushBuffer() throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (nextChar == 0)
                return;
            out.write(cb, 0, nextChar);
            nextChar = 0;
        }
    }

    
    public void write(int c) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (nextChar >= nChars)
                flushBuffer();
            cb[nextChar++] = (char) c;
        }
    }


    private int min(int a, int b) {
        if (a < b) return a;
        return b;
    }

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

            if (len >= nChars) {
                /* If the request length exceeds the size of the output buffer,
                   flush the buffer and then write the data directly.  In this
                   way buffered streams will cascade harmlessly. */
                flushBuffer();
                out.write(cbuf, off, len);
                return;
            }

            int b = off, t = off + len;
            while (b < t) {
                int d = min(nChars - nextChar, t - b);
                System.arraycopy(cbuf, b, cb, nextChar, d);
                b += d;
                nextChar += d;
                if (nextChar >= nChars)
                    flushBuffer();
            }
        }
    }

   
    public void write(String s, int off, int len) throws IOException {
        synchronized (lock) {
            ensureOpen();

            int b = off, t = off + len;
            while (b < t) {
                int d = min(nChars - nextChar, t - b);
                s.getChars(b, b + d, cb, nextChar);
                b += d;
                nextChar += d;
                if (nextChar >= nChars)
                    flushBuffer();
            }
        }
    }

   
    public void newLine() throws IOException {
        write(lineSeparator);
    }

    
    public void flush() throws IOException {
        synchronized (lock) {
            flushBuffer();
            out.flush();
        }
    }

    @SuppressWarnings("try")
    public void close() throws IOException {
        synchronized (lock) {
            if (out == null) {
                return;
            }
            try (Writer w = out) {
                flushBuffer();
            } finally {
                out = null;
                cb = null;
            }
        }
    }
}

好像沒有新穎之處。。。。。應該是我道行太太太淺，看不出= =

可以看到BufferedWriter之所以叫做buffered，是因為，裏面使用了一個char 類型的數組作為緩存（private char cb[];）。

寫操作的時候不會立刻寫入當前的字符流，而是先寫入緩存區。

java.io.BufferedWriter API 以及源碼解讀