关于PrintWriter 为啥自带缓冲还要写成PrintWriter = new PrintWriter(new BufferedWriter(new FileWriter("foo.out")))

个人理解：

看了下源码

  public PrintWriter (Writer out) {
        this(out, false);
    }

    /**
     * Creates a new PrintWriter.
     *
     * @param  out        A character-output stream
     * @param  autoFlush  A boolean; if true, the <tt>println</tt>,
     *                    <tt>printf</tt>, or <tt>format</tt> methods will
     *                    flush the output buffer
     */
    public PrintWriter(Writer out,
                       boolean autoFlush) {
        super(out);
        this.out = out;
        this.autoFlush = autoFlush;
        lineSeparator = java.security.AccessController.doPrivileged(
            new sun.security.action.GetPropertyAction("line.separator"));
    }

也就是说参数是BufferedWriter 才会有缓冲 节点流是没有的 这也是为啥BufferedWriter   api介绍到

常 Writer 将其输出立即发送到底层字符或字节流。除非要求提示输出，否则建议用 BufferedWriter 包装所有其 write() 操作可能开销很高的 Writer（如 FileWriters 和 OutputStreamWriters）。例如，

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

将缓冲 PrintWriter 对文件的输出。如果没有缓冲，则每次调用 print() 方法会导致将字符转换为字节，然后立即写入到文件，而这是极其低效的。

再看源码

 public PrintWriter(File file) throws FileNotFoundException {
        this(new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file))),
             false);
    }

public PrintWriter(OutputStream out) {
        this(out, false);
    }

public PrintWriter(OutputStream out, boolean autoFlush) {
        this(new BufferedWriter(new OutputStreamWriter(out)), autoFlush);

        // save print stream for error propagation
        if (out instanceof java.io.PrintStream) {
            psOut = (PrintStream) out;
        }
    }

最终都是把形参包装成BuferedWriter然后调用最上面的构造函数