// File: ElementaryListTester.java

// The ElementaryList class has an instance variable that is an int array
// object variable.  The constructor takes one parameter: the size (i.e., 
// number of elements) of the int array object to be created.  The class 
// methods show how to use a for statement to "traverse" an array (i.e., to 
// "visit" each element).

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

/**
 * A class to help students learn about arrays.
 */
class ElementaryList
{
	// instance vars
	private int [] list ;		// an int array object variable
	
	/**
	 * Creates a ElementaryList object with an array of a specified size.
	 * @param size the number of elements of the array object to be created
	 */
	public ElementaryList(int size)
	{
		list = new int [size] ;
	}
	
	/**
	 * Fill the array with successive powers of 2, beginning with 1
	 */
	public void fill()
	{
		for (int i = 0 ; i < list.length ; i++)
		{
			list[i] = (int)Math.pow(2,i) ;
		}
	}
	
	/**
	 * Print the elements of the array in order, from 1st to last
	 */	
	public void print()
	{
		for (int i = 0 ; i < list.length ; i++)
		{
			System.out.print( list[i] + "   " ) ;
		}
		System.out.println() ;
	}

	/**
	 * Print the elements of the array in reverse order, from last to 1st
	 */	
	public void printReversed()
	{
		for (int i = list.length - 1 ; i >= 0 ; i--)
		{
			System.out.print( list[i] + "   " ) ;
		}
		System.out.println() ;
	}
}

public class ElementaryListTester
{
	
	public static void main (String [] args)
	{
		String input = JOptionPane.showInputDialog
					   ("How many powers of 2 do you want to see?") ;
		int number = Integer.parseInt(input) ;
		
		ElementaryList myList = new ElementaryList(number) ;
	
		myList.fill();
	
		System.out.println("\nThe first " + number + " powers of 2:\n") ;
		myList.print() ;
	
		System.out.println("\nHere they are again, in reverse order:\n") ;
		myList.printReversed() ;
		
		System.exit(0) ;
	}
}

/*  sample output:
 
The first 13 powers of 2:

1   2   4   8   16   32   64   128   256   512   1024   2048   4096

Here they are again, in reverse order:

4096   2048   1024   512   256   128   64   32   16   8   4   2   1

*/