Java - DataOutputStream Class

Introduction

The Java.io.DataOutputStream class lets an application write primitive Java data types to an output stream in a portable way. An application can then use a data input stream to read the data back in.

Class declaration

Following is the declaration for Java.io.DataOutputStream class −

public class DataOutputStream
   extends FilterOutputStream
      implements DataOutput

Field

Following are the fields for Java.io.DataOutputStream class −

• protected int written − This is the number of bytes written to the data output stream so far.

• protected OutputStream out − This is the underlying output stream to be filtered.

Class constructors

Class methods

Methods inherited

This class inherits methods from the following classes −

• Java.io.FilterOutputStream
• Java.io.Object
• Java.io.DataOutput

Example

Following is an example to demonstrate DataInputStream and DataOutputStream. This example write a string to a file test.txt using DataOutputStream.writeUTF() method and then reads the same using DataInputStream.readUTF() method.

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("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("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

Hello World!! 

Example - Writing a Subarray of Bytes to a File

The following example shows the usage of Java DataOutputStream write(byte[] b, int off, int len) method.

DataOutputStreamDemo.java

package com.tutorialspoint;

import java.io.DataOutputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class DataOutputStreamDemo {
   public static void main(String[] args) {
      try {
         // Create a DataOutputStream to write to a file
         FileOutputStream fileOutput = new FileOutputStream("output.dat");
         DataOutputStream dataOutput = new DataOutputStream(fileOutput);

         // Byte array containing ASCII values of 'Hello, World!'
         byte[] message = "Hello, World!".getBytes();

         // Write only "World!" (start at index 7, length 6)
         dataOutput.write(message, 7, 6);

         // Close stream
         dataOutput.close();
         System.out.println("Data successfully written to output.dat");

      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

Data successfully written to output.dat

Explanation

• Create a file output stream (FileOutputStream) and wrap it with DataOutputStream.

• Convert a string ("Hello, World!") to a byte array using .getBytes().

• Write only "World!" from the byte array:

  • Offset (7) − Starts from 'W'

  • Length (6) − Writes 'World!'

• File (output.dat) will contain: World!

• Close the stream to save data properly.

Example - Writing and Reading a Boolean to/from a File

The following example shows the usage of Java DataOutputStream writeBoolean(boolean v) method.

DataOutputStreamDemo.java

package com.tutorialspoint;

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

public class DataOutputStreamDemo {
   public static void main(String[] args) {
      try {
         // Create a DataOutputStream to write to a file
         FileOutputStream fileOutput = new FileOutputStream("output.dat");
         DataOutputStream dataOutput = new DataOutputStream(fileOutput);

         // Write boolean values
         dataOutput.writeBoolean(true);
         dataOutput.writeBoolean(false);
         dataOutput.writeBoolean(true);

         // Close the output stream
         dataOutput.close();
         System.out.println("Boolean values successfully written to output.dat");

         // Read the boolean values back
         FileInputStream fileInput = new FileInputStream("output.dat");
         DataInputStream dataInput = new DataInputStream(fileInput);

         // Read and print the boolean values
         System.out.println("First boolean: " + dataInput.readBoolean());  // true
         System.out.println("Second boolean: " + dataInput.readBoolean()); // false
         System.out.println("Third boolean: " + dataInput.readBoolean());  // true

         // Close the input stream
         dataInput.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

Boolean values successfully written to output.dat
First boolean: true
Second boolean: false
Third boolean: true

Explanation

Writing Boolean Values

• FileOutputStream creates a file (booleanData.dat).

• DataOutputStream.writeBoolean(true) writes 1 (one byte).

• DataOutputStream.writeBoolean(false) writes 0 (one byte).

• DataOutputStream.writeBoolean(true) writes 1 (one byte).

• The file now contains { 1, 0, 1 }.

Reading Boolean Values

• FileInputStream opens the file.

• DataInputStream.readBoolean() reads each byte and converts it to true (1) or false (0).