Explain SynchronousQueue in Java concurrent collections.

The SynchronousQueue class implements the BlockingQueue interface introduced in Java 1.5.

SynchronousQueue is a blocking queue 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.

The peek operation cannot be performed 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. This queue does not permit null elements.

This class supports an optional fairness policy for ordering waiting producer and consumer threads. A queue constructed with fairness set to true grants threads access in FIFO order.

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.

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