When to use flush() in Java?

When to use flush() in JAVA?When usage of that is necessary?

It probably varies from class to class. Here is an example of flush(). What happens is that a buffer might be full, with unread data still in it Flushing that moves all the data out into its destination, emptying the stream or whatever.
You don't usually have to call flush() if you call close().

Here's one concrete example.

Let's say you have a BufferedOutputStream wrapped around a Socket's OutputStream. You want to send a load of data, but you don't want to close the connection yet.
After you're done writing everything you want to send, some data may still be in the buffer. Now you will have to flush the buffer, to force it to send all the data.

Thanks stephan, Nice example!

You don't usually have to call flush() if you call close().

Do you mean by using close(), buffer will be empty?

close() calls flush().

import java.io.*;

public class Main {

static int[] prices = { 4500, 3000, 2500 };
    static String[] books = { "JAVA","PHP","Mysql"};
    public static void main(String[] args) throws IOException {
        // TODO code application logic here

DataOutputStream out=null;
      out = new DataOutputStream(new BufferedOutputStream(new FileOutputStream("books.dat")));

for (int i = 0; i < prices.length; i ++) {

out.writeInt(prices[i]);
           out.writeUTF(books[i]);
           out.flush();
          }

DataInputStream in=null;
      in = new DataInputStream(new BufferedInputStream(new FileInputStream("books.dat")));
      int price;
      String book;

try {
          //boolean eof = false;
          while (in.available() != 0) {
              //if (!in.ready()) break;

price = in.readInt();
                     book= in.readUTF();
                     System.out.format(" %s  %d%n",book, price);
                    
                     //if (in == -1)
                          //eof = true;

}
       }
      catch (EOFException e) {

e.printStackTrace();
      }

}
   }

If I don't use flush in this code, It does not print anything.
If I use close, It only prints JAVA.
Only If i use flush, It works well. Why?

Were you calling close() inside the loop? If so, try calling it after the loop.

package fiedata;