Java - DataOutputStream flush() method

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Description

The Java DataOutputStream flush() method forces bytes to be written to the underlying stream. This method calls the flush method of its underlying output stream.

Declaration

Following is the declaration for java.io.DataOutputStream.flush() method −

public void flush()

Parameters

NA

Return Value

This method does not return any value.

Exception

IOException − If an I/O error occurs.

Example 1

The following example shows the usage of Java DataInputStream flush() method. We've created InputStream, DataInputStream, FileOutputStream and DataOutputStream reference. A String[] buf is initialized with some string values. A FileOutputStream object is created with a File. Then DataOutputStream is initialized with FileOutputStream object created before. Then string array is iterated to write string values to the dataoutputstream.

Once string arrays is fully written into the stream, we've flush the stream to store the values in the file using flush() method. Now using FileInputStream and DataInputStream, we're reading the file written earlier. Now we're checking if DataInputStream object has data using available() method. Then using readUtf() method, we're reading every value as String. Finally we're closing all the streams.

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

public class DataOutputStreamDemo {
   public static void main(String[] args) throws IOException {
      InputStream is = null;
      DataInputStream dis = null;
      FileOutputStream fos = null;
      DataOutputStream dos = null;
      String[] buf = {"Hello", "World!!"};
      
      try {
         // create file output stream
         fos = new FileOutputStream("F:\\test.txt");
         
         // create data output stream
         dos = new DataOutputStream(fos);
         
         // for each string in the buffer
         for (String j:buf) {
         
            // write string encoded as modified UTF-8
            dos.writeUTF(j);        
         }
         
         // force data to the underlying stream
         dos.flush();
         
         // create file input stream
         is = new FileInputStream("F:\\test.txt");
         
         // create new data input stream
         dis = new DataInputStream(is);
         
         // read till end of the stream
         while(dis.available()>0) {
         
            // reads characters encoded with modified UTF-8
            String k = dis.readUTF();
            
            // print
            System.out.print(k+" ");
         }
         
      } catch(Exception e) {
         // if any I/O error occurs
         e.printStackTrace();
      } finally {
         // releases all system resources from the streams
         if(is!=null)
            is.close();
         if(dos!=null)
            is.close();
         if(dis!=null)
            dis.close();
         if(fos!=null)
            fos.close();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Hello World!! 

Example 2

The following example shows the usage of Java DataInputStream flush() method. We've created InputStream, DataInputStream, FileOutputStream and DataOutputStream reference. A String[] buf is initialized with some String values. A FileOutputStream object is created with a File. Then DataOutputStream is initialized with FileOutputStream object created before. Then String array is iterated to write String values to the dataoutputstream.

Once String arrays is fully written into the stream, we're not flushing the stream to store the values in the file. Now using FileInputStream and DataInputStream, we're reading the file written earlier. Now we're checking if DataInputStream object has data using available() method. Then using readUtf() method, we're reading every value as String. Now as a special case, we're closing the stream before reading the values to see if this methods throw exception or not. As a result, we can see the available() method throws the exception.

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

public class DataInputStreamDemo {
   public static void main(String[] args) throws IOException {
      InputStream is = null;
      DataInputStream dis = null;
      FileOutputStream fos = null;
      DataOutputStream dos = null;
      String[] buf = {"Hello", "World!!"};
      
      try {
         // create file output stream
         fos = new FileOutputStream("F:\\test.txt");
         
         // create data output stream
         dos = new DataOutputStream(fos);
         
         // for each String in the buffer
         for (String j:buf) {
         
            // write string encoded as modified UTF-8
            dos.writeUTF(j);         
         }
         
         // do not force Strings to the underlying stream
         // dos.flush();
         
         // create file input stream
         is = new FileInputStream("F:\\test.txt");
         
         // create new data input stream
         dis = new DataInputStream(is);
         
         // read till end of the stream
         while(dis.available()>0) {
         
            // reads characters encoded with modified UTF-8
            String k = dis.readUTF();
            
            // print
            System.out.print(k+" ");
         }
         
      } catch(Exception e) {
         // if any I/O error occurs
         e.printStackTrace();
      } finally {
         // releases all system resources from the streams
         if(is!=null)
            is.close();
         if(dos!=null)
            is.close();
         if(dis!=null)
            dis.close();
         if(fos!=null)
            fos.close();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Hello World!!

The flush() method flushes the stream. If the stream has saved any characters from the various write() methods in a buffer, write them immediately to their intended destination. Then, if that destination is another character or byte stream, flush it. Thus one flush() invocation will flush all the buffers in a chain of Writers and OutputStreams. So it is not necessary to use flush() to reflect the changes but it is a good practice to use flush() method to clear the buffers.