ForkJoin use and abuse examples

ForkJoinUseAndAbuseExample.java

package com.example;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;

public class ForkJoinUseAndAbuseExample {

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        ForkJoinPool fgp = new ForkJoinPool(1);

        /*
         * Example of ForkJoinPool abuse. In this example ForkJoinPool is used as an Executor and although constructed
         * with parallelism=1, it will create 4 threads to complete this nested task!
         */
        System.out.printf("Example 1 returned: %d\n\n",
                CompletableFuture.supplyAsync(() -> {
                    System.out.printf("1: %s\n", Thread.currentThread().getName());
                    return CompletableFuture.supplyAsync(() -> {
                        System.out.printf("2: %s\n", Thread.currentThread().getName());
                        return CompletableFuture.supplyAsync(() -> {
                            System.out.printf("3: %s\n", Thread.currentThread().getName());
                            return CompletableFuture.supplyAsync(() -> {
                                System.out.printf("4: %s\n", Thread.currentThread().getName());
                                return 1;
                            }, fgp).join();
                        }, fgp).join();
                    }, fgp).join();
                }, fgp).join()
        );

        /*
         * Example of a nested ForkJoinTask that allegedly require 4 threads to complete, but successfully completes
         * with only a single thread thanks to the ForkJoinPool task management capabilities.
         * This example takes full advantage of the fork-join framework and is able to use resources more effectively.
         */
        System.out.printf("Example 2 returned: %d\n\n",
                fgp.invoke(ForkJoinTask.adapt(() -> {
                    System.out.printf("1: %s\n", Thread.currentThread().getName());
                    return ForkJoinTask.adapt(() -> {
                        System.out.printf("2: %s\n", Thread.currentThread().getName());
                        return ForkJoinTask.adapt(() -> {
                            System.out.printf("3: %s\n", Thread.currentThread().getName());
                            return ForkJoinTask.adapt(() -> {
                                System.out.printf("4: %s\n", Thread.currentThread().getName());
                                return 1;
                            }).fork().get();
                        }).fork().get();
                    }).fork().get();
                }))
        );
    }
}