I'm writing my own PriorityQueue class and I have:
private Queue<E>[] queues;
public PriorityQueue(int maxSize) {
queues = new Queue[maxSize+1];
size = maxSize;
}
This compiles, but when I call .add on the priorityQueue I get this error:
java.lang.NullPointerException
at PriorityQueue.add(PriorityQueue.java:13)
Here's add:
public void add(E item, int priority) {
queues[priority].offer(item);
}
you need to initialize your queues:
public PriorityQueue(int maxSize) {
queues = new Queue[maxSize+1];
size = maxSize;
for(int i = 0; i <= maxSize; i++)
queues[i] = new LinkedList<E>();
}
You can pick whatever Queue implementation you want, I just picked LinkedList because it popped in my head first...
When you do this:
queues = new Queue[maxSize+1];
you create an array of size maxSize+1 but each element of the array is still null. You have to iterate over the array and place a Queue object in each position before you can add to it.
You create a new array, but you never initialize the elements of queues to be a Queue. By default, all elements of queues are null until you initialize them.
queues[priority] returns a queue, on which you're calling .offer(). Because you're not initializing the elements of the array, you're calling .offer() on a null reference, which is causing your error.
You haven't initialized the array of queues to have any queues.
You need to put queues into the array. Also not sure why you add an additional queue unless you're just trying to pretend arrays aren't 0-based.
public PriorityQueue(int maxSize) {
queues = new Queue<E>[maxSize+1];
for (int i = 0; i < maxSize + 1; i++) {
queues[i] = new LinkedList<E>(); // Or whatever implementation you're using.
}
size = maxSize;
}