neatprimesoutput.java

Java Classic Examples是我买的两本书：《JAVA经典实例》和《java入门经典源代码》里边附送光盘里带的源码

import java.io.*;

public class NeatPrimesOutput
{
  public static void main(String[] args)
  {
    long[] primes = new long[20];     // Array to store primes
    primes[0] = 2;                    // Seed the first prime
    primes[1] = 3;                    // and the second
    int count = 2;                    // Count of primes found - up to now
                                      // which is also the array index
    long number = 5;                  // Next integer to be tested

    outer:
    for( ; count < primes.length; number += 2)
    {
      // The maximum divisor we need to try is square root of number
      long limit = (long)Math.ceil(Math.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] == 0)     // Is it an exact divisor?
          continue outer;             // yes, try the next number

      primes[count++] = number;       // We got one!
    }

    // Output the primes array using a formatted buffered stream
    FormatWriter out = new FormatWriter(
                       new BufferedWriter(
                       new FileWriter(FileDescriptor.out)), 12);

    for(int i=0; i < primes.length; i++)
    {
      if(i%5==0)                      // New line before every fifth prime
        out.println();

      out.print(primes[i]);           // Output a prime
    }
    out.close();                      // Close the stream
  }
}