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# SynchronousQueue

`public class SynchronousQueue`
`` `` `extends AbstractQueue<E>` `implements BlockingQueue<E>, Serializable`

java.lang.Object
↳ | java.util.AbstractCollection<E>
| ↳ | java.util.AbstractQueue<E>
|  | ↳ | java.util.concurrent.SynchronousQueue<E>

A BlockingQueue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa. A synchronous queue does not have any internal capacity, not even a capacity of one. You cannot `peek` at a synchronous queue because an element is only present when you try to remove it; you cannot insert an element (using any method) unless another thread is trying to remove it; you cannot iterate as there is nothing to iterate. The head of the queue is the element that the first queued inserting thread is trying to add to the queue; if there is no such queued thread then no element is available for removal and `poll()` will return `null`. For purposes of other `Collection` methods (for example `contains`), a `SynchronousQueue` acts as an empty collection. This queue does not permit `null` elements.

Synchronous queues are similar to rendezvous channels used in CSP and Ada. They are well suited for handoff designs, in which an object running in one thread must sync up with an object running in another thread in order to hand it some information, event, or task.

This class supports an optional fairness policy for ordering waiting producer and consumer threads. By default, this ordering is not guaranteed. However, a queue constructed with fairness set to `true` grants threads access in FIFO order.

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

## Summary

### Public constructors
`SynchronousQueue()` Creates a `SynchronousQueue` with nonfair access policy.
`SynchronousQueue(boolean fair)` Creates a `SynchronousQueue` with the specified fairness policy.

### Public methods
`void` | `clear()` Does nothing.
`boolean` | `contains(Object o)` Always returns `false`.
`boolean` | `containsAll(Collection<?> c)` Returns `false` unless the given collection is empty.
`int` | `drainTo(Collection<? super E> c, int maxElements)` Removes at most the given number of available elements from this queue and adds them to the given collection.
`int` | `drainTo(Collection<? super E> c)` Removes all available elements from this queue and adds them to the given collection.
`boolean` | `isEmpty()` Always returns `true`.
`Iterator<E>` | `iterator()` Returns an empty iterator in which `hasNext` always returns `false`.
`boolean` | `offer(E e)` Inserts the specified element into this queue, if another thread is waiting to receive it.
`boolean` | `offer(E e, long timeout, TimeUnit unit)` Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.
`E` | `peek()` Always returns `null`.
`E` | `poll(long timeout, TimeUnit unit)` Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
`E` | `poll()` Retrieves and removes the head of this queue, if another thread is currently making an element available.
`void` | `put(E e)` Adds the specified element to this queue, waiting if necessary for another thread to receive it.
`int` | `remainingCapacity()` Always returns zero.
`boolean` | `remove(Object o)` Always returns `false`.
`boolean` | `removeAll(Collection<?> c)` Always returns `false`.
`boolean` | `retainAll(Collection<?> c)` Always returns `false`.
`int` | `size()` Always returns zero.
`Spliterator<E>` | `spliterator()` Returns an empty spliterator in which calls to `Spliterator.trySplit()` always return `null`.
`E` | `take()` Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
`<T> T[]` | `toArray(T[] a)` Sets the zeroth element of the specified array to `null` (if the array has non-zero length) and returns it.
`Object[]` | `toArray()` Returns a zero-length array.
`String` | `toString()` Always returns `"[]"`.

### Inherited methods
From class `java.util.AbstractQueue`
`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.
From class `java.util.AbstractCollection`
`boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation). This implementation always throws an `UnsupportedOperationException`.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
`void` | `clear()` Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
`boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
`boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements contained in this collection.
`boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
`boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`abstract int` | `size()` Returns the number of elements in this collection.
`<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. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
`String` | `toString()` Returns a string representation of this collection.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.util.Queue`
`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.
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.util.concurrent.BlockingQueue`
`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 boolean` | `contains(Object o)` Returns `true` if this queue contains the specified element.
`abstract int` | `drainTo(Collection<? super E> c)` Removes all available elements from this queue and adds them to the given collection.
`abstract int` | `drainTo(Collection<? super E> c, int maxElements)` Removes at most the given number of available elements from this queue and adds them to the given collection.
`abstract boolean` | `offer(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 `false` if no space is currently available.
`abstract boolean` | `offer(E e, long timeout, TimeUnit unit)` Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
`abstract E` | `poll(long timeout, TimeUnit unit)` Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
`abstract void` | `put(E e)` Inserts the specified element into this queue, waiting if necessary for space to become available.
`abstract int` | `remainingCapacity()` Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or `Integer.MAX_VALUE` if there is no intrinsic limit.
`abstract boolean` | `remove(Object o)` Removes a single instance of the specified element from this queue, if it is present.
`abstract E` | `take()` Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
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 constructors

### SynchronousQueue

    public SynchronousQueue ()

Creates a `SynchronousQueue` with nonfair access policy.

### SynchronousQueue

    public SynchronousQueue (boolean fair)

Creates a `SynchronousQueue` with the specified fairness policy.

Parameters
`fair` | `boolean`: if true, waiting threads contend in FIFO order for access; otherwise the order is unspecified.

