Java基础之读文件——使用通道随机读取文件（RandomFileRead）

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

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

     final int PRIMESREQUIRED = 10;
       final int LONG_BYTES = 8;                                        // Number of bytes for type long
     ByteBuffer buf = ByteBuffer.allocate(LONG_BYTES*PRIMESREQUIRED);

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

     try (FileChannel inCh = (FileChannel)(Files.newByteChannel(file))){
       // Count of primes in the file
       final int PRIMECOUNT = (int)inCh.size()/LONG_BYTES;

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

       // Output the selection of random primes 5 to a line in field width of 12
       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();
     }
   }
 }

控制塔程序，本例从primes.bin文件中随机选择一些值进行提取。