//	File:  PrimeMinister2.java 

//	Finds and prints the first n prime numbers, where n is entered
//  by the user

//  Version 2 of the PrimeFinder class differs from the previous version 
//  only in boolean method isPrime(), which makes use of this recursive
//  definition of primehood: 

//  "A positive integer is prime if it is not a product of any smaller
//  prime number"

//  isPrime() checks to see if a number being tested for primehood
//  divides evenly by any of the prime numbers on a list.  If so, 
//  then the number is not prime and false is returned; otherwise, 
//  the number is prime so it is added to the list and true is returned.

//  Note that the first number tested is 2 and the list is initially
//  empty, so 2 is the first number added to the list

//  NOTE: The PrimeFinder class "depends on" the Align class to format the 
//  output in columns 

import javax.swing.JOptionPane ;
import java.util.ArrayList ;

/**
 * Finds and prints the first "n" prime numbers
 */
class PrimeFinder
{
	private int howMany ;		              // number of primes to be found
	
	private ArrayList<Integer> primesList ;   // list of prime numbers

	/**
	 *  Construct a PrimeFinder object
	 *  @param howMany the number of primes to be found
	 */
	public PrimeFinder(int howMany)
	{
		this.howMany = howMany ;
		primesList = new ArrayList<Integer>() ;  // creates empty list
	}
	
 	/**
 	 * Prints the first N prime numbers
 	 */ 										
	public void printPrimes()
	{
   		int count = 0 ;			// counts actual number of primes found
   		int number = 2 ;   		// each of a series of ints to be tested for
                                // "primehood," beginning with 2

   		System.out.println("\nThe First " +  howMany + " Prime Numbers:\n") ;
   
   		// find and print primes
   		do								    // repeat...
   		{
     		if (isPrime(number))     		// ...if number is prime ...
     		{
        		// ...print it right-aligned in columns 6 spaces wide
        		System.out.print(Align.right(number,6)) ;		
        		count++ ;                   // .......and count it
        		if (count % 10 == 0)		// ...every ten primes ...
	     		   System.out.println() ;	// ...... start new line
     		}

     		number++ ;						// ...get next test number
   		}
   		while (count < howMany) ;			// until required # of primes found
	    
	    System.out.println() ;			
	}
	
	// See if num is a prime number by trying to divide it by smaller
	// prime numbers stored in primesList
	private boolean isPrime (int num)
	{
   		for (int position = 0 ; position < primesList.size() ; position++)
   		{
   			int trialDivisor = primesList.get(position) ;
   			if (num % trialDivisor == 0)   // num is not prime...
   			  return false ;			   // ...so return false
   		}
	    primesList.add(num) ;			   // num is prime, so add to list...
      	return true ;                      // ...and return true
	}
}   // end of PrimeFinder class definition ====================================

/**
 * A test class for the PrimeFinder class.
 */
public class PrimeMinister2
{
	public static void main (String [] args)
	{
		JOptionPane.showMessageDialog
			        (null, "I will find and print the first N prime numbers!") ;
			        
		String input = JOptionPane.showInputDialog
			                       ("Enter N (number of primes to find)") ;
		
		int howMany = Integer.parseInt(input) ;
		
		PrimeFinder primes = new PrimeFinder(howMany) ;

		primes.printPrimes() ;
	}
}

/*  sample output:

The First 100 Prime Numbers:

     2     3     5     7    11    13    17    19    23    29
    31    37    41    43    47    53    59    61    67    71
    73    79    83    89    97   101   103   107   109   113
   127   131   137   139   149   151   157   163   167   173
   179   181   191   193   197   199   211   223   227   229
   233   239   241   251   257   263   269   271   277   281
   283   293   307   311   313   317   331   337   347   349
   353   359   367   373   379   383   389   397   401   409
   419   421   431   433   439   443   449   457   461   463
   467   479   487   491   499   503   509   521   523   541
   
*/