How do I send and receive messages synchronously using a queue

In this Q&A, we'll go over how to send and receive messages synchronously using TransferQueue

TransferQueue is a blocking queue interface where producers can wait for consumers to receive elements.  It has blocking and non-blocking methods.

Example below uses LinkedTransferQueue class to synchronously transfer and take elements in different threads. One can also use SynchronousQueue - it behaves the same way as LinkedTransferQueue with capacity zero.

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.LinkedTransferQueue;

import java.util.concurrent.TransferQueue;

public class LTQTest {

    public static void main(String[] args) {

        TransferQueue<Integer> tq = new LinkedTransferQueue<>();

        ExecutorService producer = Executors.newSingleThreadExecutor();

        ExecutorService consumer = Executors.newSingleThreadExecutor();

        producer.submit(() -> {

            try {

                // transfer an element.  blocking call

                // waits until consumer thread calls take()

                tq.transfer(100);

            } catch (InterruptedException e) {

                e.printStackTrace();

            }

        });

        consumer.submit(() -> {

            try {

                // blocking call.  waits until the element is in the queue

                System.out.println(tq.take());

            } catch (InterruptedException e) {

                e.printStackTrace();

            }

        });

        producer.shutdown();

        consumer.shutdown();

    }

}