# Exchanger

public class **Exchanger** extends [Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)  
A synchronization point at which threads can pair and swap elements
within pairs. Each thread presents some object on entry to the
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Exchanger.html#exchange(V)` method, matches with a partner thread,
and receives its partner's object on return. An Exchanger may be
viewed as a bidirectional form of a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/SynchronousQueue.html`.
Exchangers may be useful in applications such as genetic algorithms
and pipeline designs.

**Sample Usage:**
Here are the highlights of a class that uses an `Exchanger`
to swap buffers between threads so that the thread filling the
buffer gets a freshly emptied one when it needs it, handing off the
filled one to the thread emptying the buffer.

     class FillAndEmpty {
       Exchanger<DataBuffer> exchanger = new Exchanger<>();
       DataBuffer initialEmptyBuffer = ... a made-up type
       DataBuffer initialFullBuffer = ...

       class FillingLoop implements Runnable {
         public void run() {
           DataBuffer currentBuffer = initialEmptyBuffer;
           try {
             while (currentBuffer != null) {
               addToBuffer(currentBuffer);
               if (currentBuffer.isFull())
                 currentBuffer = exchanger.exchange(currentBuffer);
             }
           } catch (InterruptedException ex) { ... handle ... }
         }
       }

       class EmptyingLoop implements Runnable {
         public void run() {
           DataBuffer currentBuffer = initialFullBuffer;
           try {
             while (currentBuffer != null) {
               takeFromBuffer(currentBuffer);
               if (currentBuffer.isEmpty())
                 currentBuffer = exchanger.exchange(currentBuffer);
             }
           } catch (InterruptedException ex) { ... handle ...}
         }
       }

       void start() {
         new Thread(new FillingLoop()).start();
         new Thread(new EmptyingLoop()).start();
       }
     }

Memory consistency effects: For each pair of threads that
successfully exchange objects via an `Exchanger`, actions
prior to the `exchange()` in each thread
[*happen-before*](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/package-summary#MemoryVisibility)
those subsequent to a return from the corresponding `exchange()`
in the other thread.

### Public Constructor Summary

|---|---|
|   | [Exchanger](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Exchanger.html#Exchanger())() Creates a new Exchanger. |

### Public Method Summary

|---|---|
| V | [exchange](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Exchanger.html#exchange(V,%20long,%20java.util.concurrent.TimeUnit))(V x, long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit) Waits for another thread to arrive at this exchange point (unless the current thread is {@linkplain Thread#interrupt interrupted} or the specified waiting time elapses), and then transfers the given object to it, receiving its object in return. |
| V | [exchange](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Exchanger.html#exchange(V))(V x) Waits for another thread to arrive at this exchange point (unless the current thread is {@linkplain Thread#interrupt interrupted}), and then transfers the given object to it, receiving its object in return. |

### Inherited Method Summary

From class [java.lang.Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)

|---|---|
| [Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [clone](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#clone())() Creates and returns a copy of this `Object`. |
| boolean | [equals](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#equals(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) obj) Compares this instance with the specified object and indicates if they are equal. |
| void | [finalize](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#finalize())() Invoked when the garbage collector has detected that this instance is no longer reachable. |
| final [Class](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html)\<?\> | [getClass](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#getClass())() Returns the unique instance of `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html` that represents this object's class. |
| int | [hashCode](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#hashCode())() Returns an integer hash code for this object. |
| final void | [notify](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notify())() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| final void | [notifyAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notifyAll())() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#toString())() Returns a string containing a concise, human-readable description of this object. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long,%20int))(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long))(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait())() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object. |

## Public Constructors

#### public
**Exchanger**
()

Creates a new Exchanger.

## Public Methods

#### public V
**exchange**
(V x, long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit)

Waits for another thread to arrive at this exchange point (unless
the current thread is {@linkplain Thread#interrupt interrupted} or
the specified waiting time elapses), and then transfers the given
object to it, receiving its object in return.

If another thread is already waiting at the exchange point then
it is resumed for thread scheduling purposes and receives the object
passed in by the current thread. The current thread returns immediately,
receiving the object passed to the exchange by that other thread.

If no other thread is already waiting at the exchange then the
current thread is disabled for thread scheduling purposes and lies
dormant until one of three things happens:

- Some other thread enters the exchange; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread; or
- The specified waiting time elapses.

If the current thread:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting for the exchange,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeoutException.html` is thrown. If the time is less than or equal
to zero, the method will not wait at all.

##### Parameters

| x | the object to exchange |
| timeout | the maximum time to wait |
| unit | the time unit of the `timeout` argument |
|---|---|

##### Returns

- the object provided by the other thread

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread was interrupted while waiting |
| [TimeoutException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeoutException.html) | if the specified waiting time elapses before another thread enters the exchange |
|---|---|

#### public V
**exchange**
(V x)

Waits for another thread to arrive at this exchange point (unless
the current thread is {@linkplain Thread#interrupt interrupted}),
and then transfers the given object to it, receiving its object
in return.

If another thread is already waiting at the exchange point then
it is resumed for thread scheduling purposes and receives the object
passed in by the current thread. The current thread returns immediately,
receiving the object passed to the exchange by that other thread.

If no other thread is already waiting at the exchange then the
current thread is disabled for thread scheduling purposes and lies
dormant until one of two things happens:

- Some other thread enters the exchange; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread.

If the current thread:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting for the exchange,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` is thrown and the current thread's interrupted status is cleared.

<br />

##### Parameters

| x | the object to exchange |
|---|---|

##### Returns

- the object provided by the other thread

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread was interrupted while waiting |
|---|---|