How can I can solve my NullPointerException problem?

Question

Hi all,
So I'm playing around with some code and was wondering why I get the following?
Exception in thread "main" java.lang.NullPointerException
     at lecture_examples.Library.searchForTitle(Library.java:38)
     at lecture_examples.LibraryManager.main(LibraryManager.java:24)The problem seems to occur when I call searchForTitle() in the library class. Here is my code:
package lecture_examples;
public class LibraryManager
     public static void main(String []args)
          Library lib = new Library(150);
          Book book1 = new Book("Africa", "Ray Mears", "1111-1111-1111-1", "Penguin Books", 2007);
          Book book2 = new Book("North America", "Ray Mears", "2222-2222-2222-2", "Penguin Books", 2007);
          Book book3 = new Book("South America", "Ray Mears", "3333-3333-3333-3", "Penguin Books", 2007);
          Book book4 = new Book("Europe", "Ray Mears", "4444-4444-4444-4444-4", "Penguin Books", 2007);
          Book book5 = new Book("Asia", "Ray Mears", "5555-5555-5555-5555-5", "Penguin Books", 2007);
          Book book6 = new Book("Australasia", "Ray Mears", "6666-6666-6666-6666-6", "Penguin Books", 2007);
          Book book7 = new Book("India", "Ray Mears", "7777-7777-7777-7", "Penguin Books", 2007);
          Book book8 = new Book("South Pacific", "Ray Mears", "8888-8888-8888-8", "Penguin Books", 2007);
          lib.addBook(book1);
          lib.addBook(book2);
          lib.addBook(book3);
          lib.addBook(book4);
          lib.addBook(book5);
          lib.addBook(book6);
          lib.addBook(book7);
          lib.addBook(book8);
          lib.searchForTitle("India");
     //other static methods used by main
package lecture_examples;
import java.util.Vector;
public class Library
     private int currSize = 0;
     private Book[] books;
     public Library()
          books=new Book[1000];
     public Library(int maxSize)
          books=new Book[maxSize];
     public void addBook(Book newBook)
          if(currSize<books.length) //if there the book array is not full
               books[currSize++]=newBook;//Puts the newBook into an element of book array
          else
               System.out.println("Your Library is Full");//Output error message
     public Book[] searchForTitle(String aTitle)
          Vector v=new Vector();//create an expandable Vector called v.
          for(int i=0;i<books.length;i++)//loops through the array of books
               if(aTitle.equals(books.getTitle()))//compares the searched Title with each title stored in each element of the book array
                    v.add(books[i]);//if the book lexigraphically matches a book in the book array it adds the existing book to the vector
          Book[] results=new Book[v.size()];//creates an book-result array which is the same size as the vector
          for(int i=0;i<v.size();i++)
               results[i]=(Book)(v.elementAt(i)); //loops through the results vector and casts them as books into the book array
          return results;     //returns results     
     public void removeBook(String isbn)
          int index=getIndex(isbn);//gets the index of isbn and assigns it to the index integer variable
          for (int i=index;i<currSize-1;i++) //loops through the books
               books[i]=books[i+1]; //moves each book up one element in the book array
          currSize--;//decrements the number of books in the library
     public int getIndex(String isbn)
          int index = 0;
          for(int i=0;i<currSize;i++)//loops through the number of books
               if(isbn.equals(books[i].getIsbn()))//If the isbn is in the book array
                    index = i; //assign index to i
          return index;
package lecture_examples;
public class Book
     private String title = "Invalid";
     private String author = "Invalid";
     private String isbn = "Invalid";
     private String publisher = "Invalid";
     private int publicationYear = 0000;
     public Book(String title, String author, String isbn, String publisher, int publicationYear)
          this.title = title;
          this.author = author;
          this.isbn = isbn;
          this.publisher = publisher;
          this.publicationYear = publicationYear;          
     public String getTitle()
          return title;
     public String getAuthor()
          return author;
     public String getIsbn()
          return isbn;
     public String getPublisher()
          return publisher;
     public int getPublicationYear()
          return publicationYear;
     public void setTitle(String newTitle)
          title = newTitle;
     public void setAuthor(String newAuthor)
          author = newAuthor;
     public void setisbn(String newisbn)
          isbn = newisbn;
     public void setPublisher(String newPublisher)
          publisher = newPublisher;
     public void setPublicationYear(int newPublicationYear)
          publicationYear = newPublicationYear;
     public String toString()
          String fullDetails = "\nTitle = \t" + title
          + "\nAuthor = \t" + author
          + "\nISBN = \t\t" + isbn
          + "\nPublisher = \t" + publisher
          + "\nYear = \t\t" + publicationYear;
          System.out.println(fullDetails);
          return fullDetails;

Answer 1

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Exception in thread "main" java.lang.NullPointerException
     at lecture_examples.Library.searchForTitle(Library.java:38)
     at lecture_examples.LibraryManager.main(LibraryManager.java:24)Simple, line #38 in your Library.java in giving you NullPointerException. In the constructor of your Library.java, you are creating books array of size 1000, which will be initialized to an Books array with 1000 null values. Are you having a reference to a Book object at all 1000 array positions? If no, then in your searchForTitle() method you are iterating through the entire books array using a for loop and at every array object you are calling books.getTitle(). But, if your array has null values then it will throw NullPointerException. So do something like this...
if(books!=null && aTitle.equals(books[i].getTitle()))//compares the searched Title with each title stored in each element of the book array
                    v.add(books[i]);//if the book lexigraphically matches a book in the book array it adds the existing book to the vector