java.util.concurrent.CompletableFuture#allOfff方法

`java.util.concurrent.CompletableFuture#allOf`方法是一个静态方法，它接受一个`CompletableFuture<?>... cfs`参数，返回一个`CompletableFuture<Void>`对象。该方法的作用是当所有的CompletableFuture都完成时，返回一个新的CompletableFuture，该CompletableFuture的计算结果为null。如果任何一个CompletableFuture异常完成，那么返回的CompletableFuture也将以该异常完成。

以下是一个使用`CompletableFuture#allOf`方法的例子：

CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> {
    try {
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    return "Result of Future 1";
});

CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> {
    try {
        Thread.sleep(500);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    return "Result of Future 2";
});

CompletableFuture<String> future3 = CompletableFuture.supplyAsync(() -> {
    try {
        Thread.sleep(2000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    return "Result of Future 3";
});

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

allFutures.join();

System.out.println(future1.get());
System.out.println(future2.get());
System.out.println(future3.get());