Module java.base
Package java.util.concurrent

# Class ConcurrentLinkedDeque<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.concurrent.ConcurrentLinkedDeque<E>

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

All Implemented Interfaces:
  `Serializable`, `Iterable<E>`, `Collection<E>`, `Deque<E>`, `Queue<E>`

public class ConcurrentLinkedDeque<E> extends AbstractCollection<E> implements Deque<E>, Serializable
An unbounded concurrent deque based on linked nodes. Concurrent insertion, removal, and access operations execute safely across multiple threads. A `ConcurrentLinkedDeque` is an appropriate choice when many threads will share access to a common collection. Like most other concurrent collection implementations, this class does not permit the use of `null` elements.

Iterators and spliterators are weakly consistent.

Beware that, unlike in most collections, the `size` method is NOT a constant-time operation. Because of the asynchronous nature of these deques, determining the current number of elements requires a traversal of the elements, and so may report inaccurate results if this collection is modified during traversal.

Bulk operations that add, remove, or examine multiple elements, such as `addAll(java.util.Collection<? extends E>)`, `removeIf(java.util.function.Predicate<? super E>)` or `forEach(java.util.function.Consumer<? super E>)`, are not guaranteed to be performed atomically. For example, a `forEach` traversal concurrent with an `addAll` operation might observe only some of the added elements.

This class and its iterator implement all of the optional methods of the `Deque` and `Iterator` interfaces.

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a `ConcurrentLinkedDeque` happen-before actions subsequent to the access or removal of that element from the `ConcurrentLinkedDeque` in another thread.

This class is a member of the Java Collections Framework.

Since:
  1.7
See Also:
  - Serialized Form

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `ConcurrentLinkedDeque()`
  Constructs an empty deque.
  `ConcurrentLinkedDeque(Collection<? extends E> c)`
  Constructs a deque initially containing the elements of the given collection, added in traversal order of the collection's iterator.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `add(E e)`
  Inserts the specified element at the tail of this deque.
  `boolean`
  `addAll(Collection<? extends E> c)`
  Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection's iterator.
  `void`
  `addFirst(E e)`
  Inserts the specified element at the front of this deque.
  `void`
  `addLast(E e)`
  Inserts the specified element at the end of this deque.
  `void`
  `clear()`
  Removes all of the elements from this deque.
  `boolean`
  `contains(Object o)`
  Returns `true` if this deque contains the specified element.
  `Iterator<E>`
  `descendingIterator()`
  Returns an iterator over the elements in this deque in reverse sequential order.
  `E`
  `element()`
  Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque).
  `void`
  `forEach(Consumer<? super E> action)`
  Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
  `E`
  `getFirst()`
  Retrieves, but does not remove, the first element of this deque.
  `E`
  `getLast()`
  Retrieves, but does not remove, the last element of this deque.
  `boolean`
  `isEmpty()`
  Returns `true` if this collection contains no elements.
  `Iterator<E>`
  `iterator()`
  Returns an iterator over the elements in this deque in proper sequence.
  `boolean`
  `offer(E e)`
  Inserts the specified element at the tail of this deque.
  `boolean`
  `offerFirst(E e)`
  Inserts the specified element at the front of this deque.
  `boolean`
  `offerLast(E e)`
  Inserts the specified element at the end of this deque.
  `E`
  `peek()`
  Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.
  `E`
  `peekFirst()`
  Retrieves, but does not remove, the first element of this deque, or returns `null` if this deque is empty.
  `E`
  `peekLast()`
  Retrieves, but does not remove, the last element of this deque, or returns `null` if this deque is empty.
  `E`
  `poll()`
  Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.
  `E`
  `pollFirst()`
  Retrieves and removes the first element of this deque, or returns `null` if this deque is empty.
  `E`
  `pollLast()`
  Retrieves and removes the last element of this deque, or returns `null` if this deque is empty.
  `E`
  `pop()`
  Pops an element from the stack represented by this deque.
  `void`
  `push(E e)`
  Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an `IllegalStateException` if no space is currently available.
  `E`
  `remove()`
  Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).
  `boolean`
  `remove(Object o)`
  Removes the first occurrence of the specified element from this deque.
  `boolean`
  `removeAll(Collection<?> c)`
  Removes all of this collection's elements that are also contained in the specified collection (optional operation).
  `E`
  `removeFirst()`
  Retrieves and removes the first element of this deque.
  `boolean`
  `removeFirstOccurrence(Object o)`
  Removes the first occurrence of the specified element from this deque.
  `boolean`
  `removeIf(Predicate<? super E> filter)`
  Removes all of the elements of this collection that satisfy the given predicate.
  `E`
  `removeLast()`
  Retrieves and removes the last element of this deque.
  `boolean`
  `removeLastOccurrence(Object o)`
  Removes the last occurrence of the specified element from this deque.
  `boolean`
  `retainAll(Collection<?> c)`
  Retains only the elements in this collection that are contained in the specified collection (optional operation).
  `int`
  `size()`
  Returns the number of elements in this deque.
  `Spliterator<E>`
  `spliterator()`
  Returns a `Spliterator` over the elements in this deque.
  `Object[]`
  `toArray()`
  Returns an array containing all of the elements in this deque, in proper sequence (from first to last element).
  `<T> T[]`
  `toArray(T[] a)`
  Returns an array containing all of the elements in this deque, in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.

