What's the difference between a DoubleLinkedQueue and a ListQueue in Dart?

Question

The Dart core API has two classes that implement the Queue<E> interface, DoubleLinkedQueue<E> and ListQueue<E> .

The documentation of both classes is almost identical, the only difference that is explicitly mentioned is the following note in the ListQueue<E> documentation:

Operations like removeAll and removeWhere are very inefficient. If those are needed, use a DoubleLinkedQueue instead.

What is the actual difference between them implementation-wise and when should which implementation be used?

Answer 1

The DoubleLinkedQueue is basically a Queue implemented on top of a double-linked list. This means that removing elements at arbitrary positions in it is fast, since it only requires the adjustment of pointers.

The ListQueue is implemented on top of a List. First and last are indices into the list. In general this is the more efficient implementation, since it has less memory-overhead than the double-linked list.

You can see both implementations here

Most of the time you want to use the ListQueue . For that reason, the Queue interface defaults to the ListQueue (ie Queue() returns a ListQueue ).

The DoubleLinkedQueue implementation is mostly useful if you need to selectively remove elements inside the queue. It's a relatively rare scenario, and the main-reason the class is in the dart-libraries, is that the DoubleLinkedQueue existed before the ListQueue . Since we already had the DoubleLinkedQueue we kept it. If we had started out with the ListQueue we probably wouldn't have added the DoubleLinkedQueue .