Java beginner: nullpointerexception error

Question

I'm developing a library management program (it's an assignment from an online course), and I can't solve a nullpointerexception error at runtime.

This is the program so far:

//file Library.java
public class Library {

String address;
Book[] collection;
int collectionCounter = 0;

    // Add the missing implementation to this class
    public static void main(String[] args)
    {
        // Create two libraries
        Library firstLibrary = new Library("10 Main St.");
        Library secondLibrary = new Library("228 Liberty St.");
        // Add four books to the first library
        firstLibrary.addBook(new Book("The Da Vinci Code"));
        firstLibrary.addBook(new Book("Le Petit Prince"));
        firstLibrary.addBook(new Book("A Tale of Two Cities"));
        firstLibrary.addBook(new Book("The Lord of the Rings"));
    }

    //Constructor
    public Library(String libraryName)
    {
        address = libraryName;
        collectionCounter = 0;
    }

    //Methods
    public void addBook(Book newBook)
    {
        System.out.println(this.collectionCounter);
        this.collection[this.collectionCounter] = newBook;
        this.collectionCounter += 1;
    }

And the other .java file, for the Book class:

public class Book {
String title;
boolean borrowed;

    // Creates a new Book
    public Book(String bookTitle) {
        // Implement this method
        title = bookTitle;
        borrowed = false;
    }
    // Marks the book as rented
    public void rented() {
        // Implement this method
        this.borrowed = true;
    }

    // Marks the book as not rented
    public void returned() {
        // Implement this method
        this.borrowed = false;
    }

    // Returns true if the book is rented, false otherwise
    public boolean isBorrowed() {
        // Implement this method
        return this.borrowed;
    }

    // Returns the title of the book
    public String getTitle() {
        return this.title;
    }

    public static void main(String[] arguments) {

        // Small test of the Book class
        Book example = new Book("The Da Vinci Code");
        System.out.println("Title (should be The Da Vinci Code): " + example.getTitle());
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
        example.rented();
        System.out.println("Borrowed? (should be true): " + example.isBorrowed());
        example.returned();
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
    }

}

This is the program output:

0
Exception in thread "main" java.lang.NullPointerException
    at Library.addBook(Library.java:59)
    at Library.main(Library.java:14)

I understand the error is caused by the array of books, but I don't really know what to do, I had never seen an object instantiation as an argument for a method. Thanks in advance!

Answer 1

You must create your array collection in order to be able to put anything into it. Replace

Book[] collection;

by

Book[] collection = new Book[BOOK_COLLECTION_SIZE];

with BOOK_COLLECTION_SIZE being a reasonably high number.

Even better, use a List instead of an array. That way you won't have to guess your library size beforehand:

List<Book> collection = new LinkedList<Book>();

Then addBook can look like this:

public void addBook(Book newBook)
{
    this.collection.add(newBook);
}

and you can get rid of collectionCounter . If you ever really need the number of books, you can get it using this.collection.size() .

Answer 2

The problem lies in your Library class. It's addBook method uses this.collection , however this was only declared, hence still being null .

public void addBook(Book newBook)
{
    System.out.println(this.collectionCounter);
    this.collection[this.collectionCounter] = newBook;
    this.collectionCounter += 1;
}