Java基础之读文件——使用通道随机读写文件（RandomReadWrite）

控制台程序，使用通道随机读写primes_backup.bin文件。

 import static java.nio.file.StandardOpenOption.*;
 import java.nio.file.*;
 import java.nio.channels.SeekableByteChannel;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.util.EnumSet;

 public class RandomReadWrite {
   public static void main(String[] args)
   {
     Path file = Paths.get(System.getProperty("user.home")).resolve("Beginning Java Struff").resolve("primes_backup.bin");
     if(!Files.exists(file)) {
       System.out.println(file + " does not exist. Terminating program.");
       System.exit(1);
     }

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

     long[] primes = new long[PRIMESREQUIRED];
     int index = 0;                                                     // Position for a prime in the file
     final long REPLACEMENT = 99999L;                                   // Replacement for a selected prime

     try (SeekableByteChannel channel = Files.newByteChannel(file, EnumSet.of(READ, WRITE))){
       final int PRIMECOUNT = (int)channel.size()/8;
       System.out.println("Prime count = "+PRIMECOUNT);
       for(int i = 0 ; i < PRIMESREQUIRED ; ++i) {
         index = 8*(int)(PRIMECOUNT*Math.random());
         channel.position(index).read(buf);                             // Read at a random position
         buf.flip();                                                    // Flip the buffer
         primes[i] = buf.getLong();                                     // Extract the prime
         buf.flip();                                                    // Flip to ready for insertion
         buf.putLong(REPLACEMENT);                                      // Replacement into buffer
         buf.flip();                                                    // Flip ready to write
         channel.position(index).write(buf);                            // Write the replacement to file
         buf.clear();                                                   // Reset ready for next read
       }

       // Alternate loop code to avoid selecting REPLACEMENT values
 /*      for(int i = 0 ; i < PRIMESREQUIRED ; ++i)
       {
         while(true)
         {
           index = 8*(int)(PRIMECOUNT*Math.random());
             channel.position(index).read(buf);   // Read at a random position
           buf.flip();                            // Flip the buffer
           primes[i] = buf.getLong();             // Extract the prime
           if(primes[i] != REPLACEMENT) {
             break;                               // It's good so exit the inner loop
           } else {
             buf.clear();                         // Clear ready to read another
           }
         }
         buf.flip();                              // Flip to ready for insertion
         buf.putLong(REPLACEMENT);                // Replacement into buffer
         buf.flip();                              // Flip ready to write
           channel.position(index).write(buf);    // Write the replacement to file
         buf.clear();                             // Reset ready for next read
       }*/

       int count = 0;                                                   // Count of primes written
       for(long prime : primes) {
         System.out.printf("%12d", prime);
         if(++count%5 == 0) {
           System.out.println();
         }
       }
     } catch(IOException e) {
       e.printStackTrace();
     }
   }
 }