Java - RandomAccessFile read((byte[] b,int off,int len) method

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Description

The Java RandomAccessFile read((byte[] b,int off,int len) method reads up to len bytes of data from this file into an array of bytes.

read((byte[] b,int off,int len) method −

• Reads up to len bytes from the file into the byte array b, starting at index off in the array.

• Returns the number of bytes actually read, or -1 if end of file (EOF) is reached.

• This is useful for reading a portion of a file directly into a part of an existing array.

Declaration

Following is the declaration for java.io.RandomAccessFile.read((byte[] b,int off,int len) method.

public int read((byte[] b,int off,int len)

Parameters

• b − The buffer into which the data is read.

• off − The start offset in array b at which the data is written.

• len − The maximum number of bytes read.

Return Value

This method returns the total number of bytes read into the buffer, or -1 if there is no more data because the end of the file has been reached.

Exception

• IOException − If an I/O error occurs. Not thrown if end-of-file has been reached.

• NullPointerException − If b is null.

• IndexOutOfBoundsException − If off is negative, len is negative, or len is greater than b.length - off.

Example - Usage of RandomAccessFile read((byte[] b,int off,int len) method

The following example shows the usage of RandomAccessFile read((byte[] b,int off,int len) method.

RandomAccessFileDemo.java

package com.tutorialspoint;

import java.io.IOException;
import java.io.RandomAccessFile;

public class RandomAccessFileDemo {
   public static void main(String[] args) {

      try {
         byte[] b1 = {1, 2, 3};
         byte[] b2 = {1, 2, 3, 4, 5, 6, 7, 8};

         // create a new RandomAccessFile with filename test
         RandomAccessFile raf = new RandomAccessFile("test.txt", "rw");

         // write something in the file
         raf.writeUTF("Hello World");

         // set the file pointer at 0 position
         raf.seek(0);

         // read 2 bytes, starting from 1
         System.out.println("" + raf.read(b1, 1, 2));

         // set the file pointer at 0 position
         raf.seek(0);

         // read 3 bytes, starting from 4rth
         System.out.println("" + raf.read(b2, 4, 3));         
      } catch (IOException ex) {
         ex.printStackTrace();
      }
   }
}

Output

Assuming we have a text file test.txt in current directory which has the following content. This file will be used as an input for our example program −

ABCDE

Let us compile and run the above program, this will produce the following result −

2
3

Example - Reading specific number of bytes into part of an array

The following example shows the usage of RandomAccessFile read((byte[] b,int off,int len) method.

RandomAccessFileDemo.java

package com.tutorialspoint;

import java.io.RandomAccessFile;
import java.io.IOException;

public class RandomAccessFileDemo {
   public static void main(String[] args) {
      try (RandomAccessFile raf = new RandomAccessFile("example1.txt", "rw")) {
         raf.write("ABCDEFGHIJ".getBytes()); // 10 bytes written

         byte[] buffer = new byte[10];
         raf.seek(0);
         int bytesRead = raf.read(buffer, 2, 5); // Read 5 bytes into buffer starting at index 2

         System.out.println("Bytes read: " + bytesRead);
         System.out.print("Buffer content: ");
         for (byte b : buffer) {
            System.out.print((b == 0 ? '.' : (char) b) + " ");
         }
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Bytes read: 5
Buffer content: . . A B C D E . . .

Explanation

• File content: "ABCDEFGHIJ"

• Reads 5 bytes starting at the beginning of the file.

• Stores those 5 bytes starting at index 2 in the buffer.

• Other parts of the buffer remain as default (0, shown as .).

Example - Reading from a certain position in the file into a subset of a buffer

The following example shows the usage of RandomAccessFile read((byte[] b,int off,int len) method.

RandomAccessFileDemo.java

package com.tutorialspoint;

import java.io.RandomAccessFile;
import java.io.IOException;

public class RandomAccessFileDemo {
   public static void main(String[] args) {
      try (RandomAccessFile raf = new RandomAccessFile("example2.txt", "rw")) {
         raf.write("1234567890".getBytes()); // Write 10 bytes

         byte[] buffer = new byte[8];
         raf.seek(5); // Start reading from the 6th byte ('6')
         int bytesRead = raf.read(buffer, 3, 4); // Read 4 bytes into buffer starting at index 3

         System.out.println("Bytes read: " + bytesRead);
         System.out.print("Buffer content: ");
         for (byte b : buffer) {
            System.out.print((b == 0 ? '.' : (char) b) + " ");
         }
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Bytes read: 4
Buffer content: . . . 6 7 8 9 .

Explanation

• File content: "1234567890"

• Seeks to position 5 → points to '6'.

• Reads 4 bytes ('6', '7', '8', '9') and places them at buffer index 3-6.

• Buffer indices 0-2 and 7 remain as default (0).