Java DataOutputStream Tutorial with Examples

DataOutputStream
Constructors
Methods
Examples
writeBytes(String s)
writeChars(String s)
writeUTF(String str)
writeBoolean(boolean v)
writeByte(int v)
writeShort(int v)
writeChar(int v)
writeInt(int v)
writeLong(long v)
writeFloat(float v)
writeDouble(double v)
size()

1. DataOutputStream

DataOutputStream is commonly used to write primitive data to another OutputStream. An application then can use the DataInputStream to read the data back.

public class DataOutputStream extends FilterOutputStream implements DataOutput

DataOutputStream is recommended for writing tabular data, like Excel. (See the example in the middle of this article).

OrderDate	Finished	Item	Units	UnitCost	Total
2020-01-06		Pencil	95	1.99	189.05
2020-01-23		Binder	50	19.99	999.50
2020-02-09		Pencil	36	4.99	179.64
2020-02-26		Pen	27	19.99	539.73
2020-03-15		Pencil	56	2.99	167.44

ByteArrayOutputStream
FileOutputStream
FilterOutputStream
ObjectOutputStream
PipedOutputStream
BufferedOutputStream
OutputStream
PrintStream
CheckedOutputStream
CipherOutputStream
DeflaterOutputStream
DigestOutputStream
InflaterOutputStream

2. Constructors

public DataOutputStream(OutputStream out)

3. Methods

public final void writeBoolean(boolean v) throws IOException  
public final void writeByte(int v) throws IOException  
public final void writeShort(int v) throws IOException
public final void writeChar(int v) throws IOException  
public final void writeInt(int v) throws IOException  
public final void writeLong(long v) throws IOException  
public final void writeFloat(float v) throws IOException  
public final void writeDouble(double v) throws IOException  

public final void writeBytes(String s) throws IOException
public final void writeChars(String s) throws IOException  
public final void writeUTF(String str) throws IOException  

public final int size()

Other methods inherited from parent classes:

public static OutputStream nullOutputStream()  

public void write(int b) throws IOException
public void write(byte b[]) throws IOException  
public void write(byte b[], int off, int len) throws IOException  

public void flush() throws IOException  
public void close() throws IOException

4. Examples

DataOutputStream is commonly used to write structured data like Excel. Now, we will write the following data table to a file:

OrderDate	Finished	Item	Units	UnitCost	Total
2020-01-06		Pencil	95	1.99	189.05
2020-01-23		Binder	50	19.99	999.50
2020-02-09		Pencil	36	4.99	179.64
2020-02-26		Pen	27	19.99	539.73
2020-03-15		Pencil	56	2.99	167.44

First, write an Order class that simulates the single-row data of a table.

Order.java

package org.o7planning.dataoutputstream.ex;

import java.time.LocalDate;

public class Order {
    private LocalDate orderDate;
    private boolean finished;
    private String item;
    private int units;
    private float unitCost;
    private float total;

    public Order(LocalDate orderDate, boolean finished, //
            String item, int units, float unitCost, float total) {
        this.orderDate = orderDate;
        this.finished = finished;
        this.item = item;
        this.units = units;
        this.unitCost = unitCost;
        this.total = total;
    }
    public LocalDate getOrderDate() {
        return orderDate;
    }
    public boolean isFinished() {
        return finished;
    }
    public String getItem() {
        return item;
    }
    public int getUnits() {
        return units;
    }
    public float getUnitCost() {
        return unitCost;
    }
    public float getTotal() {
        return total;
    }
}

Next, use DataOutputStream to write the above data table to a file:

WriteDataFile_example1.java

package org.o7planning.dataoutputstream.ex;

import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.time.LocalDate;

public class WriteDataFile_example1 {

    // Windows: C:/somepath/data-file.txt
    private static final String filePath = "/Volumes/Data/test/data-file.txt";

    public static void main(String[] args) throws IOException {

        Order[] orders = new Order[] { //
                new Order(LocalDate.of(2020, 1, 6), true, "Pencil", 95, 1.99f, 189.05f),
                new Order(LocalDate.of(2020, 1, 23), false, "Binder", 50, 19.99f, 999.50f),
                new Order(LocalDate.of(2020, 2, 9), true, "Pencil", 36, 4.99f, 179.64f),
                new Order(LocalDate.of(2020, 2, 26), false, "Pen", 27, 19.99f, 539.73f),
                new Order(LocalDate.of(2020, 3, 15), true, "Pencil", 56, 2.99f, 167.44f) //
        };
        File outFile = new File(filePath);
        outFile.getParentFile().mkdirs();

        OutputStream outputStream = new FileOutputStream(outFile);
        DataOutputStream dataOutputStream = new DataOutputStream(outputStream);

        for (Order order : orders) {
            dataOutputStream.writeUTF(order.getOrderDate().toString());
            dataOutputStream.writeBoolean(order.isFinished());
            dataOutputStream.writeUTF(order.getItem());
            dataOutputStream.writeInt(order.getUnits());
            dataOutputStream.writeFloat(order.getUnitCost());
            dataOutputStream.writeFloat(order.getTotal());
        }
        dataOutputStream.close();
    }
}

After running the above example, we have a data file. Its content is quite confusing:

Finally, we use DataInputStream to read the above file.

