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Invoking an object`s method inside a for loop.

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Last response: in Applications
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Hey guys. Need your help again.
I have to make a Library that contains books.
In the Library class, a method called mostPagesBook, returns the book with most pages. It compiles, but errors when run:

Exception in thread "main" java.lang.NullPointerException
at Library.mostPagesBook(Library.java:32)
at TEMP.main(TEMP.java:29)
(POINTING AT THE pages = checkBook.getNumPages(); Line)


In Library:
  1. public Book mostPagesBook ()
    

  2. 	{
    

  3. 		int bookNumber = 0, pages, mostPages = 0, largestBook = 0;
    

  4. 		for (Book checkBook : bookList)
    

  5. 		{	
    

  6. 			pages = checkBook.getNumPages();
    

  7. 			if (pages > mostPages)
    

  8. 			largestBook = bookNumber;
    

  9. 			bookNumber++;												
    

  10. 		}	
    

  11. 		return bookList[largestBook];	
    

  12. 	}


in Book :
  1. public int getNumPages ()
    

  2. 	{
    

  3. 		return numPages;
    

  4. 	}


Help is most appreciated.

More about : invoking object method inside loop

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I defined it that way:

  1. public class Library
    

  2. {
    

  3. 	private final int MAX_BOOKS = 150;
    

  4. 	private int totalBooks = 0;
    

  5. 	private int borrowedBooks;	
    

  6. 	private Book[] bookList;
    

  7.  
  8. 	public Library ()
    

  9. 	{
    

  10. 		bookList = new Book[MAX_BOOKS];
    

  11. 		totalBooks = 0;
    

  12. 		borrowedBooks = 0;
    

  13. 	}
    

  14.  
  15. 	public void addBook (String bookName, String authorName, int numPages)
    

  16. 	{
    

  17. 		if (totalBooks <= bookList.length)
    

  18. 		{
    

  19. 			bookList[totalBooks] = new Book (bookName, authorName, numPages);
    

  20. 			totalBooks++;
    

  21. 		}
    

  22. 		else 
    

  23. 			System.out.println ("Limit reached (150): Library is full.");
    

  24. 	}


What should I change?
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Programming Expert

You could add a simple check inside the loop like this:

  1. if (checkBook != null)
    

  2. {
    

  3.     do_something();
    

  4. }


Your best option is to make sure that there are no null pointers. Just from the code I'm looking at here I can only assume that your Book constructor must return null somewhere. Never do this, or you'll have to put null checks everywhere. If you have to, just return a dummy instance. Your mostPagesBook() method should not have to concern itself with whether or not the Book is null.

mgwildi said:
also, "mostPages" is never reassigned with the current "pages" value, so you'll just go into that if every time

Yes, I fixed that as you said. Thanks.

randoMIZER, I have changed it to use a simple arithmetical count (I`m not sure the null was introduced in the course):
  1. public Book mostPagesBook ()
    

  2. 	{
    

  3. 		int pages, mostPages = 0, largestBook = 0;
    

  4.  
  5. 		for (int check = 0; check < totalBooks; check++)
    

  6. 		{	
    

  7. 			if (bookList[check].isAvailable())
    

  8. 			{
    

  9. 				pages = bookList[check].getNumPages();
    

  10. 				if (pages > mostPages)
    

  11. 				{
    

  12. 					mostPages = pages;
    

  13. 					largestBook = check;	
    

  14. 				}
    

  15. 			}											
    

  16. 		}	
    

  17. 		return bookList[largestBook];	
    

  18. 	}
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