Since J2SE5.0, Java's library contains a Queue interface that specifies queue operations implementing classes include LinkedList and (since J2SE 1.6) ArrayDeque. For example, Perl and Ruby allow pushing and popping an array from both ends, so one can use push and shift functions to enqueue and dequeue a list (or, in reverse, one can use unshift and pop), although in some cases these operations are not efficient.Ĭ++'s Standard Template Library provides a ' queue' templated class which is restricted to only push/pop operations. Queues may be implemented as a separate data type, or maybe considered a special case of a double-ended queue (deque) and not implemented separately. A deque implemented using a modified dynamic array.However, a small modification-keeping a pointer to the last node in addition to the first one-will enable it to implement an efficient queue. A regular singly linked list only has efficient insertion and deletion at one end.A doubly linked list has O(1) insertion and deletion at both ends, so it is a natural choice for queues.An efficient implementation is one that can perform the operations-en-queuing and de-queuing-in O(1) time. There are several efficient implementations of FIFO queues. Queue overflow results from trying to add an element onto a full queue and queue underflow happens when trying to remove an element from an empty queue.Ī bounded queue is a queue limited to a fixed number of items. Such data structures may have not specified a fixed capacity limit besides memory constraints. Most modern languages with objects or pointers can implement or come with libraries for dynamic lists.

The array size must be declared ahead of time, but some implementations simply double the declared array size when overflow occurs. This is still the conceptually simplest way to construct a queue in a high-level language, but it does admittedly slow things down a little, because the array indices must be compared to zero and the array size, which is comparable to the time taken to check whether an array index is out of bounds, which some languages do, but this will certainly be the method of choice for a quick and dirty implementation, or for any high-level language that does not have pointer syntax. If n is the size of the array, then computing indices modulo n will turn the array into a circle. The simple trick of turning the array into a closed circle and letting the head and tail drift around endlessly in that circle makes it unnecessary to ever move items stored in the array. It can also be empty, at which point removing an element will be impossible until a new element has been added again.įixed-length arrays are limited in capacity, but it is not true that items need to be copied towards the head of the queue. Regardless of how many elements are already contained, a new element can always be added. Theoretically, one characteristic of a queue is that it does not have a specific capacity. In these contexts, the queue performs the function of a buffer.Īnother usage of queues is in the implementation of breadth-first search. Queues provide services in computer science, transport, and operations research where various entities such as data, objects, persons, or events are stored and held to be processed later. Common implementations are circular buffers and linked lists. Queues are common in computer programs, where they are implemented as data structures coupled with access routines, as an abstract data structure or in object-oriented languages as classes. A queue is an example of a linear data structure, or more abstractly a sequential collection. This is equivalent to the requirement that once a new element is added, all elements that were added before have to be removed before the new element can be removed. In a FIFO data structure, the first element added to the queue will be the first one to be removed. The operations of a queue make it a first-in-first-out (FIFO) data structure. Other operations may also be allowed, often including a peek or front operation that returns the value of the next element to be dequeued without dequeuing it. The operation of adding an element to the rear of the queue is known as enqueue, and the operation of removing an element from the front is known as dequeue. By convention, the end of the sequence at which elements are added is called the back, tail, or rear of the queue, and the end at which elements are removed is called the head or front of the queue, analogously to the words used when people line up to wait for goods or services. In computer science, a queue is a collection of entities that are maintained in a sequence and can be modified by the addition of entities at one end of the sequence and the removal of entities from the other end of the sequence. Representation of a FIFO (first in, first out) queue

Fifo java queue