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The readLong() method of DataInputStream class in Java is used to read eight input bytes and returns a long value. This method reads the next eight bytes from the input stream and interprets it into long type and returns. Syntax:

public final long readLong()
                  throws IOException

Specified By: This method is specified by readLong() method of DataInput interface. Parameters: This method does not accept any parameter. Return value: This method returns the long value interpreted by the next eight bytes of the input stream. Exceptions:

EOFException - It throws EOFException if the input stream is ended before eight bytes can be read.
IOException - This method throws IOException if the stream is closed or some other I/O error occurs.

Below programs illustrate readLong() method in DataInputStream class in IO package: Program 1: Assume the existence of file "demo.txt".

Java

// Java program to illustrate
// DataInputStream readLong() method
import java.io.*;
public class GFG {
    public static void main(String[] args)
        throws IOException
    {

        // Create long array
        long[] buf = { 10000000000l,
                       20000000000l,
                       30000000000l };

        // Create file output stream
        FileOutputStream outputStream
            = new FileOutputStream("c:\\demo.txt");

        // Create data output stream
        DataOutputStream dataOutputStr
            = new DataOutputStream(outputStream);

        for (long b : buf) {
            // Write long value to
            // the dataOutputStream
            dataOutputStr.writeLong(b);
        }

        dataOutputStr.flush();

        // Create file input stream
        FileInputStream inputStream
            = new FileInputStream("c:\\demo.txt");

        // Create data input stream
        DataInputStream dataInputStr
            = new DataInputStream(inputStream);

        while (dataInputStr.available() > 0) {
            // Print long values
            System.out.println(
                dataInputStr.readLong());
        }
    }
}

Output:

Program 2: Assume the existence of file "demo.txt".

Java

// Java program to illustrate
// DataInputStream readLong() method
import java.io.*;
public class GFG {
    public static void main(String[] args)
        throws IOException
    {

        // Create long array
        long[] buf = { 1234567890L,
                       9876543210L,
                       12345678910L };

        // Create file output stream
        FileOutputStream outputStream
            = new FileOutputStream("c:\\demo.txt");

        // Create data output stream
        DataOutputStream dataOutputStr
            = new DataOutputStream(outputStream);

        for (long b : buf) {
            // Write long value to
            // the dataOutputStream
            dataOutputStr.writeLong(b);
        }

        dataOutputStr.flush();

        // Create file input stream
        FileInputStream inputStream
            = new FileInputStream("c:\\demo.txt");

        // Create data input stream
        DataInputStream dataInputStr
            = new DataInputStream(inputStream);

        while (dataInputStr.available() > 0) {
            // Print long values
            System.out.println(
                dataInputStr.readLong());
        }
    }
}

Output:

References: https://docs.oracle.com/javase/10/docs/api/java/io/DataInputStream.html#readLong()

DataInputStream readLong() method in Java with Examples

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