I have NullPointerException error

Question

Hello,
I am new to this forum, so if I make any code-of-conduct blunders please forgive me. I am doing a program in java for an intro to programming class at the University of South Alabama. The objective is to create a program that will manage a fictional inventory of software items bieng stored in an array of objects. I have a to use an instance method and driver method. (I am a complete novice, so i hope all this is making sense.)I am using the JOptionPane.showOptionDialog, and i have only been shown one example of it. In the showOptionDialog, i am giving the user options on how the array should be sorted.
This is a sample of my code that i am getting the run time error in.
     public static void viewInventory(Application a[])
          //Allows the user to choose how the inventory should be displayed.
          String sorts[]={"By Title","By Price","By Quantity","Exit"};
          int choice;
          do
               choice = JOptionPane.showOptionDialog(null,"How should the inventory be sorted?",
                    "View Inventory",JOptionPane.DEFAULT_OPTION, JOptionPane.QUESTION_MESSAGE,null,
                    sorts,sorts[0]);
               switch(choice)
                    case 0:
                         sortByTitle(a);
                         break;
                    case 1:
                         sortByPrice(a);
                         break;
                    case 2:
                         sortByQuantity(a);
                         break;
          }while(choice < 3);     
     }//End viewInventory method.
This is the error message i am given:
java.lang.NullPointerException
     at SoftwareInventory.sortByQuantity(SoftwareInventory.java:101)
     at SoftwareInventory.viewInventory(SoftwareInventory.java:59)
     at SoftwareInventory.main(SoftwareInventory.java:25)
Any help on what i might be doing wrong or some general information on the showOptionsDialog method would be greatly appreciated.
Thank you.

Answer 1

It looks like the Application array 'a' may be empty.
Before you do your switch statement add the following
line:
System.out.println("Application array has this many
items: " + a.lenght);Let us know what was printed out.ok, I was wrong when I said that the array was empty.We have not done the println in a while, I forgot that it would not just show up in a dialog box( i'm working in the jgrasp ide) I missed it earlier. I think I know what the problem is, but I'm not sure how to fix it. We have been working with arrays that are declared and "automatically" sized to the information we put in them. In this case, we have to declare an array of a certain size[100] that we can add information too.
We are to put 6 items in the array and then design methods that will allow a user to view the array, edit information in the array, or add a new item to the array.
How do I deal with an array that has several null spaces in it? (The array has to be big enough for the 100 items, it was one of the specs of the assignment)
Here is the program so far:(I hope this does not make the post so big that it is a problem)
     public static void main( String args[])
          Application inventory[] = new Application[100];
          inventory[0] = new Application("Jumpstart Toddlers", 19.99, 500);
          inventory[1] = new Application("Norton Antivirus 2003", 49.99, 1200);
          inventory[2] = new Application("Roller Coaster Tycoon", 19.99, 1000);
     inventory[3] = new Application("Quick Books Basic 2002",179.99, 750);
          inventory[4] = new Application("Jumpstart 1st Grade", 19.99,680);
     inventory[5] = new Application("300 Best Arcade Games",9.99,975);
          String options[]={"View"};//,"Add","Edit","Exit"};
          int choice;
          do
               choice = JOptionPane.showOptionDialog(null,"Choose one of the following options",
                    "Main Menu",JOptionPane.DEFAULT_OPTION, JOptionPane.PLAIN_MESSAGE, null,
                    options,options[0]);
               switch(choice)
                    case 0:
                         viewInventory(inventory);
                         break;
               /*     case 1:
                         addItem(inventory);
                         break;
                    case 2:
                         editItem(inventory);
                         break;*/
          }while(choice < 3);//choice 3 is exit condition of the loop.*/
          System.exit(0);
     }//End of main method.
     //Method for viewing the inventory.
     public static void viewInventory(Application a[])
          //Allows the user to choose how the inventory should be displayed.
          String sorts[]={"By Title","By Price","By Quantity","Exit"};
          int choice;
          do
               choice = JOptionPane.showOptionDialog(null,"How should the inventory be sorted?",
                    "View Inventory",JOptionPane.DEFAULT_OPTION, JOptionPane.QUESTION_MESSAGE,null,
                    sorts,sorts[0]);
          //     System.out.println("Application array has this many items: " + a.length);
               switch(choice)
                    case 0:
                         sortByTitle(a);
                         break;
                    case 1:
                         sortByPrice(a);
                         break;
                    case 2:
                         sortByQuantity(a);
                         break;
          }while(choice < 3);     
     }//End viewInventory method.
     //Method for sorting by the title of the application.     
     public static void sortByTitle(Application b[] )
          for (int i = 1; i < b.length -1; i++)
               for (int j = 0; j < b.length - 1; j++)
                    if (b[j].getTitle().compareTo ( b[j+1].getTitle())>0)
                         swap(b, j, j+1);
          displayArray(b);
     //Method for sorting by the price of the application.
     public static void sortByPrice(Application b[])
          for (int i = 1; i < b.length -1; i++)
               for (int j = 0; j < b.length - 1; j++)
                    if (b[j].getPrice() > b[j+1].getPrice())
                         swap(b, j, j+1);
          displayArray(b);
     //Method for sorting by the quantity of the application.
     public static void sortByQuantity(Application b[])
          for (int i = 1; i < b.length -1; i++)
               for (int j = 0; j < b.length - 1; j++)
                    if (b[j].getQuantity() > b[j+1].getQuantity())
                         swap(b, j, j+1);
          displayArray(b);
     //Method for sorting the array.
     public static void swap(Application c[], int index1, int index2)
          Application temp = new Application(c[index1].getTitle(),c[index1].getPrice(),
               c[index1].getQuantity());
          c[index1] = new Application(c[index2].getTitle(),c[index2].getPrice(),
               c[index2].getQuantity());
          c[index2] = new Application(temp.getTitle(), temp.getPrice(), temp.getQuantity());
     public static void displayArray(Application d[])
          String output =" ";
          for (int i=0; i <d.length; i++)
               output += d[i] + "\n\n";
          JTextArea text = new JTextArea(15,35);
          text.setText (output);
          JScrollPane scroller = new JScrollPane (text);
          JOptionPane.showMessageDialog(null, scroller, "dammit",JOptionPane.PLAIN_MESSAGE);