Fifo java queue11/5/2023 I just did a quick review of my copy and neither "Stack" nor "Queue" appears in the index, nor as a discrete sub-topic, but a chapter exists for Linear Lists, which he equates to "queues". One of the most prominent ones was "Algorithms + Data Structures = Programs" by Niklaus (Mr. LinkedList is a Queue.īack when Pascal was the Big Thing, before OOP, there was an explosion of works published about data structures. Queue is a Collection, and thus iterable, not a complete black box. In most multi-tasking OS's, the name of the system's collection of tasks is a "queue", but since we have to distinguish between different task states, one has to be able to not merely push and pop, but also traverse and search. We've been talking about single-access/double-access point queues, but really the abstract queue covers a lot more than that. You can use a double-ended Queue in either stack or "queue" mode. If you have a class that implements Queue and it's a LIFO ("stack"), then the head() method returns the newest element in the collection. The same data structure with different access methods.Īs I said, if you have a class that implements Queue and it's a FIFO ("Queue"), then the head() method returns the oldest element in the collection. So they still aren't all that different.īut okay, it could be kind of confusing to future readers if you used a Queue with that rule to implement a Stack. For example you might use a rule which assigns the highest priority to the most recent entry added. FIFO means the least recent entry added has the highest priority, but other rules can assign different priorities to entries based on various attributes of the entries. But okay, typically Stack processing involves taking out the entry which was put in most recently, whereas Queue processing involves taking out the entry which appears to be the highest priority according to some rule. So no, they aren't different at that level of detail. For a Stack, on the other hand, you have some processes which put entries into the Stack, to be taken out later by some processes. Monica Shiralkar wrote:But in any case, aren't Queue and Stack different data structures?įor a Queue, you have some processes which put entries into the Queue, to be taken out later by some processes. An SQS standard queue will relax the FIFO requirement in order to just give you a queue that's fast to use. Ideally you'd handle them FIFO - but the data is spread across machines in different places, so your idea of who'd next may be a little out-of-date. Imagine you have a bunch of customers around the world trying to place orders for something, and you're putting these in a queue to handle them. This is relevant if you're running code in a cloud operating worldwide. Looking at the SQS standard queue, it seems like that one is for a situation where you would kind of like a FIFO queue, but you want it to be as fast as possible, and you're willing to let it be wrong sometimes about the FIFO ordering, in order to be fast. So why have it? Well, for some applications, that minimal interface is all you need - and by abandoning the requirements of, say, a List or a Deque, they can allow other kinds of flexibility about how they work. But Queue is kind of a minimal interface that doesn't support all the operations that the other interfaces do. a LinkedList is a Queue and a Deque and a List. Some Queue implementations may implement other interfaces as well, if they do additional things. You can iterate through to see the contents, but the iteration order may not be guaranteed - it's not the primary use case. In particular, compared to a List, they don't have a get(index) or set(index) for random access. A FIFO queue is the classic example, but not the only one. They use different names based on whether you want to allow exceptions or check for nulls, but everything is either adding something to the queue, or getting something out of the queue, one element at a time. We can see that the same is also true when applied to Strings: PriorityQueue stringQueue = new PriorityQueue() ĪssertEquals("cherry", third) 6.To elaborate a bit - the key operations of a queue are to add an element, and get/remove an element. Let’s take a look at how this works with a simple unit test: PriorityQueue integerQueue = new PriorityQueue() ĭespite the order in which our integers were added to the Priority Queue, we can see that the retrieval order is changed according to the natural order of the numbers. When new elements are inserted into the Priority Queue, they are ordered based on their natural ordering, or by a defined Comparator provided when we construct the Priority Queue. One such exception to this rule is the PriorityQueue. We saw earlier that most of the Queues that we come across in Java follow the FIFO principle.
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