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Java DataInputStream Tutorial with Examples

  1. DataInputStream
  2. Constructors
  3. Methods
  4. Examples
  5. readUTF()
  6. readUTF(DataInput in)
  7. readFully(byte[] buffer)
  8. readFully(byte[] buffer, int off, int len)
  9. readBoolean()
  10. readByte()
  11. readUnsignedByte()
  12. readShort()
  13. readUnsignedShort()
  14. readChar()
  15. readInt()
  16. readLong()
  17. readFloat()
  18. readDouble()

1. DataInputStream

DataInputStream is used to read data primitives from a data source, especially data sources written by DataOutputStream.
public class DataInputStream extends FilterInputStream implements DataInput

2. Constructors

public DataInputStream(InputStream in)

3. Methods

public final boolean readBoolean() throws IOException  
public final byte readByte() throws IOException  
public final int readUnsignedByte() throws IOException  
public final short readShort() throws IOException  
public final int readUnsignedShort() throws IOException  
public final char readChar() throws IOException  
public final int readInt() throws IOException  
public final long readLong() throws IOException  
public final float readFloat() throws IOException  
public final double readDouble() throws IOException

public final String readUTF() throws IOException
public static final String readUTF(DataInput in) throws IOException

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

@Deprecated
public String readLine() throws IOException
Other methods inherited from parent classes:
public int read() throws IOException  
 
public int read(byte[] b) throws IOException  
public int read(byte[] b, int off, int len) throws IOException
 
public byte[] readAllBytes() throws IOException
public byte[] readNBytes(int len) throws IOException  
public int readNBytes(byte[] b, int off, int len) throws IOException  
 
public long skip(long n) throws IOException  
public int available() throws IOException  
 
public void close() throws IOException  
public synchronized void mark(int readlimit)  
public synchronized void reset() throws IOException  
 
public boolean markSupported()  
public long transferTo(OutputStream out) throws IOException

public static InputStream nullInputStream()

4. Examples

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DataInputStream is commonly used to read data sources written by DataOutputStream. In this example, we use DataOutputStream to write a table of data with an Excel-like structure to a file, then use DataInputStream to read this 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, we write the Order class that simulates the data of a table row:
Order.java
package org.o7planning.datainputstream.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;
    }
}
Use DataOutputStream to write the above data table to a file:
WriteDataFile_example1.java
package org.o7planning.datainputstream.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 with rather confusing content:
Next, we use DataInputStream to read the above file.
ReadDataFile_example1.java
package org.o7planning.datainputstream.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. readUTF()

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The readUTF() method is used to read a string encoded with "Modified UTF-8". The read rules are symmetric with the write rules of the DataOutput.writeUTF(String) method.
public final String readUTF() throws IOException
At the cursor position on the stream of bytes, the DataInputStream will read the next 2 bytes using the readUnsignedShort() method to determine the number of bytes used to store the UTF-8 string and get the result is utfLeng.
Then the DataInputStream reads the next utfLeng bytes and converts it to a UTF-8 string.
Modified UTF-8:

6. readUTF(DataInput in)

The static method readUTF(DataInput) is used to read a "Modified UTF-8" string from a specified DataInput object.
public static final String readUTF(DataInput in) throws IOException

7. readFully(byte[] buffer)

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Reads as many bytes as possible from this DataInputStream and assigns it to the specified buffer array.
public final void readFully(byte[] buffer) throws IOException
This method blocks until one of the following conditions occurs:
  • Read buffer.length bytes.
  • The end of the stream has been reached. --> The end of the stream is reached.
  • An IOException occurs (Different from EOFException).

8. readFully(byte[] buffer, int off, int len)

Reads as many bytes as possible from this DataInputStream and assigns it to the specified buffer array from index off to index off+len-1.
public final void readFully(byte[] buffer, int off, int len) throws IOException
This method blocks until one of the following conditions occurs:
  • Read buffer.length bytes.
  • The end of the stream has been reached. -> The end of the stream is reached.
  • An IOException occurs (Different from EOFException).

9. readBoolean()

The readBoolean() method reads a byte from this DataInputStream. If the result is non-zero the method returns true, otherwise returns false.
public final boolean readBoolean() throws IOException

10. readByte()

The readByte() method reads a byte from this DataInputStream, the returned value is between -128 and 127.
It is suitable for reading data written by the DataOutput.writeByte(byte) method.
public final byte readByte() throws IOException

11. readUnsignedByte()

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The readUnsignedShort() method reads 1 byte from this DataInputStream and converts it to an unsigned 4-bytes integer, values from 0 to 255.
public final int readUnsignedByte() throws IOException

12. readShort()

The readShort() method reads 2 bytes from this DataInputStream and converts it to a short data type.
public final short readShort() throws IOException
Conversion rule:
return (short)((firstByte << 8) | (secondByte & 0xff));

13. readUnsignedShort()

The readUnsignedShort() method reads 2 bytes from this DataInputStream and converts it to an unsigned 4-bytes integer, values from 0 to 65535.
public final int readUnsignedShort() throws IOException
Conversion rule:
return (((firstByte & 0xff) << 8) | (secondByte & 0xff))

14. readChar()

The readChar() method reads 2 bytes from this DataInputStream, and converts it to char data type.
public final char readChar() throws IOException
Conversion rule:
return (char)((firstByte << 8) | (secondByte & 0xff));

15. readInt()

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The readInt() method reads 4 bytes from this DataInputStream, and converts it to int data type.
public final int readInt() throws IOException
Conversion rule:
return (((firstByte & 0xff) << 24) | ((secondByte & 0xff) << 16) 
                                          |   ((thirdByte & 0xff) <<  8) | (fourthByte & 0xff));

16. readLong()

The readLong() method reads 8 bytes from this DataInputStream and converts it to a long data type.
public final long readLong() throws IOException
Conversion rule:
return (((long)(byte1 & 0xff) << 56) |
          ((long)(byte2 & 0xff) << 48) |
          ((long)(byte3 & 0xff) << 40) |
          ((long)(byte4 & 0xff) << 32) |
          ((long)(byte5 & 0xff) << 24) |
          ((long)(byte6 & 0xff) << 16) |
          ((long)(byte7 & 0xff) <<  8) |
          ((long)(byte8 & 0xff)));

17. readFloat()

The readFloat() method reads 4 bytes from this DataInputStream and returns the float data type.
It is suitable for reading data written by the DataOutput.writeFloat(float) method.
public final float readFloat() throws IOException

18. readDouble()

The readDouble() method reads 8 bytes from this DataInputStream and returns the double data type.
It is suitable for reading data written by the DataOutput.writeDouble(double) method.
public final double readDouble() throws IOException
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