Java IO Tutorial - Java Random Access Files

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Using a random access file, we can read from a file as well as write to the file.

Reading and writing using the file input and output streams are a sequential process.

Using a random access file, we can read or write at any position within the file.

An object of the RandomAccessFile class can do the random file access. We can read/write bytes and all primitive types values to a file.

RandomAccessFile can work with strings using its readUTF() and writeUTF() methods.

The RandomAccessFile class is not in the class hierarchy of the InputStream and OutputStream classes.

Mode

A random access file can be created in four different access modes. The access mode value is a string. They are listed as follows:

Read and Write

We create an instance of the RandomAccessFile class by specifying the file name and the access mode.

RandomAccessFile  raf = new RandomAccessFile("randomtest.txt", "rw");

A random access file has a file pointer that moves forward when we read data from it or write data to it.

The file pointer is a cursor where our next read or write will start.

Its value indicates the distance of the cursor from the beginning of the file in bytes.

We can get the value of file pointer by using its getFilePointer() method.

When we create an object of the RandomAccessFile class, the file pointer is set to zero.

We can set the file pointer at a specific location in the file using the seek() method.

The length() method of a RandomAccessFile returns the current length of the file. We can extend or truncate a file by using its setLength() method.

Example

The following code shows how to read and write Files Using a RandomAccessFile Object.

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
// w ww  .j av a  2s.  c  om
public class Main {
  public static void main(String[] args) throws IOException {
    String fileName = "randomaccessfile.txt";
    File fileObject = new File(fileName);

    if (!fileObject.exists()) {
      initialWrite(fileName);
    }
    readFile(fileName);
    readFile(fileName);
  }

  public static void readFile(String fileName) throws IOException {
    RandomAccessFile raf = new RandomAccessFile(fileName, "rw");
    int counter = raf.readInt();
    String msg = raf.readUTF();

    System.out.println(counter);
    System.out.println(msg);
    incrementReadCounter(raf);
    raf.close();
  }

  public static void incrementReadCounter(RandomAccessFile raf)
      throws IOException {
    long currentPosition = raf.getFilePointer();
    raf.seek(0);
    int counter = raf.readInt();
    counter++;
    raf.seek(0);
    raf.writeInt(counter);
    raf.seek(currentPosition);
  }

  public static void initialWrite(String fileName) throws IOException {
    RandomAccessFile raf = new RandomAccessFile(fileName, "rw");
    raf.writeInt(0);
    raf.writeUTF("Hello world!");
    raf.close();
  }
}

The code above generates the following result.

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