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();
}
}
}