What is the default buffer size if we dont specify in buffer write?

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If we dont specify what is the default buffer size in BufferedWriter. How to increase/decrease the size of it?
What is the purpose of flush?
If flush() is not used, only partial content is written to the file. Is it because of the default size of the buffer.

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THis is the bufferedwriter class, it helps to look at them, look at the bold underlined, thats answers your defualt buffer size
* @(#)BufferedWriter.java     1.26 03/12/19
* Copyright 2004 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
package java.io;
* Write text to a character-output stream, buffering characters so as to
* provide for the efficient writing of single characters, arrays, and strings.
* <p> The buffer size may be specified, or the default size may be accepted.
* The default is large enough for most purposes.
* <p> A newLine() method is provided, which uses the platform's own notion of
* line separator as defined by the system property <tt>line.separator</tt>.
* Not all platforms use the newline character ('\n') to terminate lines.
* Calling this method to terminate each output line is therefore preferred to
* writing a newline character directly.
* <p> In general, a Writer sends its output immediately to the underlying
* character or byte stream. Unless prompt output is required, it is advisable
* to wrap a BufferedWriter around any Writer whose write() operations may be
* costly, such as FileWriters and OutputStreamWriters. For example,
* <pre>
* PrintWriter out
* = new PrintWriter(new BufferedWriter(new FileWriter("foo.out")));
* </pre>
* will buffer the PrintWriter's output to the file. Without buffering, each
* invocation of a print() method would cause characters to be converted into
* bytes that would then be written immediately to the file, which can be very
* inefficient.
* @see PrintWriter
* @see FileWriter
* @see OutputStreamWriter
* @version      1.26, 03/12/19
* @author     Mark Reinhold
* @since     JDK1.1
public class BufferedWriter extends Writer {
private Writer out;
private char cb[];
private int nChars, nextChar;
private static int defaultCharBufferSize = 8192;
* Line separator string. This is the value of the line.separator
* property at the moment that the stream was created.
private String lineSeparator;
* Create a buffered character-output stream that uses a default-sized
* output buffer.
* @param out A Writer
*public BufferedWriter(Writer out) {*
*     this(out, defaultCharBufferSize);*
* Create a new buffered character-output stream that uses an output
* buffer of the given size.
* @param out A Writer
* @param sz Output-buffer size, a positive integer
* @exception IllegalArgumentException If sz is <= 0
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 =     (String) java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction("line.separator"));
/** Check to make sure that the stream has not been closed */
private void ensureOpen() throws IOException {
     if (out == null)
     throw new IOException("Stream closed");
* Flush the output buffer to the underlying character stream, without
* flushing the stream itself. This method is non-private only so that it
* may be invoked by PrintStream.
void flushBuffer() throws IOException {
     synchronized (lock) {
     ensureOpen();
     if (nextChar == 0)
          return;
     out.write(cb, 0, nextChar);
     nextChar = 0;
* Write a single character.
* @exception IOException If an I/O error occurs
public void write(int c) throws IOException {
     synchronized (lock) {
     ensureOpen();
     if (nextChar >= nChars)
          flushBuffer();
     cb[nextChar++] = (char) c;
* Our own little min method, to avoid loading java.lang.Math if we've run
* out of file descriptors and we're trying to print a stack trace.
private int min(int a, int b) {
     if (a < b) return a;
     return b;
* Write a portion of an array of characters.
* <p> Ordinarily this method stores characters from the given array into
* this stream's buffer, flushing the buffer to the underlying stream as
* needed. If the requested length is at least as large as the buffer,
* however, then this method will flush the buffer and write the characters
* directly to the underlying stream. Thus redundant
* <code>BufferedWriter</code>s will not copy data unnecessarily.
* @param cbuf A character array
* @param off Offset from which to start reading characters
* @param len Number of characters to write
* @exception IOException If an I/O error occurs
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();
* Write a portion of a String.
* <p> If the value of the <tt>len</tt> parameter is negative then no
* characters are written. This is contrary to the specification of this
* method in the {@linkplain java.io.Writer#write(java.lang.String,int,int)
* superclass}, which requires that an {@link IndexOutOfBoundsException} be
* thrown.
* @param s String to be written
* @param off Offset from which to start reading characters
* @param len Number of characters to be written
* @exception IOException If an I/O error occurs
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();
* Write a line separator. The line separator string is defined by the
* system property <tt>line.separator</tt>, and is not necessarily a single
* newline ('\n') character.
* @exception IOException If an I/O error occurs
public void newLine() throws IOException {
     write(lineSeparator);
* Flush the stream.
* @exception IOException If an I/O error occurs
public void flush() throws IOException {
     synchronized (lock) {
     flushBuffer();
     out.flush();
* Close the stream.
* @exception IOException If an I/O error occurs
public void close() throws IOException {
     synchronized (lock) {
     if (out == null)
          return;
     flushBuffer();
     out.close();
     out = null;
     cb = null;
What Flush(); does
Example, you have a file called c, your writer is b and buffereredwriter is a. so your programs calls a, a talks to b, and b talks to c. when you call the Flush method, the information is sent to the outfile which is c immediately before you even close the file, because when you write to the file, it does not write directly, it writes to a buffer, so flush actually causes the buffer to write to file. Also if you call the close method on that file without the flush, the buffer will still get flushed.
consider BufferedWriter c = new BufferedWriter(new PrintWriter("c:\\c"));
you wrap printwriter into a buffered writer, now if you close this "connection" to the file, the buffer will get flushed, noting that all the data is sitting in the buffered and not yet in the file, and this happens if something dont break...

November 30, -0001

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