# ConcurrentLinkedQueue Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) Summary:[Ctors](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#pubctors)\|[Methods](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#pubmethods)\|[Inherited Methods](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#inhmethods) # ConcurrentLinkedQueue ` public class ConcurrentLinkedQueue ` ` `` `` extends `[AbstractQueue](https://developer.android.com/reference/java/util/AbstractQueue)` `` implements `[Queue](https://developer.android.com/reference/java/util/Queue)`, `[Serializable](https://developer.android.com/reference/java/io/Serializable)` ` |---|---|---|-------------------------------------------------| | [java.lang.Object](https://developer.android.com/reference/java/lang/Object) |||| | ↳ | [java.util.AbstractCollection](https://developer.android.com/reference/java/util/AbstractCollection)\ ||| | | ↳ | [java.util.AbstractQueue](https://developer.android.com/reference/java/util/AbstractQueue)\ || | | | ↳ | java.util.concurrent.ConcurrentLinkedQueue\ |
*** ** * ** *** An unbounded thread-safe[queue](https://developer.android.com/reference/java/util/Queue)based on linked nodes. This queue orders elements FIFO (first-in-first-out). The*head* of the queue is that element that has been on the queue the longest time. The*tail* of the queue is that element that has been on the queue the shortest time. New elements are inserted at the tail of the queue, and the queue retrieval operations obtain elements at the head of the queue. A`ConcurrentLinkedQueue`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. This implementation employs an efficient*non-blocking* algorithm based on one described in[Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms](http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf)by Maged M. Michael and Michael L. Scott. Iterators are*weakly consistent* , returning elements reflecting the state of the queue at some point at or since the creation of the iterator. They do*not* throw[ConcurrentModificationException](https://developer.android.com/reference/java/util/ConcurrentModificationException), and may proceed concurrently with other operations. Elements contained in the queue since the creation of the iterator will be returned exactly once. Beware that, unlike in most collections, the`size`method is*NOT*a constant-time operation. Because of the asynchronous nature of these queues, 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(Collection)](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#addAll(java.util.Collection)),[removeIf(Predicate)](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#removeIf(java.util.function.Predicate))or[forEach(Consumer)](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#forEach(java.util.function.Consumer)), 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[Queue](https://developer.android.com/reference/java/util/Queue)and[Iterator](https://developer.android.com/reference/java/util/Iterator)interfaces. Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a`ConcurrentLinkedQueue`[*happen-before*](https://developer.android.com/reference/java/util/concurrent/package-summary#MemoryVisibility)actions subsequent to the access or removal of that element from the`ConcurrentLinkedQueue`in another thread. This class is a member of the[Java Collections Framework](https://reference/java/util/package-summary.html#CollectionsFramework). ## Summary | ### Public constructors || |-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---| | ` `[ConcurrentLinkedQueue](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#ConcurrentLinkedQueue())`() ` Creates a`ConcurrentLinkedQueue`that is initially empty. | | ` `[ConcurrentLinkedQueue](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#ConcurrentLinkedQueue(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Creates a`ConcurrentLinkedQueue`initially containing the elements of the given collection, added in traversal order of the collection's iterator. | | ### Public methods || |--------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` boolean` | ` `[add](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#add(E))`(E e) ` Inserts the specified element at the tail of this queue. | | ` boolean` | ` `[addAll](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#addAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Appends all of the elements in the specified collection to the end of this queue, in the order that they are returned by the specified collection's iterator. | | ` void` | ` `[clear](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#clear())`() ` Removes all of the elements from this queue. | | ` boolean` | ` `[contains](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#contains(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` o) ` Returns`true`if this queue contains the specified element. | | ` void` | ` `[forEach](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#forEach(java.util.function.Consumer))`(`[Consumer](https://developer.android.com/reference/java/util/function/Consumer)` action) ` | | ` boolean` | ` `[isEmpty](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#isEmpty())`() ` Returns`true`if this queue contains no elements. | | ` `[Iterator](https://developer.android.com/reference/java/util/Iterator)`` | ` `[iterator](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#iterator())`() ` Returns an iterator over the elements in this queue in proper sequence. | | ` boolean` | ` `[offer](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#offer(E))`(E