帮我用java写一个通过CompletableFuture实现任务调度的demo

好的，这里是一个通过CompletableFuture实现任务调度的Java demo，代码如下：

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

public class CompletableFutureDemo {

    public static void main(String[] args) throws ExecutionException, InterruptedException {

        CompletableFuture<Void> future1 = CompletableFuture.runAsync(() -> {
            System.out.println("Task 1 started...");
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Task 1 completed!");
        });

        CompletableFuture<Void> future2 = CompletableFuture.runAsync(() -> {
            System.out.println("Task 2 started...");
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Task 2 completed!");
        });

        CompletableFuture<Void> future3 = CompletableFuture.runAsync(() -> {
            System.out.println("Task 3 started...");
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Task 3 completed!");
        });

        CompletableFuture<Void> future4 = CompletableFuture.runAsync(() -> {
            System.out.println("Task 4 started...");
            try {
                TimeUnit.SECONDS.sleep(4);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Task 4 completed!");
        });

        CompletableFuture<Void> allFutures = CompletableFuture.allOf(future1, future2, future3, future4);
        allFutures.get();
    }
}

这个demo中，我们使用了四个CompletableFuture对象来模拟四个任务，并通过CompletableFuture.allOf()方法来等待所有任务完成。其中，每个任务都是通过runAsync()方法提交到默认的ForkJoinPool中执行的，这里我们使用了TimeUnit来模拟每个任务的执行时间。