Mega Code Archive
Using FileChannel to random access to a File
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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
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