DataOutputStream in Java

The DataOutputStream in Java used to write data in primitive type size bytes from the underlying OutputStreams. It has special methods to write primitive type size bytes.

It is a subclass of FilterOutputStream and implements the DataOutput interface. FilterOutputStream is the subclass of OutputStream. These all classes are available from Java 1.0 version onwards. OutputStream class is used to write data in binary form. DataOutput interface is used for converting data from any of the Java primitive types to a stream of bytes and writing these bytes to a binary output stream. Other implemented interfaces are:- Closeable, AutoCloseable, Flushable

public class DataOutputStream
extends FilterOutputStream
implements DataOutput

Constructors of DataOutputStream

The FileOutputStream is a basic source for all output stream classes which can directly make a connection between Java application and file. Similar to all other output stream classes, DataOutputStream class also depends on the FileOutputStream class to make a connection between Java application and file. DataOutputStream class has only one constructor which needs OnputStream as a parameter.

public DataOutputStream(OutputStream out) {

Java DataOutputStream class Methods

The writeXxx() methods are given the write the different primitive type values. Since all methods are giving for writing purposes so it doesn’t return any value and the return type is void. These all writeXxx() methods throw IOException if I/O error occurs. Methods to write different primitive types of values,

writeByte(int v)Write a byte as a 1-byte value.
writeShort(int v)Write a short as 2 bytes value, high byte first.
writeInt(int v)It writes an int as 4 bytes value, high byte first.
writeLong(long v)Write long as 8 bytes value, high byte first.
writeFloat(float v)Converts float to int using the floatToIntBits method in class Float,
and then writes int value as 4 bytes quantity, high byte first.
writeDouble(double v)Converts double to long using the doubleToLongBits method in class
Double, and then writes long value as an 8 bytes quantity, high byte first.
writeChar(int v)Write a char as 2 bytes value, high byte first.
writeBoolean(boolean v)Write a boolean as a 1-byte value.
writeUTF(String str)It writes a string using modified UTF-8 encoding in a
machine-independent manner.
writeBytes(String s)Write string as a sequence of bytes.
writeChars(String s)Write string as a sequence of bytes.

The writeUTF(String str) method also throws UTFDataFormatException if the modified UTF-8 encoding of passed string would exceed 65535 bytes in length.

Other methods of DataInputStream are given below.

Return TypeMethodDescription
voidflush()Flushes data from output stream
intsize()Returns the current value of the counter written, the number of bytes written to this data output stream so far.
voidwrite(int b)Writes the specified byte
voidwrite(byte[] b, int off, int len)Writes len bytes from the given byte array starting at offset off to the underlying output stream.

Among these methods except size(), all other methods throws IOException when I/O error occurs.

Note:- In FilterOutputStream classes, close() method itself calling flush() method. DataOutputStream class uses close() method from FilterOutputStream.

Java program example using DataOutputStream

Below application demonstrates writing data in primitive type size bytes using DataOutputStream class.


public class DOSTest{
   public static void main(String[] args) 
          throws IOException, FileNotFoundException{

      // create FileOutputStream object
      // constructor for overidden mode
      FileOutputStream fos = new FileOutputStream("output.txt");

      // constructor for append mode
      /* FileOutputStream fos = 
           new FileOutputStream("output.txt", true); */

      // create DataOutputStream object
      DataOutputStream dos = new DataOutputStream(fos);

      // write different types of primitive data
      dos.writeInt(97); // int
      System.out.print(dos.size() + " ");

      dos.writeLong(100L); // long
      System.out.print(dos.size() + " ");

      dos.writeFloat(99.9f); // float
      System.out.print(dos.size() + " ");

      dos.writeDouble(95.5); // double
      System.out.print(dos.size() + " ");

      dos.writeChar('a'); // char
      System.out.print(dos.size() + " ");

      dos.writeBoolean(true); // boolean
      System.out.print(dos.size() + " ");

      dos.writeUTF(""); // string
      System.out.print(dos.size() + " ");

      dos.writeBytes("Hello, world!"); // string
      System.out.print(dos.size() + " ");

      dos.writeChars("Java program"); // string
      System.out.print(dos.size() + " ");

      dos.write(97); // normal write() method

      // display some message for conformation
      System.out.println("Data is saved to the file");

      // close stream


4 12 16 24 26 27 44 57 81 82
Data is saved to the file

After program execution, data in output.txt file,

0000 0061 0000 0000 0000 0064 42c7 cccd
4057 e000 0000 0000 0061 0100 0f6b 6e6f
7770 726f 6772 616d 2e63 6f6d 4865 6c6c
6f2c 2077 6f72 6c64 2100 4a00 6100 7600
6100 2000 7000 7200 6f00 6700 7200 6100

Since we are using one parameter constructor of FileOutputStream which by default creates a connection in override so if we execute the same application multiple times then every time data will be overridden to the file. To append data in the file, use a two-parameter constructor, and pass true as the second parameter value.

In this program total of 82 bytes have been written to the file, so the file size is 82 bytes. You can check it. Note that when we call writeInt(97) then it takes 4 bytes but at last when we call write(97) then it only takes 1 byte because 97 belongs to byte-range so it can be treated as byte data type, compiler/JVM itself converts them.

From this program, we can say boolean takes 1 byte, and the minimum memory location size in Java is 1 byte. For storing data including double quotes (“”) JVM provides 1 byte for each character to it in a file.

Important Points

One value can be stored using different data types and they require different byte sizes. For example, we can store 97 value in different formats byte, short, int, long, float, and double, which requires 1, 2, 4, 8, 4 and 8 byte size respectively. While reading these values using DataInputStream, we must read that particular byte only else we will get unexpected results.

dos.writeByte(97); // a
dos.writeShort(97); // 0061
dos.writeInt(97); // 0000 0061
dos.writeLong(97); // 0000 0000 0000 0061
dos.writeFloat(97); // 42c2 0000
dos.writeDouble(97); // 4058 4000 0000 0000
dos.writeChar(97); // 0061
dos.writeBoolean(97); // error
dos.writeUTF(97); // error


Let us see limitations through an example, If the data is written as int type i.e. only 4 bytes then we can’t read it using readLong() which needs 8 bytes, because 8 bytes are not available. Hence it will throw EOFException. EOF => End-Of-File.

While working with DataInputStream, and DataOutputStream classes whatever writeXxx() method called in the same order readXxx() method must be called. If the order is not the same then we will get either the wrong result or EOFException. We have to first write the data using DataOutputStream then only we can read them using DataInputStream class.

Without writing primitive data in binary format using DataOutputStream class, we can’t call readXxx() methods of DataInputStream class. Both classes must be used in combination.

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