Java - RandomAccessFile setLength(long newLength) method

Description

The Java RandomAccessFile setLength(long newLength) method sets the length of this file.

Declaration

Following is the declaration for java.io.RandomAccessFile.setLength(long newLength) method.

public void setLength(long newLength)

Parameters

newLength − The desired length of the file

Return Value

This method does not return a value.

Exception

• IOException − If an I/O error occurs.

Example - Usage of RandomAccessFile setLength(long newLength) method

The following example shows the usage of RandomAccessFile setLength(long newLength) method.

RandomAccessFileDemo.java

package com.tutorialspoint;

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

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

         // print the string
         System.out.println(raf.readUTF());

         // print current length
         System.out.println(raf.length());

         // set the file length to 30
         raf.setLength(30);

         // print the new length
         System.out.println(raf.length());
         
      } 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 −

Hello World
34
30

Example - Truncate a File

The following example shows the usage of RandomAccessFile setLength(long newLength) 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("truncate.dat", "rw");

         // Write 10 bytes to the file
         raf.writeBytes("ABCDEFGHIJ");

         // Truncate the file to 5 bytes
         raf.setLength(5);

         // Read the truncated content
         raf.seek(0);
         byte[] data = new byte[5];
         raf.readFully(data);
         System.out.println("Truncated content: " + new String(data)); // Output: ABCDE

         raf.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

Truncated content: ABCDE

Explanation

• setLength(5) cuts the file to 5 bytes, discarding everything after.

• Useful when you want to remove content from the end of a file, like shrinking logs or undoing writes.

Example - Extend a File

The following example shows the usage of RandomAccessFile setLength(long newLength) 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("extend.dat", "rw");

         // Write 3 bytes to the file
         raf.writeBytes("XYZ");

         // Extend the file to 10 bytes
         raf.setLength(10);

         // Reset and read all bytes
         raf.seek(0);
         byte[] data = new byte[10];
         raf.readFully(data);

         // Print bytes as characters or placeholder for empty space
         for (byte b : data) {
            System.out.print((b == 0 ? '_' : (char) b) + " ");
         }
         // Output: X Y Z _ _ _ _ _ _ _

         raf.close();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

X Y Z _ _ _ _ _ _ _

Explanation

• setLength(10) increases file size from 3 to 10 bytes.

• The new space is filled with null bytes (0).

• This is useful for preallocating space or reserving room for future data.