  ### Methods declared in class java.util.AbstractCollection

  `containsAll, toString`

  ### Methods declared in class java.lang.Object

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

  ### Methods declared in interface java.util.Collection

  `containsAll, equals, hashCode, parallelStream, stream, toArray`

- ## Constructor Details

  - ### ConcurrentLinkedDeque

    public ConcurrentLinkedDeque()
    Constructs an empty deque.

  - ### ConcurrentLinkedDeque

    public ConcurrentLinkedDeque(Collection<? extends E> c)
    Constructs a deque initially containing the elements of the given collection, added in traversal order of the collection's iterator.
    Parameters:
      `c` - the collection of elements to initially contain
    Throws:
      `NullPointerException` - if the specified collection or any of its elements are null

- ## Method Details

  - ### addFirst

    public void addFirst(E e)
    Inserts the specified element at the front of this deque. As the deque is unbounded, this method will never throw `IllegalStateException`.
    Specified by:
      `addFirst` in interface `Deque<E>`
    Parameters:
      `e` - the element to add
    Throws:
      `NullPointerException` - if the specified element is null

  - ### addLast

    public void addLast(E e)
    Inserts the specified element at the end of this deque. As the deque is unbounded, this method will never throw `IllegalStateException`.

    This method is equivalent to `add(E)`.

    Specified by:
      `addLast` in interface `Deque<E>`
    Parameters:
      `e` - the element to add
    Throws:
      `NullPointerException` - if the specified element is null

  - ### offerFirst

    public boolean offerFirst(E e)
    Inserts the specified element at the front of this deque. As the deque is unbounded, this method will never return `false`.
    Specified by:
      `offerFirst` in interface `Deque<E>`
    Parameters:
      `e` - the element to add
    Returns:
      `true` (as specified by `Deque.offerFirst(E)`)
    Throws:
      `NullPointerException` - if the specified element is null

  - ### offerLast

    public boolean offerLast(E e)
    Inserts the specified element at the end of this deque. As the deque is unbounded, this method will never return `false`.

    This method is equivalent to `add(E)`.

    Specified by:
      `offerLast` in interface `Deque<E>`
    Parameters:
      `e` - the element to add
    Returns:
      `true` (as specified by `Deque.offerLast(E)`)
    Throws:
      `NullPointerException` - if the specified element is null

  - ### peekFirst

    public E peekFirst()
    Description copied from interface: `Deque`
    Retrieves, but does not remove, the first element of this deque, or returns `null` if this deque is empty.
    Specified by:
      `peekFirst` in interface `Deque<E>`
    Returns:
      the head of this deque, or `null` if this deque is empty

  - ### peekLast

    public E peekLast()
    Description copied from interface: `Deque`
    Retrieves, but does not remove, the last element of this deque, or returns `null` if this deque is empty.
    Specified by:
      `peekLast` in interface `Deque<E>`
    Returns:
      the tail of this deque, or `null` if this deque is empty

  - ### getFirst

    public E getFirst()
    Description copied from interface: `Deque`
    Retrieves, but does not remove, the first element of this deque. This method differs from `peekFirst` only in that it throws an exception if this deque is empty.
    Specified by:
      `getFirst` in interface `Deque<E>`
    Returns:
      the head of this deque
    Throws:
      `NoSuchElementException` - if this deque is empty