ReadDataFile_example1.java

package org.o7planning.dataoutputstream.ex;

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

public class ReadDataFile_example1 {

    // Windows: C:/somepath/data-file.txt
    private static final String filePath = "/Volumes/Data/test/data-file.txt";

    public static void main(String[] args) throws IOException {

        File file = new File(filePath);

        InputStream inputStream = new FileInputStream(file);
        DataInputStream dataInputStream = new DataInputStream(inputStream);
        int row = 0;
        
        System.out.printf("|%3s | %-10s | %10s | %-15s | %8s| %10s | %10s |%n", //
                "No", "Order Date", "Finished?", "Item", "Units", "Unit Cost", "Total");
        
        System.out.printf("|%3s | %-10s | %10s | %-15s | %8s| %10s | %10s |%n", //
                "--", "---------", "----------", "----------", "------", "---------", "---------");

        while (dataInputStream.available() > 0) {
            row++;
            String orderDate = dataInputStream.readUTF();
            boolean finished = dataInputStream.readBoolean();
            String item = dataInputStream.readUTF();
            int units = dataInputStream.readInt();
            float unitCost = dataInputStream.readFloat();
            float total = dataInputStream.readFloat();

            System.out.printf("|%3d | %-10s | %10b | %-15s | %8d| %,10.2f | %,10.2f |%n", //
                               row, orderDate, finished, item, units, unitCost, total);

        }
        dataInputStream.close();
    }
}

Output:

5. writeBytes(String s)

Convert the given String to a sequence of characters, then cast each character to a byte and write to this DataOutputStream using the writeByte(byte) method.

public final void writeBytes(String s) throws IOException

Calling the writeBytes(String) method is equivalent to:

int len = s.length();
for (int i = 0 ; i < len ; i++) {
    out.write((byte)s.charAt(i));
}
incCount(len);

6. writeChars(String s)

Convert the given String to a sequence of characters, then write each character in turn to this DataOutputStream using the writeChar(char) method.

public final void writeChars(String s) throws IOException

Calling the writeChars(String) method is equivalent to:

int len = s.length();
for (int i = 0 ; i < len ; i++) {
    int v = s.charAt(i);
    writeChar(v);
}

7. writeUTF(String str)

Write a String to this DataOutputStream with "Modified UTF-8" encoding.

public final void writeUTF(String str) throws IOException

First, two bytes are written to the output stream as if by the writeShort method giving the number of bytes to follow. This value is the number of bytes actually written out, not the length of the string.

For example, call the writeUTF("ÂBC") method to write the string "ÂBC". As is known, UTF-8 uses 1, 2, 3 or 4 bytes to store a character.

'Â' --> 2 bytes
'B' --> 1 byte
'C' --> 1 byte

First, 2 bytes are used to store information about the number of bytes needed to store this string. The next 4 bytes are used to write the string "ÂBC" in "Modified UTF-8" encoding.

UTF-8
DataInputStream

Modified UTF-8:
https://docs.oracle.com/en/java/javase/15/docs/api/java.base/java/io/DataInput.html#modified-utf-8

8. writeBoolean(boolean v)

public final void writeBoolean(boolean v) throws IOException

The writeBoolean(boolean) method writes a boolean value to this DataOutputStream. If this value is true then (byte)1 will be written to, otherwise (byte)0 will be written to.

9. writeByte(int v)

public final void writeByte(int v) throws IOException

Write a byte to this DataOutputStream.

10. writeShort(int v)

public final void writeShort(int v) throws IOException

The writeShort(int) method writes 2 bytes to this DataOutputStream.

The two bytes written to are:

(byte)(0xff & (v >> 8))
(byte)(0xff & v)

11. writeChar(int v)

public final void writeChar(int v) throws IOException

The writeChar(int) method writes 2 bytes to this DataOutputStream.

The two bytes written to are:

(byte)(0xff & (v >> 8))
(byte)(0xff & v)

12. writeInt(int v)

public final void writeInt(int v) throws IOException

The writeInt(int) method writes 4 bytes to this DataOutputStream.

The four bytes written to are:

(byte)(0xff & (v >> 24))
(byte)(0xff & (v >> 16))
(byte)(0xff & (v >>  8))
(byte)(0xff & v)

13. writeLong(long v)

public final void writeLong(long v) throws IOException

The writeLong(long) method writes 8 bytes to this DataOutputStream.

The eight bytes written to are:

(byte)(0xff & (v >> 56))
(byte)(0xff & (v >> 48))
(byte)(0xff & (v >> 40))
(byte)(0xff & (v >> 32))
(byte)(0xff & (v >> 24))
(byte)(0xff & (v >> 16))
(byte)(0xff & (v >>  8))
(byte)(0xff & v)

14. writeFloat(float v)

public final void writeFloat(float v) throws IOException

The writeFloat(float) method writes 4 bytes to this DataOutputStream.

15. writeDouble(double v)

public final void writeDouble(double v) throws IOException

The writeDouble(double) method writes 8 bytes to this DataOutputStream.

16. size()

Returns the number of bytes written to this DataOutputStream, or Integer.MAX_VALUE if the value is actually greater.

public final int size()

Example:

DataOutputStream_size_ex1.java

ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream dos = new DataOutputStream(baos);

dos.writeByte(111);
System.out.println("Number of bytes: " + dos.size()); // 1

// 1 char = 2 bytes.
dos.writeChars("ÂBC");
System.out.println("Number of bytes: " + dos.size()); // 1 + 2*3 = 7

// 1 char = 2 bytes.
dos.writeChar('A');
System.out.println("Number of bytes: " + dos.size()); // 7 + 2 = 9

// In UTF-8:
// Two bytes of length information is added before the UTF-8 String (See writeUTF methods).
// UTF-8 String: "ÂBC" =4 bytes; 'Â' = 2 bytes, 'B' = 1 byte, 'C' = 1 byte.
dos.writeUTF("ÂBC");
System.out.println("Number of bytes: " + dos.size()); // 9 + 2 + 4 = 15