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POsorting method crashes when running
 

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  1. POsorting method crashes when running
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    <p>So I have been trying to run my LibraryTest.java program but it crashes when it has to use sortBooksByTitle() and sortBooksByNumPages(). The two sorting methods compile, but when I try to run the test class, it crashes. These are my three java files.</p> <p><strong>Book.java</strong></p> <pre><code>public class Book { private String author; private String title; private int numPages; public Book() { title = "EMPTY"; } public Book(String titleIn, String authorIn, int numPagesIn) { title = titleIn; author = authorIn; numPages = numPagesIn; } public String getAuthor() { return author; } public String getTitle() { return title; } public int getNumPages() { return numPages; } public String toString() { return title + " by " + author + " (" + numPages + " pages)"; } } </code></pre> <p><strong>Library.java</strong></p> <pre><code>import java.util.Random; import java.util.Arrays; public class Library { private Book[] array; private int count; private Random randomBook = new Random(); public Library(int numBooks) { array = new Book[numBooks]; count = 0; } public int getCount() { return count; } public void addBook(Book b) { //check if program can add new book if (count &lt; array.length) { array[count] = b; count++; } //if array is full, a message is thrown else { System.out.println("The Library is full!"); } } //Adds content of a library to another library. public void addLibrary(Library l) { for (Book b : l.array) { addBook(b); } } //Returns a book after receiving a String input. public Book getBook(String book) { for (int i = 0; i &lt; array.length - 1; i++) { String titleBook = array[i].getTitle(); if (titleBook.equals(book)) { return array[i]; } } Book newBook = new Book(); return newBook; } //Returns the book located in the array index given by the input. public Book getBook(int index) { if (index &lt; array.length) { System.out.printf("num: %d", index); return array[index - 1]; } Book newBook = new Book(); return newBook; } //Uses the random number generator and returns a book located in the array which //index is the random number obtained. public Book getBook() { int num = randomBook.nextInt(array.length); //System.out.printf("random num: %d", num); return array[num]; } //Sorts books alphabetically. public void sortBooksByNumPages() { for (int i = 0; i &lt; array.length - 1; i++) { for (int j = i + 1; j &lt; array.length; j++) { if (array[i].getNumPages() &gt; array[j].getNumPages()) { Book temp = array[i]; array[i] = array[j]; array[j] = temp; } } } } //Sorts books by number of pages in ascending order. public void sortBooksByTitle() { for (int i = 0; i &lt; array.length - 1; i++) { for (int n = i; n &lt; array.length; n++) { if (array[n].getTitle().compareToIgnoreCase(array[i].getTitle()) &lt; 0) { Book temp = array[i]; array[i] = array[n]; array[n] = temp; } } } } //Output each book's information. public String toString() { String s = "Number of books: " + count + "\n"; for (int i = 0; i &lt; array.length; i++) { s = s + array[i] + "\n"; } return s; } } </code></pre> <p><strong>LibraryTest.java</strong></p> <pre><code>public class LibraryTest { public static void main(String args[]) { Library lib = new Library(3); Book b1 = new Book("Java: How to Program", "Deitel and Deitel", 1496); lib.addBook(b1); Book b2 = new Book("A Brief History of Time", "Stephen Hawking", 212); lib.addBook(b2); Book b3 = new Book("The Art of War", "Sun Tzu", 384); lib.addBook(b3); Book b4 = new Book("Ender's Game", "Orson Scott Card", 352); // This addBook call should fail since the Library lib is full lib.addBook(b4); Book b5 = new Book("The Singularity is Near", "Ray Kurzweil", 672); Library lib2 = new Library(10); lib2.addBook(b4); lib2.addBook(b5); System.out.print("\n\nOriginal library contents\n"); // This should display that there are 3 books in the library &amp; info System.out.print(lib); System.out.print("\n\nAfter combining libraries\n"); lib2.addLibrary(lib); lib = lib2; // This should display that there are 5 books in the library &amp; info System.out.print(lib); System.out.print("\n\nSorted by title\n"); try { lib.sortBooksByTitle(); } catch (Exception e) { System.out.println(e.toString()); System.out.println(e.getMessage()); } // This should display the books in alphabetical order by title System.out.print(lib); /* * System.out.print("\n\nSorted by number of pages\n"); * lib.sortBooksByNumPages(); // This should display the books in * increasing order of the number of //pages each one has * System.out.print(lib); */ // This should display Ender's Game System.out.print("\n\nBook 2:\n" + lib.getBook(1)); // This should display the EMPTY book System.out.print("\n\nBook 20:\n" + lib.getBook(20)); System.out.print("\n\nBook 'The Art of War':\n" + lib.getBook("The Art of War")); // This should randomly display a book from the library (potentially //different each time) System.out.print("\n\nRandom book:\n" + lib.getBook()); } } </code></pre> <p>Again the 3 files compile just fine but crash when I try to run. Help me please. Thank you.</p>
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    1. USPrasanth
    1. USClayton Clark
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      1. POsorting method crashes when running
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    1. COCan you edit the post and show the error?
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      1. POsorting method crashes when running
      1. USJohn Szakmeister
    2. COWell when I run the test class, this is what I get: Sorted by Title Exception in thread "main" java.lang.NullPointerException at Library.sortBooksByTitle(Library.java:113) at LibraryTest.main(LibraryTest.java:36) Library.java:113 is the line if (array[n].getTitle().compareToIgnoreCase(array[i].getTitle()) < 0) And LibraryTest.java:36 is lib.sortBooksByTitle();
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      1. POsorting method crashes when running
      1. USClayton Clark
 

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