## Public methods

### clear

    public void clear ()

Does nothing. A `SynchronousQueue` has no internal capacity.

### contains

    public boolean contains (Object o)

Always returns `false`. A `SynchronousQueue` has no internal capacity.

Parameters
`o` | `Object`: the element

Returns
`boolean` | `false`

### containsAll

    public boolean containsAll (Collection<?> c)

Returns `false` unless the given collection is empty. A `SynchronousQueue` has no internal capacity.

Parameters
`c` | `Collection`: the collection

Returns
`boolean` | `false` unless given collection is empty

### drainTo

    public int drainTo (Collection<? super E> c,
                    int maxElements)

Removes at most the given number of available elements from this queue and adds them to the given collection. A failure encountered while attempting to add elements to collection `c` may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain 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.

Parameters
`c` | `Collection`: the collection to transfer elements into
`maxElements` | `int`: the maximum number of elements to transfer

Returns
`int` | the number of elements transferred

Throws
`UnsupportedOperationException` |
`ClassCastException` |
`NullPointerException` |
`IllegalArgumentException` |

### drainTo

    public int drainTo (Collection<? super E> c)

Removes all available elements from this queue and adds them to the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection `c` may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain 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.

Parameters
`c` | `Collection`: the collection to transfer elements into

Returns
`int` | the number of elements transferred

Throws
`UnsupportedOperationException` |
`ClassCastException` |
`NullPointerException` |
`IllegalArgumentException` |

### isEmpty

    public boolean isEmpty ()

Always returns `true`. A `SynchronousQueue` has no internal capacity.

Returns
`boolean` | `true`

### iterator

    public Iterator<E> iterator ()

Returns an empty iterator in which `hasNext` always returns `false`.

Returns
`Iterator<E>` | an empty iterator

### offer

    public boolean offer (E e)

Inserts the specified element into this queue, if another thread is waiting to receive it.

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

Returns
`boolean` | `true` if the element was added to this queue, else `false`

Throws
`NullPointerException` | if the specified element is null

### offer

    public boolean offer (E e,
                    long timeout,
                    TimeUnit unit)

Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.

Parameters
`e` | `E`: the element to add
`timeout` | `long`: how long to wait before giving up, in units of `unit`
`unit` | `TimeUnit`: a `TimeUnit` determining how to interpret the `timeout` parameter

Returns
`boolean` | `true` if successful, or `false` if the specified waiting time elapses before a consumer appears

Throws
`InterruptedException` |
`NullPointerException` |

### peek

    public E peek ()

Always returns `null`. A `SynchronousQueue` does not return elements unless actively waited on.

Returns
`E` | `null`

### poll

    public E poll (long timeout,
                    TimeUnit unit)

Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.

Parameters
`timeout` | `long`: how long to wait before giving up, in units of `unit`
`unit` | `TimeUnit`: a `TimeUnit` determining how to interpret the `timeout` parameter

Returns
`E` | the head of this queue, or `null` if the specified waiting time elapses before an element is present

Throws
`InterruptedException` |

### poll

    public E poll ()

Retrieves and removes the head of this queue, if another thread is currently making an element available.

Returns
`E` | the head of this queue, or `null` if no element is available

### put

    public void put (E e)

Adds the specified element to this queue, waiting if necessary for another thread to receive it.

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

Throws
`InterruptedException` |
`NullPointerException` |

### remainingCapacity

    public int remainingCapacity ()

Always returns zero. A `SynchronousQueue` has no internal capacity.

Returns
`int` | zero

### remove

    public boolean remove (Object o)

Always returns `false`. A `SynchronousQueue` has no internal capacity.

Parameters
`o` | `Object`: the element to remove

Returns
`boolean` | `false`

### removeAll

    public boolean removeAll (Collection<?> c)

Always returns `false`. A `SynchronousQueue` has no internal capacity.

Parameters
`c` | `Collection`: the collection

Returns
`boolean` | `false`

### retainAll

    public boolean retainAll (Collection<?> c)

Always returns `false`. A `SynchronousQueue` has no internal capacity.

Parameters
`c` | `Collection`: the collection

Returns
`boolean` | `false`

### size

    public int size ()

Always returns zero. A `SynchronousQueue` has no internal capacity.

Returns
`int` | zero

### spliterator

    public Spliterator<E> spliterator ()

Returns an empty spliterator in which calls to `Spliterator.trySplit()` always return `null`.

Returns
`Spliterator<E>` | an empty spliterator

### take

    public E take ()

Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.

Returns
`E` | the head of this queue

Throws
`InterruptedException` |

### toArray

    public T[] toArray (T[] a)

Sets the zeroth element of the specified array to `null` (if the array has non-zero length) and returns it.

Parameters
`a` | `T`: the array

Returns
`T[]` | the specified array

Throws
`NullPointerException` | if the specified array is null

### toArray

    public Object[] toArray ()

Returns a zero-length array.

Returns
`Object[]` | a zero-length array

### toString

    public String toString ()

Always returns `"[]"`.

Returns
`String` | `"[]"`

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