DVD分类程序中的线程“ main”中的异常java.lang.NullPointerException

Question

I am attempting to create a program that puts DVD information into an array and sorts them alphabetically based on title using the compareTo method from Comparable.

I have a boolean method which finds out whether the two titles are the same:

public boolean equals (Object other)
{
    return (title.equals(((DVD)other).getTitle()));
}

and my compareTo method looks like this:

public int compareTo (Object other)
{
    int result;

    String otherTitle = ((DVD)other).getTitle();

    result = title.compareTo(otherTitle);

    return result;
}

These two methods are in the DVD class which I have implementing Comparable.

The Sorting algorithm itself I took used directly from an example that sorts names alphabetically but here that is as well:

public class DVDSorting 
{
    public static void selectionSort (Comparable[] list)
    {
        int min;
        Comparable temp;

        for (int index = 0; index < list.length-1; index++)
        {
            min = index;
            for (int scan = index+1; scan < list.length; scan++)
                if (list[scan].compareTo(list[min]) < 0)
                    min = scan;

            temp = list[min];
            list[min] = list[index];
            list[index] = temp;
        }
    }

    public static void insertionSort(Comparable[] list)
    {
        for (int index = 1; index < list.length; index++)
        {
            Comparable key = list[index];
            int position = index;
            while (position > 0 && key.compareTo(list[position-1]) < 0)
            {
                list[position] = list[position-1];
                position--;
            }

            list[position] = key;
        }
    }
}

I then made a DVDCollection class that initializes the array and contains a method to add DVD's.

In my driver class to test I am able to insert DVD's and print out what has been inserted but when I attempt to sort them I get a null pointer exception even though I don't see anything initialized to null. Here is my driver:

public class Movies
{
    public static void main (String[] args)
    {
        DVDCollection movies = new DVDCollection();

        movies.addDVD("The Godfather", "Francis Ford Coppola", 1972, 24.95, true);
        movies.addDVD("District 9", "Neill Blomkamp", 2009, 19.95, false);
        movies.addDVD("Iron Man", "Jon Favreau", 2008, 15.95, false);
        movies.addDVD("All About Eve", "Joseph Mankiewicz", 1950, 17.50, false);
        movies.addDVD("The Matrix", "Andy & Lana Wachowski", 1999, 19.95, true);
        movies.addDVD("Clash of the Titans", "Louis Leterrier", 2010, 19.95, true);

        DVDSorting.selectionSort(movies.collection);
        System.out.println (movies);

        movies.addDVD("Iron Man 2", "Jon Favreau", 2010, 22.99, false);
        movies.addDVD("Casablanca", "Michael Curtiz", 1942, 19.95, false);
        movies.addDVD("Clash of the Titans", "Desmond Davis", 1981, 5.00, false);


        DVDSorting.selectionSort(movies.collection);
        System.out.println (movies);
    }
}

I used movies.collection in the sorting call because when I attempted to use just movies like examples I have seen, it said selectionSort was not applicable for movies So i made collection a public variable and just used that.

Here's the error.

Exception in thread "main" java.lang.NullPointerException
    at DVDSorting.selectionSort(DVDSorting.java:13)
    at Movies.main(Movies.java:15)

It tells me the error is in the DVDSorting class but since that's copied from another source I have a feeling the actual error is in my compareTo method in my DVD class.

I'm very sorry if my question doesn't follow proper etiquette of the site, it's my first time posting. Any help would be greatly appreciated.

Answer 1

My guess is that either list or list[scan] is null at some point in the selectionSort() method. Perhaps try verifying that movies.collection (from your main method) is actually storing the values that you think it is.

This is a good task for the Eclipse debugger.