Nested List class for Simulation Queue

[Cheddz]

Hello, all. This is my first post to the forums, and I hope someone out there can help me! For a client, I am developing a (fairly) sophisticated discrete-event simulation written entirely in Java. For anyone familiar with simulations, before an entity can be serviced, it must enter a queue. In version 0.1 of the simulation there was only one queue, a Queue1 class, with a nested List class inside of it, which just initiated an empty list. The Queue1 class added or removed entities from the queue. The List class looked as follows:

class List {
	// Global variables for List
	public Commodity work;
	public List next;
	
	public List() {
		work = null;
		next = null;
	}
}

Now, I need another queue. We can call it Queue2, for kicks. If I try to add another class List {} inside Queue2, it says List has already been defined.

So, I tried creating a generic Queue class with the list nested, and had my Queue classes, Queue1 and Queue2, extend Queue. However, both queues are still feeding off of the same List. How do I create new lists in each individual queue class so that I can maintain different lists simultaneously? If you would like the code for one of the Queue classes, I'd be glad to post that, as well. Thanks!

[mrmatt1111]

Why aren't you just using a LinkedList?

LinkedList<Commodity> queue1 = new LinkedList<Commodity>();
LinkedList<Commodity> queue2 = new LinkedList<Commodity>();

Edit:

If threads are involved: ConcurrentLinkedQueue

[Cheddz]

Well that's definitely an implementation I can look into, but the List is currently called if a double length == 0, thus creating a new list. For some reason, the second process is reading off the same length value as the first process, and thus never evaluates to true, and passes right by it. However, length is a private variable in both classes. The enqueue method looks as follows:

	public void enqueue(Commodity commodityToAdd) {
		if(commodityToAdd == null) 
			return;
		
		List ptr = head;
		
		if(isEmpty()) {
			head = new List();
			ptr = head;
		}
		else {
			while(ptr.next != null) 
				ptr = ptr.next;
			
			ptr.next = new List();
			ptr = ptr.next;
		}
		
		ptr.next = null;
		ptr.work = commodityToAdd;
		length++;
	}

There is also a dequeue method which should decrement the length variable.