// 構造函數
BufferedWriter(Writer out) 
BufferedWriter(Writer out, int sz) 
void  close()               // 關閉此流，但要先刷新它。
void  flush()               // 刷新該流的緩沖。
void  newLine()              // 寫入一個行分隔符。
void  write(char[] cbuf, int off, int len) // 寫入字符數組的某一部分。
void  write(int c)             // 寫入單個字符。
void  write(String s, int off, int len)  // 寫入字符串的某一部分。 

 package java.io;
  public class BufferedWriter extends Writer {
    // 輸出流對象
    private Writer out;
    // 保存“緩沖輸出流”數據的字符數組
    private char cb[];
   // nChars 是cb緩沖區中字符的總的個數
   // nextChar 是下一個要讀取的字符在cb緩沖區中的位置
   private int nChars, nextChar;
   // 默認字符緩沖區大小
   private static int defaultCharBufferSize = ;
   // 行分割符
   private String lineSeparator;
   // 構造函數，傳入“Writer對象”，默認緩沖區大小是k
   public BufferedWriter(Writer out) {
     this(out, defaultCharBufferSize);
   }
   // 構造函數，傳入“Writer對象”，指定緩沖區大小是sz
   public BufferedWriter(Writer out, int sz) {
     super(out);
    if (sz <= 0)
       throw new IllegalArgumentException("Buffer size <= ");
     this.out = out;
     cb = new char[sz];
     nChars = sz;
     nextChar = 0;
     lineSeparator = java.security.AccessController.doPrivileged(
       new sun.security.action.GetPropertyAction("line.separator"));
   }
   // 確保“BufferedWriter”是打開狀態
   private void ensureOpen() throws IOException {
     if (out == null)
       throw new IOException("Stream closed");
   }
   // 對緩沖區執行flush()操作，將緩沖區的數據寫入到Writer中
   void flushBuffer() throws IOException {
     synchronized (lock) {
       ensureOpen();
      if (nextChar == 0)
         return;
       out.write(cb, 0, nextChar);
       nextChar = 0;
     }
   }
   // 將c寫入到緩沖區中。先將c轉換為char，然后將其寫入到緩沖區。
   public void write(int c) throws IOException {
     synchronized (lock) {
       ensureOpen();
       // 若緩沖區滿了，則清空緩沖，將緩沖數據寫入到輸出流中。
       if (nextChar >= nChars)
         flushBuffer();
       cb[nextChar++] = (char) c;
     }
   }
   // 返回a，b中較小的數
   private int min(int a, int b) {
     if (a < b) return a;
     return b;
   }
   // 將字符數組cbuf寫入到緩沖中，從cbuf的off位置開始寫入，寫入長度是len。
   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();
       }
     }
   }
   // 將字符串s寫入到緩沖中，從s的off位置開始寫入，寫入長度是len。
   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();
       }
     }
   }
   // 將換行符寫入到緩沖中
   public void newLine() throws IOException {
     write(lineSeparator);
   }
   // 清空緩沖區數據
   public void flush() throws IOException {
     synchronized (lock) {
       flushBuffer();
       out.flush();
     }
   }
   public void close() throws IOException {
     synchronized (lock) {
       if (out == null) {
         return;
       }
       try {
         flushBuffer();
       } finally {
         out.close();
         out = null;
         cb = null;
       }
     }
   }
 }

import java.io.BufferedWriter;
 import java.io.File;
 import java.io.OutputStream;
 import java.io.FileWriter;
 import java.io.IOException;
 import java.io.FileNotFoundException;
 import java.lang.SecurityException;
 import java.util.Scanner;
 /**
 * BufferedWriter 測試程序
 *
 * 
 */
 public class BufferedWriterTest {
  private static final int LEN = 5;
   // 對應英文字母“abcdefghijklmnopqrstuvwxyz”
   //private static final char[] ArrayLetters = "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) {
     testBufferedWriter() ;
   }
   /**
   * BufferedWriter的API測試函數
   */
   private static void testBufferedWriter() {
     // 創建“文件輸出流”對應的BufferedWriter
     // 它對應緩沖區的大小是16，即緩沖區的數據>=16時，會自動將緩沖區的內容寫入到輸出流。
     try {
       File file = new File("bufferwriter.txt");
       BufferedWriter out =
          new BufferedWriter(
            new FileWriter(file));
      // 將ArrayLetters數組的前10個字符寫入到輸出流中
      out.write(ArrayLetters, 0, 10);
       // 將“換行符\n”寫入到輸出流中
       out.write('\n');
       out.flush();
       //readUserInput() ;
       out.close();
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (SecurityException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
   }
   /**
   * 讀取用戶輸入
   */
   private static void readUserInput() {
     System.out.println("please input a text:");
     Scanner reader=new Scanner(System.in);
     // 等待一個輸入
     String str = reader.next();
     System.out.printf("the input is : %s\n", str);
   }
 }