Question

I try to program radix sort, but I have NullPointerException when I run this code

public class ThreeRadixSort {

    public Queue getSortedData(Queue queue){
        Queue array[] = new Queue[10];

        for(int i=1;i<=3;i++){

        while(queue.front != null){
            Student student = queue.dequeue();

            if( i == 1) 
                array[(int)(student.id%10)].enqueue(student);
            else if( i == 2)
                array[(int)((student.id%100)/10)].enqueue(student);
            else    
                array[(int)(student.id/100)].enqueue(student);

        }

        for(int j=0;j<=9;j++){

        while( array[j] != null && array[j].front != null)
            queue.enqueue(array[j].dequeue());
        }
        }   
        return queue;
    }
}

The Exception show when the implement reach to

    array[(int)(student.id%10)].enqueue(student);

Answer 1

The problem here is that when you initialize your array of Queue, each spot is initialized to null. So right now, you are trying to call the enqueue method on null. You need to loop through each position of the array and assign it a new Queue() or however you initialize those.

So for example:

for (int i = 0; i < array.length; i++) {
  array[i] = new Queue();
}