e) ` Inserts the specified element at the tail of this queue. | | ` E` | ` `[peek](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#peek())`() ` Retrieves, but does not remove, the head of this queue, or returns`null`if this queue is empty. | | ` E` | ` `[poll](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#poll())`() ` Retrieves and removes the head of this queue, or returns`null`if this queue is empty. | | ` boolean` | ` `[remove](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#remove(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` o) ` Removes a single instance of the specified element from this queue, if it is present. | | ` boolean` | ` `[removeAll](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#removeAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Removes all of this collection's elements that are also contained in the specified collection (optional operation). | | ` boolean` | ` `[removeIf](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#removeIf(java.util.function.Predicate))`(`[Predicate](https://developer.android.com/reference/java/util/function/Predicate)` filter) ` Removes all of the elements of this collection that satisfy the given predicate. | | ` boolean` | ` `[retainAll](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#retainAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Retains only the elements in this collection that are contained in the specified collection (optional operation). | | ` int` | ` `[size](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#size())`() ` Returns the number of elements in this queue. | | ` `[Spliterator](https://developer.android.com/reference/java/util/Spliterator)`` | ` `[spliterator](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#spliterator())`() ` Returns a[Spliterator](https://developer.android.com/reference/java/util/Spliterator)over the elements in this queue. | | ` T[]` | ` `[toArray](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#toArray(T[]))`(T[] a) ` Returns an array containing all of the elements in this queue, in proper sequence; the runtime type of the returned array is that of the specified array. | | ` `[Object[]](https://developer.android.com/reference/java/lang/Object) | ` `[toArray](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#toArray())`() ` Returns an array containing all of the elements in this queue, in proper sequence. | | ` `[String](https://developer.android.com/reference/java/lang/String) | ` `[toString](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#toString())`() ` Returns a string representation of this collection. | | ### Inherited methods | |-----------------------|---| | From class` `[java.util.AbstractQueue](https://developer.android.com/reference/java/util/AbstractQueue)` ` |------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` boolean` | ` `[add](https://developer.android.com/reference/java/util/AbstractQueue#add(E))`(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](https://developer.android.com/reference/java/util/AbstractQueue#addAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Adds all of the elements in the specified collection to this queue. | | ` void` | ` `[clear](https://developer.android.com/reference/java/util/AbstractQueue#clear())`() ` Removes all of the elements from this queue. | | ` E` | ` `[element](https://developer.android.com/reference/java/util/AbstractQueue#element())`() ` Retrieves, but does not remove, the head of this queue. | | ` E` | ` `[remove](https://developer.android.com/reference/java/util/AbstractQueue#remove())`() ` Retrieves and removes the head of this queue. | || | From class` `[java.util.AbstractCollection](https://developer.android.com/reference/java/util/AbstractCollection)` ` |-----------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` boolean` | ` `[add](https://developer.android.com/reference/java/util/AbstractCollection#add(E))`(E e) ` Ensures that this collection contains the specified element (optional operation). | | ` boolean` | ` `[addAll](https://developer.android.com/reference/java/util/AbstractCollection#addAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Adds all of the elements in the specified collection to this collection (optional operation). | | ` void` | ` `[clear](https://developer.android.com/reference/java/util/AbstractCollection#clear())`() ` Removes all of the elements from this collection (optional operation). | | ` boolean` | ` `[contains](https://developer.android.com/reference/java/util/AbstractCollection#contains(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` o) ` Returns`true`if this collection contains the specified element. | | ` boolean` | ` `[containsAll](https://developer.android.com/reference/java/util/AbstractCollection#containsAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Returns`true`if this collection contains all of the elements in the specified collection. | | ` boolean` | ` `[isEmpty](https://developer.android.com/reference/java/util/AbstractCollection#isEmpty())`() ` Returns`true`if this collection contains no elements. | | ` abstract `[Iterator](https://developer.android.com/reference/java/util/Iterator)`` | ` `[iterator](https://developer.android.com/reference/java/util/AbstractCollection#iterator())`() ` Returns an iterator over the elements contained in this collection. | | ` boolean` | ` `[remove](https://developer.android.com/reference/java/util/AbstractCollection#remove(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` o) ` Removes a