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Java / Java 21 Collection Framework features Interview questions

What is the difference between SequencedCollection and the Queue interface?

SequencedCollection and Queue both deal with ordered access, but they model different things: SequencedCollection is about having elements arranged with a defined first-to-last order and being able to reach either end, while Queue is specifically about FIFO-style processing semantics.

SequencedCollectionQueue
Access to both first and last endsPrimarily designed around one end for insertion, another for removal
Has a reversed() viewNo reversed() method of its own
Implemented by List and DequeImplemented by LinkedList, PriorityQueue, ArrayDeque
Does not extend QueueDoes not extend SequencedCollection

Notably, Queue itself does not extend SequencedCollection - PriorityQueue, for example, implements Queue but doesn't have a meaningful "first-to-last encounter order" in the iteration sense, since its iteration order isn't the same as its priority-based poll order, so it correctly stays outside the sequenced hierarchy.

Queue and SequencedCollection relate to each other by:
PriorityQueue doesn't implement SequencedCollection because:

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