  - ### getLast

    public E getLast()
    Description copied from interface: `Deque`
    Retrieves, but does not remove, the last element of this deque. This method differs from `peekLast` only in that it throws an exception if this deque is empty.
    Specified by:
      `getLast` in interface `Deque<E>`
    Returns:
      the tail of this deque
    Throws:
      `NoSuchElementException` - if this deque is empty

  - ### pollFirst

    public E pollFirst()
    Description copied from interface: `Deque`
    Retrieves and removes the first element of this deque, or returns `null` if this deque is empty.
    Specified by:
      `pollFirst` in interface `Deque<E>`
    Returns:
      the head of this deque, or `null` if this deque is empty

  - ### pollLast

    public E pollLast()
    Description copied from interface: `Deque`
    Retrieves and removes the last element of this deque, or returns `null` if this deque is empty.
    Specified by:
      `pollLast` in interface `Deque<E>`
    Returns:
      the tail of this deque, or `null` if this deque is empty

  - ### removeFirst

    public E removeFirst()
    Description copied from interface: `Deque`
    Retrieves and removes the first element of this deque. This method differs from `pollFirst` only in that it throws an exception if this deque is empty.
    Specified by:
      `removeFirst` in interface `Deque<E>`
    Returns:
      the head of this deque
    Throws:
      `NoSuchElementException` - if this deque is empty

  - ### removeLast

    public E removeLast()
    Description copied from interface: `Deque`
    Retrieves and removes the last element of this deque. This method differs from `pollLast` only in that it throws an exception if this deque is empty.
    Specified by:
      `removeLast` in interface `Deque<E>`
    Returns:
      the tail of this deque
    Throws:
      `NoSuchElementException` - if this deque is empty

  - ### offer

    public boolean offer(E e)
    Inserts the specified element at the tail of this deque. As the deque is unbounded, this method will never return `false`.
    Specified by:
      `offer` in interface `Deque<E>`
    Specified by:
      `offer` in interface `Queue<E>`
    Parameters:
      `e` - the element to add
    Returns:
      `true` (as specified by `Queue.offer(E)`)
    Throws:
      `NullPointerException` - if the specified element is null

  - ### add

    public boolean add(E e)
    Inserts the specified element at the tail of this deque. As the deque is unbounded, this method will never throw `IllegalStateException` or return `false`.
    Specified by:
      `add` in interface `Collection<E>`
    Specified by:
      `add` in interface `Deque<E>`
    Specified by:
      `add` in interface `Queue<E>`
    Overrides:
      `add` in class `AbstractCollection<E>`
    Parameters:
      `e` - element whose presence in this collection is to be ensured
    Returns:
      `true` (as specified by `Collection.add(E)`)
    Throws:
      `NullPointerException` - if the specified element is null

  - ### poll

    public E poll()
    Description copied from interface: `Deque`
    Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.

    This method is equivalent to `Deque.pollFirst()`.

    Specified by:
      `poll` in interface `Deque<E>`
    Specified by:
      `poll` in interface `Queue<E>`
    Returns:
      the first element of this deque, or `null` if this deque is empty

  - ### peek

    public E peek()
    Description copied from interface: `Deque`
    Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.

    This method is equivalent to `Deque.peekFirst()`.

    Specified by:
      `peek` in interface `Deque<E>`
    Specified by:
      `peek` in interface `Queue<E>`
    Returns:
      the head of the queue represented by this deque, or `null` if this deque is empty

  - ### remove

    public E remove()
    Description copied from interface: `Deque`
    Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque). This method differs from `poll()` only in that it throws an exception if this deque is empty.

    This method is equivalent to `Deque.removeFirst()`.

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

  - ### pop

    public E pop()
    Description copied from interface: `Deque`
    Pops an element from the stack represented by this deque. In other words, removes and returns the first element of this deque.

    This method is equivalent to `Deque.removeFirst()`.

    Specified by:
      `pop` in interface `Deque<E>`
    Returns:
      the element at the front of this deque (which is the top of the stack represented by this deque)
    Throws:
      `NoSuchElementException` - if this deque is empty

  - ### element

    public E element()
    Description copied from interface: `Deque`
    Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque). This method differs from `peek` only in that it throws an exception if this deque is empty.

    This method is equivalent to `Deque.getFirst()`.

