public class Book 
{
	private static int books = 0;
	String isbn;
	
	public String getisbn()
	{
		return isbn;
	}
	
	public void setisbn(String isbn)
	{
		this.isbn = isbn;
	}
	
	public Book()
	{
		books++;
	}
	public Book(String isbn)
	{
		this();
		this.setisbn(isbn);
	}
	
	public static void main(String[] args)
	{
		Book b1 = new Book("3-11-011786-X");
		Book b2 = (Book) b1;
	
		b2.setisbn("3-8258-8188-1");
		
		System.out.println("b1 =  " + b1.getisbn());
		System.out.println("b2 =  " + b2.getisbn());
		
		if	(!(b1.getisbn() == b2.getisbn()))
		{
			System.out.println("OK: " + books);
		}
		else
		{
			System.out.println("NOT OK: " + books);
		}
	}
}