Interface Queue<E>

All Superinterfaces:

Collection<E>, Iterable<E>

All Known Subinterfaces:

Deque<E>

All Known Implementing Classes:

AbstractQueue, ArrayDeque, LinkedList, PriorityQueue

public interface Queue<E>
extends Collection<E>

This kind of collection provides advanced operations compared to basic collections, such as insertion, extraction, and inspection.

Generally, a queue orders its elements by means of first-in-first-out. However, a priority queue orders its elements according to a comparator specified or the elements' natural order. Furthermore, a stack orders its elements last-in-first out.

A typical queue does not allow null to be inserted as its element, while some implementations such as LinkedList allow it. But null should not be inserted even in these implementations, since the method poll returns null to indicate that there is no element left in the queue.

Queue does not provide blocking queue methods, which would block until the operation of the method is allowed. See the BlockingQueue interface for information about blocking queue methods.

Method Summary

Modifier and Type	Method	Description
E	element()	Gets but does not remove the element at the head of the queue.
boolean	offer(E o)	Inserts the specified element into the queue provided that the condition allows such an operation.
E	peek()	Gets but does not remove the element at the head of the queue.
E	poll()	Gets and removes the element at the head of the queue, or returns null if there is no element in the queue.
E	remove()	Gets and removes the element at the head of the queue.

Methods inherited from interface Collection

add, addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray

Modifier and Type	Method	Description
boolean	add(E object)	Attempts to add object to the contents of this Collection (optional).
boolean	addAll(Collection<? extends E> collection)	Attempts to add all of the objects contained in Collection to the contents of this Collection (optional).
void	clear()	Removes all elements from this Collection, leaving it empty (optional).
boolean	contains(Object object)	Tests whether this Collection contains the specified object.
boolean	containsAll(Collection<?> collection)	Tests whether this Collection contains all objects contained in the specified Collection.
boolean	equals(Object object)	Compares the argument to the receiver, and returns true if they represent the same object using a class specific comparison.
int	hashCode()	Returns an integer hash code for the receiver.
boolean	isEmpty()	Returns if this Collection contains no elements.
Iterator<E>	iterator()	Returns an instance of Iterator that may be used to access the objects contained by this Collection.
boolean	remove(Object object)	Removes one instance of the specified object from this Collection if one is contained (optional).
boolean	removeAll(Collection<?> collection)	Removes all occurrences in this Collection of each object in the specified Collection (optional).
boolean	retainAll(Collection<?> collection)	Removes all objects from this Collection that are not also found in the Collection passed (optional).
int	size()	Returns a count of how many objects this Collection contains.
Object[]	toArray()	Returns a new array containing all elements contained in this Collection.
<T> T[]	toArray(T[] array)	Returns an array containing all elements contained in this Collection.

Method Details

offer

boolean offer(E o)

Inserts the specified element into the queue provided that the condition allows such an operation. The method is generally preferable to Collection.add(E), since the latter might throw an exception if the operation fails.

Parameters:

o - the specified element to insert into the queue.

Returns:

true if the operation succeeds and false if it fails.

poll

E poll()

Gets and removes the element at the head of the queue, or returns null if there is no element in the queue.

Returns:

the element at the head of the queue or null if there is no element in the queue.

remove

E remove()

Gets and removes the element at the head of the queue. Throws a NoSuchElementException if there is no element in the queue.

Returns:

the element at the head of the queue.

Throws:

NoSuchElementException - if there is no element in the queue.

peek

E peek()

Gets but does not remove the element at the head of the queue.

Returns:

the element at the head of the queue or null if there is no element in the queue.

element

E element()

Gets but does not remove the element at the head of the queue. Throws a NoSuchElementException if there is no element in the queue.

Returns:

the element at the head of the queue.

Throws:

NoSuchElementException - if there is no element in the queue.