1 import static java.lang.Math.ceil;
 2 import static java.lang.Math.sqrt;
 3 import static java.lang.Math.min;
 4 import static java.nio.file.StandardOpenOption.*;
 5 import java.nio.file.*;
 6 import java.nio.channels.*;
 7 import java.nio.*;
 8 import java.util.*;
 9 import java.io.IOException;
10
11 public class PrimesToFile3 {
12   public static void main(String[] args) {
13     int primesRequired = 100;                                          // Default count
14     if (args.length > 0) {
15       try {
16         primesRequired = Integer.valueOf(args[0]).intValue();
17       } catch (NumberFormatException e) {
18         System.out.println("Prime count value invalid. Using default of " + primesRequired);
19       }
20     }
21
22       long[] primes = new long[primesRequired];                        // Array to store primes
23
24       getPrimes(primes);
25       Path file = createFilePath("Beginning Java Struff","primesAgain.txt");
26       writePrimesFile(primes,file);
27     }
28       //Calculate enough primes to fill the array
29       private static long[] getPrimes(long[] primes) {
30           primes[0] = 2L;                                                  // Seed the first prime
31           primes[1] = 3L;                                                  // and the second
32           // Count of primes found ?up to now, which is also the array index
33           int count = 2;
34           // Next integer to be tested
35           long number = 5L;
36
37           outer:
38           for (; count < primes.length; number += 2) {
39
40             // The maximum divisor we need to try is square root of number
41             long limit = (long)ceil(sqrt((double)number));
42
43             // Divide by all the primes we have up to limit
44             for (int i = 1 ; i < count && primes[i] <= limit ; ++i)
45               if (number % primes[i] == 0L)                                // Is it an exact divisor?
46                 continue outer;                                            // yes, try the next number
47
48             primes[count++] = number;                                      // We got one!
49           }
50           return primes;
51         }
52     //Create the path for the named file in the specified directory
53     //in the user home directory
54     private static Path createFilePath(String directory, String fileName) {
55         Path file = Paths.get(System.getProperty("user.home")).resolve(directory).resolve(fileName);
56         try {
57           Files.createDirectories(file.getParent());                       // Make sure we have the directory
58         } catch (IOException e) {
59           e.printStackTrace();
60           System.exit(1);
61         }
62         System.out.println("New file is: " + file);
63         return file;
64     }
65
66     //Write the array contents to file
67     private static void writePrimesFile(long[] primes, Path file) {
68         final int BUFFERSIZE = 1024;                                        // Byte buffer size
69         try (WritableByteChannel channel = Files.newByteChannel( file, EnumSet.of(WRITE, CREATE))){
70           ByteBuffer buf = ByteBuffer.allocate(BUFFERSIZE);
71           String primeStr = null;
72           int primesWritten = 0;
73         while (primesWritten < primes.length) {
74             while (primesWritten < primes.length) {
75               primeStr = "prime = " + primes[primesWritten];
76               if ((buf.position() + 2*primeStr.length() + 16) > buf.limit()) {
77                 break;
78               }
79               buf.asDoubleBuffer().put(0, (double)primeStr.length());
80               buf.position(buf.position() + 8);
81               buf.position(buf.position() + 2*buf.asCharBuffer().put(primeStr).position());
82               buf.asLongBuffer().put(primes[primesWritten++]);
83               buf.position(buf.position() + 8);
84             }
85             buf.flip();
86             channel.write(buf);
87             buf.clear();
88           }
89          System.out.println("File written is " + ((FileChannel)channel).size() + " bytes.");
90         } catch (IOException e) {
91           e.printStackTrace();
92         }
93     }
94 }