Question

There is probably a simple solution to this problem I’ve been having all night. At least I hope there is. When attempting to offer an object to my subQueues LinkedList, I receive a NullPointerException. My program prints out the correct “head” Objects and “digit” integers, but then the Exception is thrown and the program ends.

My program, in short, is supposed to take a mainQueue LinkedList of integers, look at them all one-by-one, and sort them. Its examining the last digit of each integer, and placing them into corresponding subQueues. As of now, I am only to the ones place. After I get past this dilemma, I’ll be able to work out the tens, hundreds, etc.

Ex)

mainQueue = { 12 50 215 100 85 539 16 35 }
subQueue[0] = { 50 100 }
subQueue[1] = { }
subQueue[2] = { 12 }
subQueue[3] = { }
subQueue[4] = { }
subQueue[5] = { 215 85 35 } 
subQueue[6] = { 16 }
subQueue[7] = { }
subQueue[8] = { }
subQueue[9] = { 539 }

So what am I doing wrong here? Like I said, once I get by this little problem, the rest of the program should be a breeze. Any help is appreciated, thanks!

public class Sorting
 {
   private LinkedList mainQueue;
   private LinkedList[] subQueues;
   private final int SIZE = 10;
   private int maxDigits; //maximum number of digits

   //The constructor instantiates the mainQueue using the LinkedList,
   //subQueue array as an array of LinkedList using SIZE(10),
   //and initializes maxDigits = 0;
   public Sorting()
   {
    mainQueue = new LinkedList();
    for (int i=0; i<SIZE; i++)
    {
        subQueues = new LinkedList[i];
    }

    // I have also tried:
    // subQueues = new LinkedList[SIZE];   
    //I get the same runtime error.

    maxDigits = 0;
 }

   public void sortNumbers()
    {
    while (mainQueue.isEmpty() == false)
    {
        Object head = mainQueue.peek();
        mainQueue.remove();
        String digitLine = "" + head;
        int digit = Integer.parseInt(digitLine.substring(digitLine.length()-1, digitLine.length()));

        System.out.println(head);
        System.out.println(digit);

        subQueues[digit].offer(head);
    }
    }
}

Answer 1

You’re not correctly building your subQueues it looks like.
If you want an array of SIZE linked-lists, try this:

subQueues = new LinkedList[ SIZE ];
for ( int i = 0; i < SIZE; ++i ) {
    subQueues[i] = new LinkedList();
}

Note that this is using raw types as your code is, though preferably you should use parameterized types.