How implementation of java.util.queue uses LIFO?

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In Java doc:

[...] Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out)

How implementation of java.util.queue uses LIFO instead of FIFO?

Stack and LinkedList offered here are just a collections. Queue is not a collection. It is a part of concurrency package and can be used with threadpools.

I have just verified again and read javadoc that you have quoted. I think that the only option to use LIFO queue is to use priority queue with custom comparator that compare elements according to the insertion time in reverse order.

Queue (Java Platform SE 8 ), Packages that use Queue Classes in java.util that implement Queue Returns a view of a Deque as a Last-in-first-out (Lifo) Queue . Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out) How implementation of java.util.queue uses LIFO instead of FIFO?

You can use any Deque as a LIFO queue using Collections.asLifoQueue method:

Queue<Integer> arrayLifoQueue = Collections.asLifoQueue(new ArrayDeque<Integer>());
Queue<Integer> linkedListLifoQueue = Collections.asLifoQueue(new LinkedList<Integer>());

Uses of Interface java.util.Queue (Java Platform SE 7 ), trace of a stack implementation using array doubling Queue.java implements a FIFO queue of strings using a linked list. Implementing a collection that supports iteration in this way requires implementing Java's java.util. Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out). Whatever the ordering used, the head of the queue is that element which would be removed by a call to remove() or poll().

You can use a java.util.LinkedList and use the pop() and push() methods and use it like a stack, which is a LIFO queue.

Stacks and Queues, The Queue interface is available in java.util package and extends the Collection It is an ordered list of objects with its use limited to insert elements at the end of the from the start of list i.e. it follows the FIFO or the First-In-First-Out principle. if it is used, or according to the priority in case of priority queue implementation. The Stack class has been supplanted by more modern classes, as explained in the Javadoc: A more complete and consistent set of LIFO stack operations is provided by the Deque interface and its implementations, which should be used in preference to this class. See the current solution in Answer by Brett Ryan and Answer by Ivan.

Deque can be used as LIFO or FIFO

Queue Interface In Java, There are many collection classes in Java and all of them extend the java.util. First, let's instantiate one using the ArrayDeque implementation, which also As stated earlier, a Queue generally respects FIFO, which means that it'll return the  The Queue is used to insert elements at the end of the queue and removes from the beginning of the queue. It follows FIFO concept. The Java Queue supports all methods of Collection interface including insertion, deletion etc. LinkedList, ArrayBlockingQueue and PriorityQueue are the most frequently used implementations.

Implementation of the Queue can base on FIFO, priorities and LIFO - that is what official documentation says.

When a programmer first sees "Queue" he automatically thinks "it must be FIFO order" (or eventually prioritized order). But as documentation says there must be possibility to use Queue interface for LIFO ordering. Let me explain you how it can be done.

// FIFO queue usage
Queue<Integer> queue = new LinkedList<>();
queue.add(1);
queue.add(2);

queue.remove(); // returns 1
queue.remove(); // returns 2

// LIFO queue usage
Queue<Integer> queue = Collections.asLifoQueue(new ArrayDeque<>());
queue.add(1);
queue.add(2);

queue.remove(); // returns 2
queue.remove(); // returns 1

As you can see depending on the implementation, Queue interface can be used also as a LIFO.

Java Collections: Queue and Deque Interfaces, 1 I still remember: the exercise was to reverse a Stack using only methods on the Stack interface and no utility methods in java.util.Collections or other "static  The purpose of this article is to explain what a stack is and how to implement one, and present three practical use cases. Let’s get started! Clone the accompanying repository for this article here.

What's the point of implementing a Stack using two queues , In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every Queue implementation​  Whatever the ordering used, the head of the queue is that element which would be removed by a call to remove() or poll(). In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every Queue implementation must specify its ordering properties.

java.util.Queue, In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds The offer method is designed for use when failure is a normal, rather than exceptional occurrence, for example, Methods inherited from interface java.util. Where is LIFO used: Certain data structures like Stacks and other variants of Stacks uses LIFO approach for processing data. Sometimes computers use LIFO when data is extracted from an array or data buffer. When it is required to get the most recent information entered, the LIFO approach is used.

Queue (Java 2 Platform SE 5.0), In computer science, a stack is an abstract data type that serves as a collection of elements, For the use of the term LIFO in accounting, see LIFO (accounting). For the use of Following is an example program in Java language, using that class. import java.util.Stack; class Section 2.2.1: Stacks, Queues, and Deques, pp. There are several ways to implement a stack using one or two queues by tweaking their enqueue and dequeue operations. 1. Using two queues: The idea is to implement push operation of queue in such a way that the last entered item always ends up at the front of the queue. To achieve this, we need an additional queue.

Comments
  • What is your question? Queue as a data structure is normally FIFO and Stack as a data structure is normally LIFO.
  • Do you want to implement FIFO queue using LIFO datastructure?
  • +1 for question. In my book I have met following confusing quote: Queue is a base interface for containers that holds a sequence of elements for processing. For example, the classes implementing Queue can be LIFO (last in, first out— as in stack data structure) or FIFO (first in, first out—as in queue data structure).
  • docs.oracle.com/javase/7/docs/api/java/util/ArrayDeque.html
  • I do not have example right now. Just search for examples of priority queue usage. You will find a lot. Just remember that objects that you are going to enqueue should contain field "timestamp" and Comparator that you will implement will use this field in its compareTo() method.
  • could you double check the link? I think someone else might have bought the domain.