A Queue is a data abstraction similar to a Stack, but
whereas a Stack pops the most recent element entered, i.e. the youngest, a Queue removes the earliest element entered, i.e. the oldest.
A Queue is efficiently constructed by keeping a reference to the last cell of the list as well as the first.
For the moment, we will leave the construction of a Queue as an exercise.
The operations on a queue are typically called:
enqueue: add an element to the queue
dequeue: remove oldest element in the queue
The following diagrams show how a Queue would work:
A Queue with four elements
The Queue above after dequeue(); item a is returned.
The Queue above after enqueue(e).
The empty Queue
For the empty Queue, it is adequate to have only one of the references set to null; it depends on how emptiness is checked.
Linked lists are not the only way to construct Stacks and Queues; arrays may be used instead:
Source files containing array implementations of Stack and Queue:
These are worth examining, as they show how to reallocate arrays in case the stack or queue grows too large.
In the case of a queue, Queue.java shows how to reuse existing space in a queue in a circular fashion, so that reallocation is not done unless absolutely necessary.
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