java.util

## Class AbstractQueue<E>

- java.lang.Object
- - java.util.AbstractCollection<E>
  - - java.util.AbstractQueue<E>

- Type Parameters:
    `E` - the type of elements held in this collection
  All Implemented Interfaces:
    Iterable<E>, Collection<E>, Queue<E>
  Direct Known Subclasses:
    ArrayBlockingQueue, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue, SynchronousQueue

      public abstract class AbstractQueue<E>
      extends AbstractCollection<E>
      implements Queue<E>

  This class provides skeletal implementations of some `Queue` operations. The implementations in this class are appropriate when the base implementation does not allow `null` elements. Methods `add`, `remove`, and `element` are based on `offer`, `poll`, and `peek`, respectively, but throw exceptions instead of indicating failure via `false` or `null` returns.

  A `Queue` implementation that extends this class must minimally define a method `Queue.offer(E)` which does not permit insertion of `null` elements, along with methods `Queue.peek()`, `Queue.poll()`, `Collection.size()`, and `Collection.iterator()`. Typically, additional methods will be overridden as well. If these requirements cannot be met, consider instead subclassing `AbstractCollection`.

  This class is a member of the Java Collections Framework.

  Since:
    1.5
  Author:
    Doug Lea

- - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `AbstractQueue()` Constructor for use by subclasses.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and 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.
    `boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this queue.
    `void` | `clear()` Removes all of the elements from this queue.
    `E` | `element()` Retrieves, but does not remove, the head of this queue.
    `E` | `remove()` Retrieves and removes the head of this queue.

    - ### Methods inherited from class java.util.AbstractCollection

      `contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray, toString`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.Queue

      `offer, peek, poll`

    - ### Methods inherited from interface java.util.Collection

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

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### Constructor Detail

    - #### AbstractQueue

          protected AbstractQueue()

      Constructor for use by subclasses.

  - ### Method Detail

    - #### add

          public 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.

      This implementation returns `true` if `offer` succeeds, else throws an `IllegalStateException`.

      Specified by:
        `add` in interface `Collection<E>`
      Specified by:
        `add` in interface `Queue<E>`
      Overrides:
        `add` in class `AbstractCollection<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

    - #### remove

          public 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.

      This implementation returns the result of `poll` unless the queue is empty.

      Specified by:
        `remove` in interface `Queue<E>`
      Returns:
        the head of this queue
      Throws:
        `NoSuchElementException` - if this queue is empty

    - #### element

          public 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.

      This implementation returns the result of `peek` unless the queue is empty.

      Specified by:
        `element` in interface `Queue<E>`
      Returns:
        the head of this queue
      Throws:
        `NoSuchElementException` - if this queue is empty

    - #### clear

          public void clear()

      Removes all of the elements from this queue. The queue will be empty after this call returns.

      This implementation repeatedly invokes `poll` until it returns `null`.

      Specified by:
        `clear` in interface `Collection<E>`
      Overrides:
        `clear` in class `AbstractCollection<E>`

    - #### addAll

          public boolean addAll(Collection<? extends E> c)

      Adds all of the elements in the specified collection to this queue. Attempts to addAll of a queue to itself result in `IllegalArgumentException`. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

      This implementation iterates over the specified collection, and adds each element returned by the iterator to this queue, in turn. A runtime exception encountered while trying to add an element (including, in particular, a `null` element) may result in only some of the elements having been successfully added when the associated exception is thrown.

      Specified by:
        `addAll` in interface `Collection<E>`
      Overrides:
        `addAll` in class `AbstractCollection<E>`
      Parameters:
        `c` - collection containing elements to be added to this queue
      Returns:
        `true` if this queue changed as a result of the call
      Throws:
        `ClassCastException` - if the class of an element of the specified collection prevents it from being added to this queue
        `NullPointerException` - if the specified collection contains a null element and this queue does not permit null elements, or if the specified collection is null
        `IllegalArgumentException` - if some property of an element of the specified collection prevents it from being added to this queue, or if the specified collection is this queue
        `IllegalStateException` - if not all the elements can be added at this time due to insertion restrictions
      See Also:
        `add(Object)`

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