single instance of the specified element from this collection, if it is present (optional operation). | | ` boolean` | ` `[removeAll](https://developer.android.com/reference/java/util/AbstractCollection#removeAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Removes all of this collection's elements that are also contained in the specified collection (optional operation). | | ` boolean` | ` `[retainAll](https://developer.android.com/reference/java/util/AbstractCollection#retainAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Retains only the elements in this collection that are contained in the specified collection (optional operation). | | ` abstract int` | ` `[size](https://developer.android.com/reference/java/util/AbstractCollection#size())`() ` Returns the number of elements in this collection. | | ` T[]` | ` `[toArray](https://developer.android.com/reference/java/util/AbstractCollection#toArray(T[]))`(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. | | ` `[Object[]](https://developer.android.com/reference/java/lang/Object) | ` `[toArray](https://developer.android.com/reference/java/util/AbstractCollection#toArray())`() ` Returns an array containing all of the elements in this collection. | | ` `[String](https://developer.android.com/reference/java/lang/String) | ` `[toString](https://developer.android.com/reference/java/util/AbstractCollection#toString())`() ` Returns a string representation of this collection. | || | From class` `[java.lang.Object](https://developer.android.com/reference/java/lang/Object)` ` |--------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` `[Object](https://developer.android.com/reference/java/lang/Object) | ` `[clone](https://developer.android.com/reference/java/lang/Object#clone())`() ` Creates and returns a copy of this object. | | ` boolean` | ` `[equals](https://developer.android.com/reference/java/lang/Object#equals(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` obj) ` Indicates whether some other object is "equal to" this one. | | ` void` | ` `[finalize](https://developer.android.com/reference/java/lang/Object#finalize())`() ` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. | | ` final `[Class](https://developer.android.com/reference/java/lang/Class)`` | ` `[getClass](https://developer.android.com/reference/java/lang/Object#getClass())`() ` Returns the runtime class of this`Object`. | | ` int` | ` `[hashCode](https://developer.android.com/reference/java/lang/Object#hashCode())`() ` Returns a hash code value for the object. | | ` final void` | ` `[notify](https://developer.android.com/reference/java/lang/Object#notify())`() ` Wakes up a single thread that is waiting on this object's monitor. | | ` final void` | ` `[notifyAll](https://developer.android.com/reference/java/lang/Object#notifyAll())`() ` Wakes up all threads that are waiting on this object's monitor. | | ` `[String](https://developer.android.com/reference/java/lang/String) | ` `[toString](https://developer.android.com/reference/java/lang/Object#toString())`() ` Returns a string representation of the object. | | ` final void` | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait(long,%20int))`(long timeoutMillis, int nanos) ` Causes the current thread to wait until it is awakened, typically by being*notified* or*interrupted*, or until a certain amount of real time has elapsed. | | ` final void` | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait(long))`(long timeoutMillis) ` Causes the current thread to wait until it is awakened, typically by being*notified* or*interrupted*, or until a certain amount of real time has elapsed. | | ` final void` | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait())`() ` Causes the current thread to wait until it is awakened, typically by being*notified* or*interrupted*. | || | From interface` `[java.util.Queue](https://developer.android.com/reference/java/util/Queue)` ` |---------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` abstract boolean` | ` `[add](https://developer.android.com/reference/java/util/Queue#add(E))`(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](https://developer.android.com/reference/java/util/Queue#element())`() ` Retrieves, but does not remove, the head of this queue. | | ` abstract boolean` | ` `[offer](https://developer.android.com/reference/java/util/Queue#offer(E))`(E e) ` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. | | ` abstract E` | ` `[peek](https://developer.android.com/reference/java/util/Queue#peek())`() ` Retrieves, but does not remove, the head of this queue, or returns`null`if this queue is empty. | | ` abstract E` | ` `[poll](https://developer.android.com/reference/java/util/Queue#poll())`() ` Retrieves and removes the head of this queue, or returns`null`if this queue is empty. | | ` abstract E` | ` `[remove](https://developer.android.com/reference/java/util/Queue#remove())`() ` Retrieves and removes the head of this queue. | || | From interface` `[java.util.Collection](https://developer.android.com/reference/java/util/Collection)` ` |----------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` abstract boolean` | ` `[add](https://developer.android.com/reference/java/util/Collection#add(E))`(E e) ` Ensures that this collection contains the specified element (optional operation). | | ` abstract boolean` | ` `[addAll](https://developer.android.com/reference/java/util/Collection#addAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Adds all of the elements in the specified collection to this collection (optional operation). | | ` abstract