Java RandomAccessFile: Seek, Read and Write at Arbitrary Positions

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

// Open for reading only
try (RandomAccessFile raf = new RandomAccessFile("data.bin", "r")) {
    System.out.println("File length: " + raf.length() + " bytes");
    System.out.println("Current pointer: " + raf.getFilePointer()); // 0
}

// Open for reading and writing (creates file if absent)
try (RandomAccessFile raf = new RandomAccessFile("output.bin", "rw")) {
    raf.writeInt(42);
    raf.writeDouble(3.14);
    raf.writeUTF("Hello");

    System.out.println("After writes, pointer at: " + raf.getFilePointer());
    System.out.println("File length: " + raf.length());

    // Seek back to the start and read back
    raf.seek(0);
    System.out.println(raf.readInt());    // 42
    System.out.println(raf.readDouble()); // 3.14
    System.out.println(raf.readUTF());    // Hello
}

import java.io.*;
import java.nio.charset.StandardCharsets;
import java.util.Arrays;

public class PersonRecord {
    // Fixed-length record layout:
    //   id     : 4 bytes  (int)
    //   age    : 4 bytes  (int)
    //   name   : 32 bytes (fixed-length padded UTF-8)
    // Total    : 40 bytes per record

    static final int RECORD_SIZE = 40;
    static final int NAME_BYTES   = 32;

    static void writeRecord(RandomAccessFile raf, int index,
                            int id, int age, String name) throws IOException {
        raf.seek((long) index * RECORD_SIZE);
        raf.writeInt(id);
        raf.writeInt(age);

        // Write name as fixed 32-byte field (pad with 0s)
        byte[] nameBytes = Arrays.copyOf(
            name.getBytes(StandardCharsets.UTF_8), NAME_BYTES);
        raf.write(nameBytes);
    }

    static String readName(RandomAccessFile raf, int index) throws IOException {
        raf.seek((long) index * RECORD_SIZE + 8); // skip id (4) + age (4)
        byte[] nameBytes = new byte[NAME_BYTES];
        raf.readFully(nameBytes);
        // Trim trailing null bytes
        int len = 0;
        while (len < NAME_BYTES && nameBytes[len] != 0) len++;
        return new String(nameBytes, 0, len, StandardCharsets.UTF_8);
    }

    public static void main(String[] args) throws IOException {
        try (RandomAccessFile raf = new RandomAccessFile("persons.bin", "rw")) {
            // Write 3 records
            writeRecord(raf, 0, 1, 30, "Alice");
            writeRecord(raf, 1, 2, 25, "Bob");
            writeRecord(raf, 2, 3, 35, "Carol");

            // Read record 1 (Bob) directly — O(1) access
            System.out.println("Record 1 name: " + readName(raf, 1)); // Bob

            // Update record 0 (Alice) age in-place
            raf.seek(0L * RECORD_SIZE + 4); // skip id, position at age
            raf.writeInt(31);

            // Read record 2 (Carol) name
            System.out.println("Record 2 name: " + readName(raf, 2)); // Carol

            System.out.println("Total records: " + raf.length() / RECORD_SIZE); // 3
        }
    }
}

import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.*;

// FileChannel equivalent of RandomAccessFile random access
try (FileChannel fc = FileChannel.open(
        Path.of("data.bin"),
        StandardOpenOption.READ, StandardOpenOption.WRITE,
        StandardOpenOption.CREATE)) {

    // Write at position 0
    ByteBuffer buf = ByteBuffer.allocate(8);
    buf.putInt(42).putDouble(3.14).flip(); // ← flip to switch from write to read mode...
    // Actually for separate writes:
    ByteBuffer ibuf = ByteBuffer.allocate(4);
    ibuf.putInt(42);
    ibuf.flip();
    fc.write(ibuf, 0);  // write int at position 0

    // Read back at position 0
    ibuf.clear();
    fc.read(ibuf, 0);
    ibuf.flip();
    System.out.println("Value: " + ibuf.getInt()); // 42

    // Get a RandomAccessFile's channel
    try (RandomAccessFile raf = new RandomAccessFile("existing.bin", "rw");
         FileChannel channel = raf.getChannel()) {
        // now use channel for high-throughput I/O
        System.out.println("Channel size: " + channel.size());
    }
}