Difference between BlockingQueue and TransferQueue

Solution 1:

From TransferQueue JavaDocs:

A BlockingQueue in which producers may wait for consumers to receive elements. A TransferQueue may be useful for example in message passing applications in which producers sometimes (using method transfer(E)) await receipt of elements by consumers invoking take or poll, while at other times enqueue elements (via method put) without waiting for receipt.

In other words, when you use BlockingQueue, you can only put element into queue (and block if queue is full). With TransferQueue, you can also block until other thread receives your element (you must use new transfer method for that). This is the difference. With BlockingQueue, you cannot wait until other thread removes your element (only when you use SynchronousQueue, but that isn't really a queue).

Other than this, TransferQueue is also a BlockingQueue. Check out new available methods in TransferQueue: http://download.oracle.com/javase/7/docs/api/java/util/concurrent/TransferQueue.html (transfer, tryTransfer, hasWaitingConsumer, getWaitingConsumerCount).

Collections Framework Enhancements in Java SE 7 says explicitly:

The interface TransferQueue has been added. It is a refinement of the BlockingQueue interface in which producers can wait for consumers to receive elements. One implementation of the new interface is also included in this release, LinkedTransferQueue.

Solution 2:

In short, BlockingQueue guarantees that the element made by producer must be in the queue, while TransferQueue gets one step further, it guarantees that the element "consumed" by some consumer.