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

  - ### push

    public void push(E e)
    Description copied from interface: `Deque`
    Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an `IllegalStateException` if no space is currently available.

    This method is equivalent to `Deque.addFirst(E)`.

    Specified by:
      `push` in interface `Deque<E>`
    Parameters:
      `e` - the element to push
    Throws:
      `NullPointerException` - if the specified element is null and this deque does not permit null elements

  - ### removeFirstOccurrence

    public boolean removeFirstOccurrence(Object o)
    Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first element `e` such that `o.equals(e)` (if such an element exists). Returns `true` if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).
    Specified by:
      `removeFirstOccurrence` in interface `Deque<E>`
    Parameters:
      `o` - element to be removed from this deque, if present
    Returns:
      `true` if the deque contained the specified element
    Throws:
      `NullPointerException` - if the specified element is null

  - ### removeLastOccurrence

    public boolean removeLastOccurrence(Object o)
    Removes the last occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the last element `e` such that `o.equals(e)` (if such an element exists). Returns `true` if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).
    Specified by:
      `removeLastOccurrence` in interface `Deque<E>`
    Parameters:
      `o` - element to be removed from this deque, if present
    Returns:
      `true` if the deque contained the specified element
    Throws:
      `NullPointerException` - if the specified element is null

  - ### contains

    public boolean contains(Object o)
    Returns `true` if this deque contains the specified element. More formally, returns `true` if and only if this deque contains at least one element `e` such that `o.equals(e)`.
    Specified by:
      `contains` in interface `Collection<E>`
    Specified by:
      `contains` in interface `Deque<E>`
    Overrides:
      `contains` in class `AbstractCollection<E>`
    Parameters:
      `o` - element whose presence in this deque is to be tested
    Returns:
      `true` if this deque contains the specified element

  - ### isEmpty

    public boolean isEmpty()
    Returns `true` if this collection contains no elements.
    Specified by:
      `isEmpty` in interface `Collection<E>`
    Overrides:
      `isEmpty` in class `AbstractCollection<E>`
    Returns:
      `true` if this collection contains no elements

  - ### size

    public int size()
    Returns the number of elements in this deque. If this deque contains more than `Integer.MAX_VALUE` elements, it returns `Integer.MAX_VALUE`.

    Beware that, unlike in most collections, this method is NOT a constant-time operation. Because of the asynchronous nature of these deques, determining the current number of elements requires traversing them all to count them. Additionally, it is possible for the size to change during execution of this method, in which case the returned result will be inaccurate. Thus, this method is typically not very useful in concurrent applications.

    Specified by:
      `size` in interface `Collection<E>`
    Specified by:
      `size` in interface `Deque<E>`
    Returns:
      the number of elements in this deque

  - ### remove

    public boolean remove(Object o)
    Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first element `e` such that `o.equals(e)` (if such an element exists). Returns `true` if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

    This method is equivalent to `removeFirstOccurrence(Object)`.

    Specified by:
      `remove` in interface `Collection<E>`
    Specified by:
      `remove` in interface `Deque<E>`
    Overrides:
      `remove` in class `AbstractCollection<E>`
    Parameters:
      `o` - element to be removed from this deque, if present
    Returns:
      `true` if the deque contained the specified element
    Throws:
      `NullPointerException` - if the specified element is null

  - ### addAll

    public boolean addAll(Collection<? extends E> c)
    Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection's iterator. Attempts to `addAll` of a deque to itself result in `IllegalArgumentException`.
    Specified by:
      `addAll` in interface `Collection<E>`
    Specified by:
      `addAll` in interface `Deque<E>`
    Overrides:
      `addAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - the elements to be inserted into this deque
    Returns:
      `true` if this deque changed as a result of the call
    Throws:
      `NullPointerException` - if the specified collection or any of its elements are null
      `IllegalArgumentException` - if the collection is this deque
    See Also:
      - `AbstractCollection.add(Object)`

  - ### clear

    public void clear()
    Removes all of the elements from this deque.
    Specified by:
      `clear` in interface `Collection<E>`
    Overrides:
      `clear` in class `AbstractCollection<E>`

  - ### toArray

    public Object[] toArray()
    Returns an array containing all of the elements in this deque, in proper sequence (from first to last element).

    The returned array will be "safe" in that no references to it are maintained by this deque. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array.

