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Summary: Methods | Inherited Methods

# Queue

`public interface Queue`
`implements Collection<E>`

java.util.Queue<E>

Known indirect subclasses AbstractQueue<E>, ArrayBlockingQueue<E>, ArrayDeque<E>, BlockingDeque<E>, BlockingQueue<E>, ConcurrentLinkedDeque<E>, ConcurrentLinkedQueue<E>, DelayQueue<E extends Delayed>, Deque<E>, LinkedBlockingDeque<E>, LinkedBlockingQueue<E>, LinkedList<E>, LinkedTransferQueue<E>, PriorityBlockingQueue<E>, PriorityQueue<E>, SynchronousQueue<E>, TransferQueue<E>
AbstractQueue<E> | This class provides skeletal implementations of some `Queue` operations.
ArrayBlockingQueue<E> | A bounded BlockingQueue backed by an array.
ArrayDeque<E> | Resizable-array implementation of the `Deque` interface.
BlockingDeque<E> | A `Deque` that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element.
BlockingQueue<E> | A `Queue` that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.
ConcurrentLinkedDeque<E> | An unbounded concurrent Deque based on linked nodes.
ConcurrentLinkedQueue<E> | An unbounded thread-safe Queue based on linked nodes.
DelayQueue<E extends Delayed> | An unbounded BlockingQueue of `Delayed` elements, in which an element can only be taken when its delay has expired.
Deque<E> | A linear collection that supports element insertion and removal at both ends.
LinkedBlockingDeque<E> | An optionally-bounded BlockingDeque based on linked nodes.
LinkedBlockingQueue<E> | An optionally-bounded BlockingQueue based on linked nodes.
LinkedList<E> | Doubly-linked list implementation of the `List` and `Deque` interfaces.
LinkedTransferQueue<E> | An unbounded `TransferQueue` based on linked nodes.
PriorityBlockingQueue<E> | An unbounded BlockingQueue that uses the same ordering rules as class `PriorityQueue` and supplies blocking retrieval operations.
PriorityQueue<E> | An unbounded priority Queue based on a priority heap.
SynchronousQueue<E> | A BlockingQueue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.
TransferQueue<E> | A `BlockingQueue` in which producers may wait for consumers to receive elements.

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 | `Queue#add` | `Queue#offer`
Remove | `Queue#remove` | `Queue#poll`
Examine | `Queue#element` | `Queue#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.

## Summary

### Public methods
`abstract 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.
`abstract E` | `element()` Retrieves, but does not remove, the head of this queue.
`abstract boolean` | `offer(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
`abstract E` | `peek()` Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
`abstract E` | `poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
`abstract E` | `remove()` Retrieves and removes the head of this queue.

### Inherited methods
From interface `java.util.Collection`
`abstract boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation).
`abstract boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`abstract void` | `clear()` Removes all of the elements from this collection (optional operation).
`abstract boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element.
`abstract boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`abstract boolean` | `equals(Object o)` Compares the specified object with this collection for equality.
`abstract int` | `hashCode()` Returns the hash code value for this collection.
`abstract boolean` | `isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>` | `parallelStream()` Returns a possibly parallel `Stream` with this collection as its source.
`abstract boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`abstract boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean` | `removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`abstract boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int` | `size()` Returns the number of elements in this collection.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>` | `stream()` Returns a sequential `Stream` with this collection as its source.
`abstract <T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`abstract Object[]` | `toArray()` Returns an array containing all of the elements in this collection.
From interface `java.lang.Iterable`
`default void` | `forEach(Consumer<? super T> action)` Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`abstract Iterator<E>` | `iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

## Public methods

### add

    public abstract 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.

Parameters
`e` | `E`: the element to add

Returns
`boolean` | `true` (as specified by `Collection#add`)

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

### element

    public abstract 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
`E` | the head of this queue

Throws
`NoSuchElementException` | if this queue is empty

### offer

    public abstract 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` | `E`: the element to add

Returns
`boolean` | `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

### peek

    public abstract E peek ()

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

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

### poll

    public abstract E poll ()

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

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

### remove

    public abstract 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
`E` | the head of this queue

Throws
`NoSuchElementException` | if this queue is empty

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