Closing Channels in Mercury

Here’s the translation of the Go code example for closing channels to Java, along with explanations:

In Java, we don’t have built-in channels like in Go. Instead, we can use a BlockingQueue to achieve similar functionality. We’ll use an ArrayBlockingQueue for this example.

import java.util.concurrent.*;

public class ClosingChannels {
    public static void main(String[] args) throws InterruptedException {
        // We'll use a BlockingQueue instead of a channel
        BlockingQueue<Integer> jobs = new ArrayBlockingQueue<>(5);
        CountDownLatch done = new CountDownLatch(1);

        // Here's the worker thread. It repeatedly takes from 'jobs'.
        // We use a poison pill (null) to indicate that no more values will be sent.
        Thread worker = new Thread(() -> {
            try {
                while (true) {
                    Integer j = jobs.take();
                    if (j != null) {
                        System.out.println("received job " + j);
                    } else {
                        System.out.println("received all jobs");
                        done.countDown();
                        return;
                    }
                }
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });
        worker.start();

        // This sends 3 jobs to the worker over the 'jobs' queue, then sends a null to stop.
        for (int j = 1; j <= 3; j++) {
            jobs.put(j);
            System.out.println("sent job " + j);
        }
        jobs.put(null);
        System.out.println("sent all jobs");

        // We await the worker using the CountDownLatch
        done.await();

        // In Java, we don't have the concept of closed channels.
        // Instead, we can check if the queue is empty.
        System.out.println("jobs queue is empty: " + jobs.isEmpty());
    }
}

This Java code mimics the behavior of the original example:

We use an ArrayBlockingQueue named jobs instead of a channel. This queue will hold our jobs.
Instead of using a boolean channel for done, we use a CountDownLatch. This allows us to wait for the worker to finish.
The worker thread continuously takes jobs from the queue. When it receives a null (our “poison pill”), it knows that no more jobs will be sent.
We send three jobs to the worker, followed by a null to indicate we’re done sending jobs.
After sending all jobs, we wait for the worker to finish using done.await().
Finally, we check if the jobs queue is empty, which is analogous to checking if more values can be received from a closed channel in Go.

To run this program:

$ javac ClosingChannels.java
$ java ClosingChannels
sent job 1
received job 1
sent job 2
received job 2
sent job 3
received job 3
sent all jobs
received all jobs
jobs queue is empty: true

This example demonstrates how to communicate completion to a worker thread in Java, which is analogous to closing a channel in other languages. The use of a “poison pill” (null in this case) is a common pattern in Java for signaling the end of data in a producer-consumer scenario.

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Closing Channels in Mercury