DataOutputStream in Java | Methods, Example

DataOutputStream in Java is a filter output stream that provides methods for writing Java’s standard data types.

It enables you conveniently to write strings and all primitive data types such as boolean, int, float, long, etc to a stream.

Java DataOutputStream first converts primitive-type values or strings into bytes and then writes them to the underlying output stream in an appropriate way.

Using data input stream, we can then read the data back in. Thus, DataOutputStream works as wrappers on the existing output stream to filter data in the original stream.

Java DataOutputStream class declaration


DataOutputStream class extends FilterOutputStream class that extends OutputStream. It implements the interface DataOutput to use methods defined in the DataOutput interface.

The general declaration for DataOutputStream class in Java is given below:

public class DataOutputStream
   extends FilterOutputStream
      implements DataOutput

DataOutputStream class also implements Closeable, Flushable, and AutoCloseable interfaces. It was added in Java 1.0 version. It is present in the java.io.DataOutputStream package.

Constructor of DataOutputStream class


DataOutputStream class defines only a single constructor in Java that is as follows:

1. DataOutputStream​(OutputStream outputStream): This constructor creates a DataOutputStream object that uses the specified underlying OutputStream. Here, outputStream defines the output stream to which data will be written.


A data output stream object for output can be created as follows:

FileOutputStream fos = new FileOutputStream(String filename);
DataOutputStream dos = new DataOutputStream(fos);

These two statements basically wrap dos on fos and use it as a filter. Look at the figure below.

DataOutputStream in Java

To append data to an existing file, use the following syntax:

FileOutputStream fos = new FileOutputStream(new File(String filename), true)
DataOutputStream dos = new DataOutputStream(fos);
or,
FileOutputStream fos = new FileOutputStream(String filename, true);
DataOutputStream dos = new DataOutputStream(fos);

DataOutputStream Methods in Java


In addition to methods inherited by OutputStream and FilterOutputStream superclasses, DataOutputStream class uses also methods defined by DataOutput interface that make it unique.

These methods convert values of a primitive data type into a byte sequence and then writes it to the underlying stream.


A list of important methods provided by DataOutput interface is as follows:

Method Description
1. void writeBoolean(boolean b): This method writes a boolean to the output stream as a 1-byte value.
2. void writeByte(int v): This method writes out a byte to the output stream as a 1-byte value.
3. void writeBytes(String s): This method writes the lower byte of characters in a string to the underlying output stream as a sequence of bytes.
4. void writeChar(char c): This method writes a character composed of 2 bytes to the underlying output stream, high byte first.
5. void writeChars(String s): This method writes a sequence of characters in a string s to the underlying output stream, 2 bytes per character.
6. void writeDouble(double v): This method writes a double value to the underlying output stream.
7. void writeFloat(float v): This method writes a float value to the underlying output stream.
8. void writeInt(int v): It writes an int value to the underlying output stream.
9. void writeLong(long v): This method writes a long value to the underlying output stream.
10. void writeShort(int v): This method writes a short value to the underlying output stream.
11. void writeUTF(String str): This method writes a string to the underlying output stream using UTF-8 format.

All of these methods can throw an exception named IOException if any error occurs.

Java DataOutputStream Example Program


1. Let’s take a simple example program where we will perform reading and writing operations using DataInputStream and DataOutputStream. Look at the following source code below.

Program source code 1:

package javaProgram;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
public class DataOutputStreamEx {
public static void main(String[] args) throws IOException
{
 String filepath = "D:\\myfileout.dat";
// Create a FileOutputStream object to connect with myfileout.dat file.
    FileOutputStream fos = new FileOutputStream(filepath);
  
// Create a DataOutputStream object to wrap on fos.  
    DataOutputStream dos = new DataOutputStream(fos);

// Write following primitive data to the "myfileout.dat" file.
      dos.writeUTF("Welcome to Scientech Easy");
      dos.writeInt(124);
      dos.writeDouble(125.25);
      dos.writeBoolean(true);
      dos.writeChar('A');
   
  dos.close();
  fos.close();
      
// Read data from the "myfileout.dat" file.
     FileInputStream fis = new FileInputStream(filepath);
     DataInputStream dis = new DataInputStream(fis);
   
  System.out.println(dis.readUTF()); 
  System.out.println(dis.readInt());
  System.out.println(dis.readDouble());
  System.out.println(dis.readBoolean());
  System.out.println(dis.readChar());
  
  dis.close();
  fis.close();
 }
}
Output:
           Welcome to Scientech Easy
           124
           125.25
           true
           A

This program first creates “myfileout.dat” file on the mentioned filepath and then writes the string and primitive data types into it using data output stream. At the end of writing, streams are closed using close() method.

Now the program also constructs a data input stream object and connects it to “myfileout.dat” file. It then reads the following data from the file and displays them on the console. At last, it closes the streams.

Note: The main method declares that it throws an exception named IOException. Therefore, we do not use Java try-catch block.


Hope that this tutorial has covered all the important points related to DataOutputStream class in Java with example program. I hope that you will have understood all the basic points of data output stream.
Thanks for reading!!!
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