How do I produce and consume data in priority order

In this Q&A, we'll go over how to consume data elements in priority order.

By default ExecutorService delivers tasks to consumer threads in FIFO order.  We can change that default behavior by using PriorityBlockingQueue.  Priority for a task can be defined by specifying appropriate Comparator.  See below example

package com.javahowdoi.thread;

import java.util.concurrent.*;

public class PriorityQueueDemo {

    private static class QueueItem {

        final private int i;

        public QueueItem(int i) {

            this.i = i;

        }

        public int getI() {return i;}

    }

    private static class LoopingTask implements Runnable {

        final BlockingQueue<QueueItem> bq ;

        public LoopingTask(BlockingQueue<QueueItem> bq ) {

            this.bq = bq;

        }

        @Override

        public void run() {

            try {

                while(true) {

                    System.out.println(bq.take().getI());

                }

            } catch(Exception e) {}

        }

    }

    public static void main(String[] args) {

        BlockingQueue<QueueItem> pbq = new PriorityBlockingQueue<>(10,

                (QueueItem o1, QueueItem o2) -> {

                    if( o1.getI() > o2.getI() )

                        return -1;

                    if( o1.getI() == o2.getI() )

                        return 0;

                    return 1;

                }

                );

        ExecutorService tpe = Executors.newSingleThreadExecutor();

        QueueItem c1 = new QueueItem(10);

        QueueItem c2 = new QueueItem(20);

        pbq.add(c1);

        pbq.add(c2);

        tpe.submit( new LoopingTask(pbq) );

        tpe.shutdown();

    }

}