    This method acts as bridge between array-based and collection-based APIs.

    Specified by:
      `toArray` in interface `Collection<E>`
    Overrides:
      `toArray` in class `AbstractCollection<E>`
    Returns:
      an array containing all of the elements in this deque

  - ### toArray

    public <T> T[] toArray(T[] a)
    Returns an array containing all of the elements in this deque, in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the deque fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this deque.

    If this deque fits in the specified array with room to spare (i.e., the array has more elements than this deque), the element in the array immediately following the end of the deque is set to `null`.

    Like the `toArray()` method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

    Suppose `x` is a deque known to contain only strings. The following code can be used to dump the deque into a newly allocated array of `String`:

         String[] y = x.toArray(new String[0]);

    Note that `toArray(new Object[0])` is identical in function to `toArray()`.
    Specified by:
      `toArray` in interface `Collection<E>`
    Overrides:
      `toArray` in class `AbstractCollection<E>`
    Type Parameters:
      `T` - the component type of the array to contain the collection
    Parameters:
      `a` - the array into which the elements of the deque are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose
    Returns:
      an array containing all of the elements in this deque
    Throws:
      `ArrayStoreException` - if the runtime type of the specified array is not a supertype of the runtime type of every element in this deque
      `NullPointerException` - if the specified array is null

  - ### iterator

    public Iterator<E> iterator()
    Returns an iterator over the elements in this deque in proper sequence. The elements will be returned in order from first (head) to last (tail).

    The returned iterator is weakly consistent.

    Specified by:
      `iterator` in interface `Collection<E>`
    Specified by:
      `iterator` in interface `Deque<E>`
    Specified by:
      `iterator` in interface `Iterable<E>`
    Specified by:
      `iterator` in class `AbstractCollection<E>`
    Returns:
      an iterator over the elements in this deque in proper sequence

  - ### descendingIterator

    public Iterator<E> descendingIterator()
    Returns an iterator over the elements in this deque in reverse sequential order. The elements will be returned in order from last (tail) to first (head).

    The returned iterator is weakly consistent.

    Specified by:
      `descendingIterator` in interface `Deque<E>`
    Returns:
      an iterator over the elements in this deque in reverse order

  - ### spliterator

    public Spliterator<E> spliterator()
    Returns a `Spliterator` over the elements in this deque.

    The returned spliterator is weakly consistent.

    The `Spliterator` reports `Spliterator.CONCURRENT`, `Spliterator.ORDERED`, and `Spliterator.NONNULL`.

    Specified by:
      `spliterator` in interface `Collection<E>`
    Specified by:
      `spliterator` in interface `Iterable<E>`
    Implementation Note:
      The `Spliterator` implements `trySplit` to permit limited parallelism.
    Returns:
      a `Spliterator` over the elements in this deque
    Since:
      1.8

  - ### removeIf

    public boolean removeIf(Predicate<? super E> filter)
    Description copied from interface: `Collection`
    Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
    Specified by:
      `removeIf` in interface `Collection<E>`
    Parameters:
      `filter` - a predicate which returns `true` for elements to be removed
    Returns:
      `true` if any elements were removed
    Throws:
      `NullPointerException` - if the specified filter is null

  - ### removeAll

    public boolean removeAll(Collection<?> c)
    Description copied from class: `AbstractCollection`
    Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.
    Specified by:
      `removeAll` in interface `Collection<E>`
    Overrides:
      `removeAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be removed from this collection
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `NullPointerException` - if this collection contains one or more null elements and the specified collection does not support null elements (optional), or if the specified collection is null
    See Also:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### retainAll

    public boolean retainAll(Collection<?> c)
    Description copied from class: `AbstractCollection`
    Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.
    Specified by:
      `retainAll` in interface `Collection<E>`
    Overrides:
      `retainAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be retained in this collection
    Returns:
      `true` if this collection changed as a result of the call
    Throws:
      `NullPointerException` - if this collection contains one or more null elements and the specified collection does not permit null elements (optional), or if the specified collection is null
    See Also:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### forEach

    public void forEach(Consumer<? super E> action)
    Description copied from interface: `Iterable`
    Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

    The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.

    Specified by:
      `forEach` in interface `Iterable<E>`
    Parameters:
      `action` - The action to be performed for each element
    Throws:
      `NullPointerException` - if the specified action is null