I am a beginner-intermediate java programmer and I am getting a null pointer exception from my arraylist. I am writing a bookstore program for APCS and when i add the book, it is supposed to add to the arraylist in the inventory class. But when i call a method to search for a book (e.g. by title), it shows that there isn't anything in the arraylist.

//Here is my inventory class --> it has all methods for adding the book or searching for one The searching methods are in getBookByTitle, getBookByAuthor, and getBookByISBN and the method for adding a book is addBook

package webbazonab;

//Inventory Class

//Bharath Senthil
//Ansh Sikka

import java.util.ArrayList;
public class Inventory{
private ArrayList<Book> allBooks = new ArrayList<Book>();
    private String bookTitles;
    private String bookAuthors;
    private String bookPrices;
    private String bookCopies;
    private String ISBNs;


public Inventory()
{
}
//@param double price, int copies, String bookTitle, String Author, String isbnNumber
public void addBooks(Book addedBook){
    allBooks.add(addedBook);

}
public boolean isAvailable(){
    for(Book myBook : allBooks){
        if(myBook.copiesLeft() == 0)
            return false;

    }
    return true;
}
    public String populateTitle(){
        for (Book titleBooks : allBooks){
            bookTitles = titleBooks.getTitle() + "\n";
            return bookTitles;
        }
        return bookTitles;
    }
    public String populateAuthor(){
        for(Book authorBooks : allBooks){
            bookAuthors = authorBooks.getAuthor() + "\n";
            return bookAuthors;
        }
        return bookAuthors;
    }
    public String populatePrice(){
        for (Book pricedBooks : allBooks){
            bookPrices = String.valueOf(pricedBooks.getPrice()) + "\n";

        }
       return "$" + bookPrices;      
    }

/**
 *
 * @return
 */
public String populateCopies(){
        for (Book amtBooks : allBooks){
            bookCopies = String.valueOf(amtBooks.copiesLeft()) + "\n";
            return bookCopies;
        }
        return bookCopies;
    }
    public String populateISBN(){
        for (Book isbnNums : allBooks){
            ISBNs = isbnNums.getIsbn() + "\n";
            return ISBNs;
        }
        return ISBNs;
    }

    @SuppressWarnings("empty-statement")
    public Book getBookByTitle(String titleSearch) {
        for(Book titleBook : allBooks) {
            if (titleBook.getTitle().equals(titleSearch)) {
                return titleBook;
            }
        }
        return null;
    }
    public Book getBookByISBN(String isbnSearch){
        for(Book isbnBookSearches : allBooks){
    if(isbnBookSearches.getIsbn().equals(isbnSearch)){
                    return isbnBookSearches;
            }
        }
        return null;
}
    public Book getBookByAuthor(String authorSearch){
        for(Book authorBookSearches : allBooks){
            if(authorBookSearches.getAuthor().equals(authorSearch)){
                return authorBookSearches;
            }
        }
        return null;
}
    public void sort(){
        for(int i = 0; i < allBooks.size(); i++)
        {
    for(int k = 0; k < allBooks.size(); k++)
    {
                if(((Book) allBooks.get(i)).getIsbn().compareTo(((Book) allBooks.get(k)).getIsbn()) < 1)
                {
        Book temp = (Book) allBooks.get(k);
        allBooks.set(k, allBooks.get(i));
        allBooks.set(i, temp);
                }
                else if(((Book) allBooks.get(i)).getIsbn().compareTo(((Book) allBooks.get(k)).getIsbn()) > 1)
                {
                    Book temp = (Book) allBooks.get(i);
                    allBooks.set(i, allBooks.get(k));
        allBooks.set(k, temp);
                }
            }
        }
        }
    public ArrayList<Book> getBooks(){
        return allBooks;
    }

}

//The exception occurs when i call the method here (in another class):
        Inventory lib = new Inventory();
        jTextField12.setText(lib.getBookByAuthor(authorSearch).getTitle());

Here is my book class if you need it

package webbazonab;

//Webbazon AB 
//Project By: Ansh Sikka and Bharath Senthil

public class Book
{

private double myPrice;
private String myTitle;
private String bookAuthor;
private String isbn;
private int myCopies;   

public Book(double price, int copies, String bookTitle, String Author, String isbnNumber)
{
    myPrice = price;
    myCopies = copies;
    myTitle = bookTitle;
    bookAuthor = Author;
    isbn = isbnNumber;

}
public double getPrice()
{
    return myPrice;
}

public String getIsbn()
{
    return isbn;
}
public String getTitle()
{
    return myTitle;
}
public String getAuthor()
{
    return bookAuthor;
}
public int copiesLeft(){
    return myCopies;
}
    public String notFound(){
        return "The book you searched for could not be found!";
    }
public String toString()
{
    return "Title: " + getTitle() + "\nAuthor: " + getAuthor() + "\nNumber of Available Books: " + copiesLeft() + "\nPrice: $" + getPrice(); 
}

}

Thanks!