Abstract supertype of datastructures that can be used as FIFO queues.

A Queue has a well-defined front and back. Elements may be added to the back of the queue by offer(), and removed from the front of the queue by accept().

Note that many Queues are also Lists, but there is no defined relationship between the order of elements in the list and the direction of the queue. In particular, the front of the queue may be first element of the list, or it may be the last element of the list.