# Interface Queue<E>

Type Parameters:
  `E` - the type of elements held in this queue

All Superinterfaces:
  `Collection<E>, Iterable<E>`

All Known Subinterfaces:
  `BlockingDeque<E>, BlockingQueue<E>, Deque<E>, TransferQueue<E>`

All Known Implementing Classes:
  `AbstractQueue, ArrayBlockingQueue, ArrayDeque, ConcurrentLinkedDeque, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, LinkedList, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue, SynchronousQueue`

public interface Queue<E> extends Collection<E>
A collection designed for holding elements prior to processing. Besides basic `Collection` operations, queues provide additional insertion, extraction, and inspection operations. Each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (either `null` or `false`, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restricted `Queue` implementations; in most implementations, insert operations cannot fail.

Summary of Queue methods
| Throws exception | Returns special value
Insert | `add(e)` | `offer(e)`
Remove | `remove()` | `poll()`
Examine | `element()` | `peek()`

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. 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()`. 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.

The `offer` method inserts an element if possible, otherwise returning `false`. This differs from the `Collection.add` method, which can fail to add an element only by throwing an unchecked exception. The `offer` method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.

The `remove()` and `poll()` methods remove and return the head of the queue. Exactly which element is removed from the queue is a function of the queue's ordering policy, which differs from implementation to implementation. The `remove()` and `poll()` methods differ only in their behavior when the queue is empty: the `remove()` method throws an exception, while the `poll()` method returns `null`.

The `element()` and `peek()` methods return, but do not remove, the head of the queue.

The `Queue` interface does not define the blocking queue methods, which are common in concurrent programming. These methods, which wait for elements to appear or for space to become available, are defined in the `BlockingQueue` interface, which extends this interface.

`Queue` implementations generally do not allow insertion of `null` elements, although some implementations, such as `LinkedList`, do not prohibit insertion of `null`. Even in the implementations that permit it, `null` should not be inserted into a `Queue`, as `null` is also used as a special return value by the `poll` method to indicate that the queue contains no elements.

`Queue` implementations generally do not define element-based versions of methods `equals` and `hashCode` but instead inherit the identity based versions from class `Object`, because element-based equality is not always well-defined for queues with the same elements but different ordering properties.

This interface is a member of the Java Collections Framework.

Since:
  1.5

## Method Summary

Modifier and Type
Method
Description
`boolean`
`add(E e)`
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
`E`
`element()`
Retrieves, but does not remove, the head of this queue.
`boolean`
`offer(E e)`
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
`E`
`peek()`
Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
`E`
`poll()`
Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
`E`
`remove()`
Retrieves and removes the head of this queue.

### Methods declared in interface Collection

`addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, remove, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray`

### Methods declared in interface Iterable

`forEach`

## Method Details

### add

boolean add(E e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.

Specified by:
  `add` in interface `Collection<E>`
Parameters:
  `e` - the element to add
Returns:
  `true` (as specified by `Collection.add(E)`)
Throws:
  `IllegalStateException` - if the element cannot be added at this time due to capacity restrictions
  `ClassCastException` - if the class of the specified element prevents it from being added to this queue
  `NullPointerException` - if the specified element is null and this queue does not permit null elements
  `IllegalArgumentException` - if some property of this element prevents it from being added to this queue

### offer

boolean offer(E e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. When using a capacity-restricted queue, this method is generally preferable to `add(E)`, which can fail to insert an element only by throwing an exception.

Parameters:
  `e` - the element to add
Returns:
  `true` if the element was added to this queue, else `false`
Throws:
  `ClassCastException` - if the class of the specified element prevents it from being added to this queue
  `NullPointerException` - if the specified element is null and this queue does not permit null elements
  `IllegalArgumentException` - if some property of this element prevents it from being added to this queue

### remove

E remove()
Retrieves and removes the head of this queue. This method differs from `poll()` only in that it throws an exception if this queue is empty.

Returns:
  the head of this queue
Throws:
  `NoSuchElementException` - if this queue is empty

### poll

E poll()
Retrieves and removes the head of this queue, or returns `null` if this queue is empty.

Returns:
  the head of this queue, or `null` if this queue is empty

### element

E element()
Retrieves, but does not remove, the head of this queue. This method differs from `peek` only in that it throws an exception if this queue is empty.

Returns:
  the head of this queue
Throws:
  `NoSuchElementException` - if this queue is empty

### peek

E peek()
Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.

Returns:
  the head of this queue, or `null` if this queue is empty