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Java - RandomAccessFile read(byte[] b) method
Description
The Java RandomAccessFile read(byte[] b) method reads up to b.length bytes of data from this file into an array of bytes.
read(byte[] b) method −
Reads up to b.length bytes from the file into the given byte array b.
Returns the number of bytes actually read, or -1 if the end of file (EOF) is reached.
Declaration
Following is the declaration for java.io.RandomAccessFile.read(byte[] b) method.
public int read(byte[] b)
Parameters
b − The buffer into which the data is 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 this 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.
Example - Usage of RandomAccessFile read(byte[] b) method
The following example shows the usage of RandomAccessFile read(byte[] b) 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 the first 8 bytes and print the number of bytes read System.out.println("" + raf.read(b1)); // set the file pointer at 0 position raf.seek(0); // read the first 8 bytes and print the number of bytes read System.out.println("" + raf.read(b2)); } 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 −
3 8
Example - Read all data from a file into a byte array
The following example shows the usage of RandomAccessFile read(byte[] b) 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("read_bytes1.txt", "rw")) { raf.write("HelloWorld".getBytes()); byte[] buffer = new byte[10]; raf.seek(0); // move pointer to start of file int bytesRead = raf.read(buffer); System.out.println("Bytes read: " + bytesRead); System.out.println("Content: " + new String(buffer)); } catch (IOException e) { e.printStackTrace(); } } }
Output
Let us compile and run the above program, this will produce the following result−
Bytes read: 10 Content: HelloWorld
Explanation
Writes "HelloWorld" (10 bytes) to the file.
Uses a 10-byte buffer to read all bytes in one go.
Converts and prints the result as a string.
Example - Partial read when buffer is smaller than file content
The following example shows the usage of RandomAccessFile read(byte[] b) 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("read_bytes2.txt", "rw")) { raf.write("abcdefghij".getBytes()); // 10 bytes byte[] buffer = new byte[4]; raf.seek(6); // Move to 7th byte (index 6) int bytesRead = raf.read(buffer); System.out.println("Bytes read: " + bytesRead); System.out.println("Content read from offset 6: " + new String(buffer, 0, bytesRead)); } catch (IOException e) { e.printStackTrace(); } } }
Output
Let us compile and run the above program, this will produce the following result−
Bytes read: 4 Content read from offset 6: ghij
Explanation
Writes "abcdefghij" (10 bytes) to file.
Seeks to byte index 6, which is 'g'.
Reads 4 bytes (g, h, i, j) into a smaller buffer.
Shows how to read a part of a file using seek + buffer.