CF.java

import java.util.Collection;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        System.out.println("App started");
//        TASKS
        final Runnable runnable11 = () -> {
            System.out.println("List 1, task 1");
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };
        final Runnable runnable12 = () -> System.out.println("List 1, task 2");
        final Runnable runnable21 = () -> {
            System.out.println("List 2, task 1");
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            throw new RuntimeException("Error");
        };
        final Runnable runnable22 = () -> System.out.println("List 2, task 2");


        final List<CompletableFuture<Void>> completableFutures = runList(runnable11, runnable12);
        final List<CompletableFuture<Void>> completableFutures1 = runList(runnable21, runnable22);

        Stream.of(completableFutures, completableFutures1)
                .flatMap(Collection::stream)
                .forEach(CompletableFuture::join);
    }

    private static List<CompletableFuture<Void>> runList(Runnable... runnables) {
        return Stream.of(runnables).
                map(runnable -> CompletableFuture.runAsync(runnable)
                        .whenComplete((finished, exc) -> handleTaskResult(exc))
                        .exceptionally(Main::handleExceptionallyResult))
                .collect(Collectors.toList());
    }

    private static Void handleExceptionallyResult(Throwable exc) {
        System.err.println("Task endded with error " + exc.getMessage());
        return null;
    }

    private static void handleTaskResult(Throwable exc) {
        if (exc == null) {
            System.out.println("Task completed");
        } else {
            System.err.println("ERROR in runnable " + exc.getMessage());
        }
    }
}