void` | ` `[clear](https://developer.android.com/reference/java/util/Collection#clear())`() ` Removes all of the elements from this collection (optional operation). | | ` abstract boolean` | ` `[contains](https://developer.android.com/reference/java/util/Collection#contains(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` o) ` Returns`true`if this collection contains the specified element. | | ` abstract boolean` | ` `[containsAll](https://developer.android.com/reference/java/util/Collection#containsAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Returns`true`if this collection contains all of the elements in the specified collection. | | ` abstract boolean` | ` `[equals](https://developer.android.com/reference/java/util/Collection#equals(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` o) ` Compares the specified object with this collection for equality. | | ` abstract int` | ` `[hashCode](https://developer.android.com/reference/java/util/Collection#hashCode())`() ` Returns the hash code value for this collection. | | ` abstract boolean` | ` `[isEmpty](https://developer.android.com/reference/java/util/Collection#isEmpty())`() ` Returns`true`if this collection contains no elements. | | ` abstract `[Iterator](https://developer.android.com/reference/java/util/Iterator)`` | ` `[iterator](https://developer.android.com/reference/java/util/Collection#iterator())`() ` Returns an iterator over the elements in this collection. | | ` default `[Stream](https://developer.android.com/reference/java/util/stream/Stream)`` | ` `[parallelStream](https://developer.android.com/reference/java/util/Collection#parallelStream())`() ` Returns a possibly parallel`Stream`with this collection as its source. | | ` abstract boolean` | ` `[remove](https://developer.android.com/reference/java/util/Collection#remove(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` o) ` Removes a single instance of the specified element from this collection, if it is present (optional operation). | | ` abstract boolean` | ` `[removeAll](https://developer.android.com/reference/java/util/Collection#removeAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Removes all of this collection's elements that are also contained in the specified collection (optional operation). | | ` default boolean` | ` `[removeIf](https://developer.android.com/reference/java/util/Collection#removeIf(java.util.function.Predicate))`(`[Predicate](https://developer.android.com/reference/java/util/function/Predicate)` filter) ` Removes all of the elements of this collection that satisfy the given predicate. | | ` abstract boolean` | ` `[retainAll](https://developer.android.com/reference/java/util/Collection#retainAll(java.util.Collection))`(`[Collection](https://developer.android.com/reference/java/util/Collection)` c) ` Retains only the elements in this collection that are contained in the specified collection (optional operation). | | ` abstract int` | ` `[size](https://developer.android.com/reference/java/util/Collection#size())`() ` Returns the number of elements in this collection. | | ` default `[Spliterator](https://developer.android.com/reference/java/util/Spliterator)`` | ` `[spliterator](https://developer.android.com/reference/java/util/Collection#spliterator())`() ` Creates a[Spliterator](https://developer.android.com/reference/java/util/Spliterator)over the elements in this collection. | | ` default `[Stream](https://developer.android.com/reference/java/util/stream/Stream)`` | ` `[stream](https://developer.android.com/reference/java/util/Collection#stream())`() ` Returns a sequential`Stream`with this collection as its source. | | ` abstract T[]` | ` `[toArray](https://developer.android.com/reference/java/util/Collection#toArray(T[]))`(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[]](https://developer.android.com/reference/java/lang/Object) | ` `[toArray](https://developer.android.com/reference/java/util/Collection#toArray())`() ` Returns an array containing all of the elements in this collection. | | ` default T[]` | ` `[toArray](https://developer.android.com/reference/java/util/Collection#toArray(java.util.function.IntFunction))`(`[IntFunction](https://developer.android.com/reference/java/util/function/IntFunction)` generator) ` Returns an array containing all of the elements in this collection, using the provided`generator`function to allocate the returned array. | || | From interface` `[java.lang.Iterable](https://developer.android.com/reference/java/lang/Iterable)` ` |----------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` default void` | ` `[forEach](https://developer.android.com/reference/java/lang/Iterable#forEach(java.util.function.Consumer))`(`[Consumer](https://developer.android.com/reference/java/util/function/Consumer)` 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](https://developer.android.com/reference/java/util/Iterator)`` | ` `[iterator](https://developer.android.com/reference/java/lang/Iterable#iterator())`() ` Returns an iterator over elements of type`T`. | | ` default `[Spliterator](https://developer.android.com/reference/java/util/Spliterator)`` | ` `[spliterator](https://developer.android.com/reference/java/lang/Iterable#spliterator())`() ` Creates a[Spliterator](https://developer.android.com/reference/java/util/Spliterator)over the elements described by this`Iterable`. | || ## Public constructors ### ConcurrentLinkedQueue Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public ConcurrentLinkedQueue () ``` Creates a`ConcurrentLinkedQueue`that is initially empty.
