Java - RandomAccessFile setLength(long newLength) method

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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.