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javadequejava-collections-api

Deque interface in the Collections hierarchy

In the Oracle tutorial about Collection interfaces, the interface hierarchy suggests that a Deque directly extends Collection.

enter image description here

But Deque extends Queue (there doesn't seem to be a change in this relationship across Java releases).

Any reason why this is not reflected in the hierarchy picture (e.g. the same way SortedSet is shown to extend Set)?

Solution

  • Taken from The Deque Interface - The Java™ Tutorials:

    The Deque interface is a richer abstract data type than both Stack and Queue because it implements both stacks and queues at the same time.

    You're right, Deque indeed extends Queue in Java...
    But even though this actual relation exists, I reckon that the point of omitting it it this diagram is that, conceptually, a Deque could be seen as not only a specialisation of a Queue.

    A Deque provides you with both the last-in-first-out principle of a Stack and the first-in-first-out one that a Queue offers, and representing only the strict actual hierarchy of these classes and interfaces could be misleading.
    For example, LinkedList implements Deque... and yet, you'd probably never place it under Deque on a schema explaining the different types of Collections and the relations between them. :)