Using FileChannel to random access to a File : FileChannel « File « Java Tutorial

1. Java Tutorial
2. File
3. FileChannel

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.ByteBuffer;
import java.nio.LongBuffer;
import java.nio.channels.FileChannel;

public class MainClass {
  public static void main(String[] args) throws Exception {
    createFile();

    File aFile = new File("C:/primes.bin");
    FileInputStream inFile = new FileInputStream(aFile);

    FileChannel inChannel = inFile.getChannel();

    final int PRIMESREQUIRED = 10;
    ByteBuffer buf = ByteBuffer.allocate(8 * PRIMESREQUIRED);

    long[] primes = new long[PRIMESREQUIRED];
    int index = 0; // Position for a prime in the file

    // Count of primes in the file
    final int PRIMECOUNT = (int) inChannel.size() / 8;

    // Read the number of random primes required
    for (int i = 0; i < PRIMESREQUIRED; i++) {
      index = 8 * (int) (PRIMECOUNT * Math.random());
      inChannel.read(buf, index); // Read the value
      buf.flip();
      primes[i] = buf.getLong(); // Save it in the array
      buf.clear();
    }

    for (long prime : primes) {
      System.out.printf("%12d", prime);
    }
    inFile.close(); // Close the file and the channel

  }

  private static void createFile() throws Exception {
    long[] primes = new long[] { 1, 2, 3, 5, 7 };
    File aFile = new File("C:/primes.bin");
    FileOutputStream outputFile = new FileOutputStream(aFile);
    FileChannel file = outputFile.getChannel();
    final int BUFFERSIZE = 100;
    ByteBuffer buf = ByteBuffer.allocate(BUFFERSIZE);
    LongBuffer longBuf = buf.asLongBuffer();
    int primesWritten = 0;
    while (primesWritten < primes.length) {
      longBuf.put(primes, primesWritten, min(longBuf.capacity(), primes.length - primesWritten));
      buf.limit(8 * longBuf.position());

      file.write(buf);
      primesWritten += longBuf.position();
      longBuf.clear();
      buf.clear();
    }
    System.out.println("File written is " + file.size() + "bytes.");
    outputFile.close();
  }

  private static int min(int a, int b) {
    if (a > b) {
      return b;
    } else {
      return a;
    }

  }
}

File written is 40bytes.
           3           5           1           7           7           5           7           1           7           1