CompletableFuture.thenComposeAsync example

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

public class Test {

    public static void main(String argv[]) throws Exception {

        int nThreads = 1;
        Executor executor = Executors.newFixedThreadPool( nThreads );

        CompletableFuture.completedFuture(true)
            .thenComposeAsync((unused)->{

                System.err.println("About to enqueue task");
                CompletableFuture<Boolean> innerFuture = new CompletableFuture<>();
                executor.execute(() -> {

                    // pretend this is some really expensive computation done asynchronously

                    System.err.println("Inner task");
                    innerFuture.complete(true);
                });
                System.err.println("Task enqueued");

                return innerFuture;
            }, executor).get();

        // This is never reached.  The inner runnable (with pretend... in its comment) is
        // starved because the single threaded executor is blocking in the call to thenComposeAsync
        System.err.println("All done");
        System.exit(0);
    }

}