### ConcurrentLinkedQueue Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public ConcurrentLinkedQueue (Collection c) ``` Creates a`ConcurrentLinkedQueue`initially containing the elements of the given collection, added in traversal order of the collection's iterator.
| Parameters || |-----|----------------------------------------------------------------------| | `c` | `Collection`: the collection of elements to initially contain
| | Throws || |------------------------------------------------------------------------------------------------|-------------------------------------------------------------| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if the specified collection or any of its elements are null | ## Public methods ### add Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean add (E e) ``` Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never throw[IllegalStateException](https://developer.android.com/reference/java/lang/IllegalStateException)or return`false`.
| Parameters || |-----|--------------------------------| | `e` | `E`: the element to add
| | Returns || |-----------|---------------------------------------------------------------------------------------------------------------------| | `boolean` | `true`(as specified by[Collection.add](https://developer.android.com/reference/java/util/Collection#add(E)))
| | Throws || |------------------------------------------------------------------------------------------------|----------------------------------| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if the specified element is null | ### addAll Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean addAll (Collection c) ``` Appends all of the elements in the specified collection to the end of this queue, in the order that they are returned by the specified collection's iterator. Attempts to`addAll`of a queue to itself result in`IllegalArgumentException`.
| Parameters || |-----|------------------------------------------------------------------| | `c` | `Collection`: the elements to be inserted into this queue
| | Returns || |-----------|------------------------------------------------------------| | `boolean` | `true`if this queue changed as a result of the call
| | Throws || |--------------------------------------------------------------------------------------------------------|-------------------------------------------------------------| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if the specified collection or any of its elements are null | | [IllegalArgumentException](https://developer.android.com/reference/java/lang/IllegalArgumentException) | if the collection is this queue | ### clear Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` 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](https://developer.android.com/reference/java/util/Queue#poll())until it returns`null`.
### contains Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean contains (Object o) ``` Returns`true`if this queue contains the specified element. More formally, returns`true`if and only if this queue contains at least one element`e`such that`o.equals(e)`.
| Parameters || |-----|---------------------------------------------------------------------| | `o` | `Object`: object to be checked for containment in this queue
| | Returns || |-----------|-----------------------------------------------------------| | `boolean` | `true`if this queue contains the specified element
| ### forEach Added in[API level 24](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public void forEach (Consumer action) ```

| Parameters || |----------|-------------------| | `action` | `Consumer`
| | Throws || |------------------------------------------------------------------------------------------------|---| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | | ### isEmpty Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean isEmpty () ``` Returns`true`if this queue contains no elements.
| Returns || |-----------|-------------------------------------------------| | `boolean` | `true`if this queue contains no elements
| ### iterator Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public Iterator iterator () ``` Returns an iterator over the elements in this queue in proper sequence. The elements will be returned in order from first (head) to last (tail). The returned iterator is[*weakly consistent*](https://developer.android.com/reference/java/util/concurrent/package-summary#Weakly).
| Returns || |-----------------------------------------------------------------------------|-----------------------------------------------------------------------| | [Iterator](https://developer.android.com/reference/java/util/Iterator)`` | an iterator over the elements in this queue in proper sequence
| ### offer Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean offer (E e) ``` Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never return`false`.
| Parameters || |-----|--------------------------------| | `e` | `E`: the element to add
| | Returns || |-----------|---------------------------------------------------------------------------------------------------------------| | `boolean` | `true`(as specified by[Queue.offer](https://developer.android.com/reference/java/util/Queue#offer(E)))
| | Throws || |------------------------------------------------------------------------------------------------|----------------------------------| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if the specified element is null | ### peek Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public 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 Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public 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 Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean remove (Object o) ``` Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element`e`such that`o.equals(e)`, if this queue contains one or more such elements. Returns`true`if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).
