Exception in thread "main" java.lang.NullPointerException

Hi,

Im getting this error in my main class but not sure why. Hopefully someone can spot my error. The main class is:

Code:

---

public class BookMain{

public static void main(String[] args){

// Create Book
Book book = new Book("Data Structures");

// Create Chapters
Chapter ch1 = new Chapter();
Chapter ch2 = new Chapter();
Chapter ch3 = new Chapter();
Chapter ch4 = new Chapter();

ch1.setChapterTitle("Introduction");
ch2.setChapterTitle("Chapter 1");
ch3.setChapterTitle("Chapter 2");
ch4.setChapterTitle("Chapter 3");

// Create Pages
Page pg1 = new Page();
Page pg2 = new Page();
Page pg3 = new Page();
Page pg4 = new Page();

pg1.setPageNumber(1);
pg2.setPageNumber(2);
pg3.setPageNumber(3);
pg4.setPageNumber(4);

// errors occur here!
ch1.addPage(pg1);
ch2.addPage(pg2);
ch3.addPage(pg3);
ch4.addPage(pg4);

book.addChapter(ch1);
book.addChapter(ch2);
book.addChapter(ch3);
book.addChapter(ch4);

        }
}

---

and the other classes which have instantiations within main look like this:

Code:

---

import java.util.ArrayList;


public class Book{

// Attributes

private ArrayList<Chapter> chapters;

        String Title;

        public Book(String ti){
        Title = ti;
        }


        // Add chapter

        public void addChapter(Chapter c){
        chapters.add(c);

        }

        // Size of book in chapters

        public void size(){

        System.out.println("This book contains  " +
                                        chapters.size() + "  chapters.");

        }

        //size of book in pages

        public void numPages(){

        int total = 0;

                for(Chapter ch : chapters){

                total += (int) ch.size();

                }

        System.out.println("the book has  " + total + "  pages.");


        }

        // Get Title

        public void getTitle(){

        System.out.println("Now reading: " + Title);

        }

}

---

Code:

---


import java.util.ArrayList;

public class Chapter{


        // Attributes
       
        private ArrayList<Page> pages;
        String chapterName;

        // set chapter name

        public void setChapterTitle(String ti){

        chapterName = ti;

        }

        // get chapter name

        public void getChapterTitle(){

        System.out.println(" " + chapterName);

        }


        // Add page

        public void addPage(Page p){

        pages.add(p);

        }

        //How many pages

        public int size(){

        System.out.println("This chapter has  " +
                                        pages.size() + "  pages.");
                               
                                return pages.size();

        }

}

---

Code:

---

public class Page{

// Attributes

        int pageNumber;


        // Set page Number

        public void setPageNumber(int pgNum){

        pageNumber = pgNum;

        }

        // Get page Number
       
        public void getPageNumber(){

        System.out.println("The page number is  " + pageNumber);

        }

}

---

The error occurs when I try add an object into another's inner ArrayList. E.g when I try add ch1 to book with book.addChapter(ch1);

Any ideas?

Cheers

BIOS

You need to instantiate the ArrayLists (set the reference to the objects using new)
Creating Objects (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

Hi DoWhile,

Thanks alot for the reply. Aha! That's great. I'm new to collections and this is basically an exercise in using them :P So i declared the ArrayList objects but didn't instantiate them! OOps. Will rectify.

Cheers

BIOS