Storing User Input in Arraylist Java

Question

I'm trying to build a library type-program in java, but I'm having some trouble with storing user input in my ArrayList. I have an ArrayList called 'list' that stores the title, author, publisher, genre, year and pages of the book and I need to allow users to input books to the ArrayList to later be able to view all the books stored in the ArrayList.

When I run the program it's throwing this error:

Exception in thread "main" java.lang.NullPointerException
    at thelibrary.addBooks(Book.java:73)
    at Menu.bookmenu(Menu.java:68)
    at Menu.main(Menu.java:27)

Where am I going wrong?

    public static void addBooks() {
        Scanner newBooks = new Scanner(System.in);
        ArrayList<Book> list = null;

        for (int i = 0; i < 1; i++) {

System.out.println("\n\nAdd a new book to the library:");

        list.add(new Book(title, author, publisher, genre, year, pages));
        }

        System.out.println("You have successfully added a new book to the library!");
}

Possible duplicate of What is a NullPointerException, and how do I fix it? — Thomas Fritsch
– Thomas Fritsch, Commented Nov 20, 2017 at 21:50

Evan Weissburg · Accepted Answer · 2017-11-20 21:48:42Z

1

You declared list to be of type ArrayList, but you never called the ArrayList constructor (you set it equal to null, instead).

Instead of: ArrayList<Book> list = null;

Use ArrayList<Book> list = new ArrayList<Book>();.

Additionally, list dies when it leaves the scope of addBooks(). Try this, instead:

class thelibrary {
    public static ArrayList<Book> list = new ArrayList<Book>();

    public static void allBooks() {

            Book obj1 = new Book("Harry Potter and the Philosopher's Stone","JK Rowling","Bloomsbury","Genre",1997,223);
            Book obj2 = new Book("Alexander Hamilton","Ron Chernow","Head of Zeus","Biograophy",2016,818);
            Book obj3 = new Book("To Kill a Mockingbird","Harper Lee","Arrow","Southern Gothic",1960,309);

            list.add(obj1);
            list.add(obj2);
            list.add(obj3);

            for(Book ob : list) {

                System.out.println("Title: " + ob.title);
                System.out.println("Author: " + ob.author);
                System.out.println("Publisher: " + ob.publisher);
                System.out.println("Genre: " + ob.genre);
                System.out.println("Year: " + ob.year);
                System.out.println("Pages: " + ob.pages);

                System.out.println(" - - - - - - - - - - - - - - - - - - - - - - - - ");
            }

            Menu.bookmenu();
        }

        public static void addBooks() {
            Scanner newBooks = new Scanner(System.in);

            for (int i = 0; i < 1; i++) {
            System.out.println("\n\nAdd a new book to the library:");

            System.out.println("\n>Please enter book title: ");
            String title = newBooks.nextLine();

            System.out.println(">Please enter book author: ");
            String author = newBooks.nextLine();

            System.out.println(">Please enter book publisher: ");
            String publisher = newBooks.nextLine();

            System.out.println(">Please enter book genre: ");
            String genre = newBooks.nextLine();

            System.out.println(">Please enter book release year: ");
            int year = newBooks.nextInt();

            System.out.println(">Please enter number of book pages: ");
            int pages = newBooks.nextInt();

            list.add(new Book(title, author, publisher, genre, year, pages));

            System.out.println("You have successfully added a new book to the library!");
            Menu.bookmenu();   
        }
    }

edited Nov 20, 2017 at 21:48

answered Nov 20, 2017 at 21:36

Evan Weissburg

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3 Comments

courtneyjones Over a year ago

Thank you for your answer. I've changed this, but it still isn't storing any user input. Do you have any other ideas?

courtneyjones Over a year ago

Yes. In my menu's if-statement in order to redirect the user view what's stored in the ArrayList I've had to use 'thelibrary.addBook(null);' will this have something to do with it? I don't really know how else to call that part because I'm brand new to java!

Evan Weissburg Over a year ago

Oh -- there's another issue. Once you leave the scope of addBooks, the variable list stops existing. If you try printing the contents of list inside addBooks, it'll work. If you want to be able to output from somewhere else, see my updated answer (give me a couple minutes)

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