Java基础之写文件——将素数写入文件中（PrimesToFile）

控制台程序，计算素数、创建文件路径、写文件。

 import static java.lang.Math.ceil;
 import static java.lang.Math.sqrt;
 import static java.lang.Math.min;
 import static java.nio.file.StandardOpenOption.*;
 import java.nio.file.*;
 import java.nio.channels.*;
 import java.nio.*;
 import java.util.*;
 import java.io.IOException;

 public class PrimesToFile {
   public static void main(String[] args) {
     int primesRequired = 100;                                          // Default count
     if (args.length > 0) {
       try {
         primesRequired = Integer.valueOf(args[0]).intValue();
       } catch (NumberFormatException e) {
         System.out.println("Prime count value invalid. Using default of " + primesRequired);
       }
     }

       long[] primes = new long[primesRequired];                        // Array to store primes

       getPrimes(primes);
       Path file = createFilePath("Beginning Java Struff","primes.bin");
       writePrimesFile(primes,file);
     }
       //Calculate enough primes to fill the array
       private static long[] getPrimes(long[] primes) {
           primes[0] = 2L;                                                  // Seed the first prime
           primes[1] = 3L;                                                  // and the second
           // Count of primes found ?up to now, which is also the array index
           int count = 2;
           // Next integer to be tested
           long number = 5L;

           outer:
           for (; count < primes.length; number += 2) {

             // The maximum divisor we need to try is square root of number
             long limit = (long)ceil(sqrt((double)number));

             // Divide by all the primes we have up to limit
             for (int i = 1 ; i < count && primes[i] <= limit ; ++i)
               if (number % primes[i] == 0L)                                // Is it an exact divisor?
                 continue outer;                                            // yes, try the next number

             primes[count++] = number;                                      // We got one!
           }
           return primes;
         }
     //Create the path for the named file in the specified directory
     //in the user home directory
     private static Path createFilePath(String directory, String fileName) {
         Path file = Paths.get(System.getProperty("user.home")).resolve(directory).resolve(fileName);
         try {
           Files.createDirectories(file.getParent());                       // Make sure we have the directory
         } catch (IOException e) {
           e.printStackTrace();
           System.exit(1);
         }
         System.out.println("New file is: " + file);
         return file;
     }

     //Write the array contents to file
     private static void writePrimesFile(long[] primes, Path file) {
         final int BUFFERSIZE = 100;                                        // Byte buffer size
         try (WritableByteChannel channel = Files.newByteChannel(file, EnumSet.of(WRITE, CREATE))) {
           ByteBuffer buf = ByteBuffer.allocate(BUFFERSIZE);
           LongBuffer longBuf = buf.asLongBuffer();                         // View buffer for type long
           int primesWritten = 0;                                           // Count of primes written to file
           while (primesWritten < primes.length) {
             longBuf.put(primes,                                            // Array to be written
                         primesWritten,                                     // Index of 1st element to write
                         min(longBuf.capacity(), primes.length - primesWritten));
             buf.limit(8*longBuf.position());                               // Update byte buffer position
             channel.write(buf);
             primesWritten += longBuf.position();
             longBuf.clear();
             buf.clear();
           }
           System.out.println("File written is " + ((FileChannel)channel).size() + " bytes.");
         } catch (IOException e) {
           e.printStackTrace();
         }
     }
 }