| Parameters || |-----|--------------------------------------------------------------------| | `o` | `Object`: element to be removed from this queue, if present
| | Returns || |-----------|------------------------------------------------------------| | `boolean` | `true`if this queue changed as a result of the call
| ### removeAll Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean removeAll (Collection c) ``` 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.
| Parameters || |-----|----------------------------------------------------------------------------------------| | `c` | `Collection`: collection containing elements to be removed from this collection
| | Returns || |-----------|-----------------------------------------------------------------| | `boolean` | `true`if this collection changed as a result of the call
| | Throws || |------------------------------------------------------------------------------------------------|---| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | | ### removeIf Added in[API level 24](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean removeIf (Predicate filter) ``` 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.
| Parameters || |----------|-------------------------------------------------------------------------------| | `filter` | `Predicate`: a predicate which returns`true`for elements to be removed
| | Returns || |-----------|-------------------------------------------| | `boolean` | `true`if any elements were removed
| | Throws || |------------------------------------------------------------------------------------------------|---| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | | ### retainAll Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public boolean retainAll (Collection c) ``` 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.
| Parameters || |-----|---------------------------------------------------------------------------------------| | `c` | `Collection`: collection containing elements to be retained in this collection
| | Returns || |-----------|-----------------------------------------------------------------| | `boolean` | `true`if this collection changed as a result of the call
| | Throws || |------------------------------------------------------------------------------------------------|---| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | | ### size Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public int size () ``` Returns the number of elements in this queue. If this queue contains more than`Integer.MAX_VALUE`elements, 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 queues, determining the current number of elements requires an O(n) traversal. Additionally, if elements are added or removed during execution of this method, the returned result may be inaccurate. Thus, this method is typically not very useful in concurrent applications.
| Returns || |-------|---------------------------------------------| | `int` | the number of elements in this queue
| ### spliterator Added in[API level 24](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public Spliterator spliterator () ``` Returns a[Spliterator](https://developer.android.com/reference/java/util/Spliterator)over the elements in this queue. The returned spliterator is[*weakly consistent*](https://developer.android.com/reference/java/util/concurrent/package-summary#Weakly). The`Spliterator`reports[Spliterator.CONCURRENT](https://developer.android.com/reference/java/util/Spliterator#CONCURRENT),[Spliterator.ORDERED](https://developer.android.com/reference/java/util/Spliterator#ORDERED), and[Spliterator.NONNULL](https://developer.android.com/reference/java/util/Spliterator#NONNULL).
##### Implementation Note: - The`Spliterator`implements`trySplit`to permit limited parallelism. | Returns || |-----------------------------------------------------------------------------------|------------------------------------------------------| | [Spliterator](https://developer.android.com/reference/java/util/Spliterator)`` | a`Spliterator`over the elements in this queue
| ### toArray Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public T[] toArray (T[] a) ``` Returns an array containing all of the elements in this queue, in proper sequence; the runtime type of the returned array is that of the specified array. If the queue 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 queue. If this queue fits in the specified array with room to spare (i.e., the array has more elements than this queue), the element in the array immediately following the end of the queue is set to`null`. Like the[toArray()](https://developer.android.com/reference/java/util/concurrent/ConcurrentLinkedQueue#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 queue known to contain only strings. The following code can be used to dump the queue 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()`.

| Parameters || |-----|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | `a` | `T`: the array into which the elements of the queue are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose
| | Returns || |-------|--------------------------------------------------------------| | `T[]` | an array containing all of the elements in this queue
| | Throws || |------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------| | [ArrayStoreException](https://developer.android.com/reference/java/lang/ArrayStoreException) | if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue | | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if the specified array is null | ### toArray Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public Object[] toArray () ``` Returns an array containing all of the elements in this queue, in proper sequence. The returned array will be "safe" in that no references to it are maintained by this queue. (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.
| Returns || |----------------------------------------------------------------------|--------------------------------------------------------------| | [Object[]](https://developer.android.com/reference/java/lang/Object) | an array containing all of the elements in this queue
| ### toString Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public String toString () ``` Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets (`"[]"`). Adjacent elements are separated by the characters`", "`(comma and space). Elements are converted to strings as by[String.valueOf(Object)](https://developer.android.com/reference/java/lang/String#valueOf(java.lang.Object)).
| Returns || |--------------------------------------------------------------------|---------------------------------------------------| | [String](https://developer.android.com/reference/java/lang/String) | a string representation of this collection
|