完了可能将来の使用法

CompletableFuture を使用して複数の非同期操作の完了を待機する方法?

CompletableFuture には、複数の非同期操作の完了を待機するために使用できる allOf というメソッドが用意されています。 allOf メソッドは、可変数の CompletableFuture オブジェクトを引数として受け取り、すべての入力 CompletableFutureCompletableFuture を返します。 /code> オブジェクトが完了しました。allOf that can be used to wait for multiple asynchronous operations to complete. The allOf method takes a variable number of CompletableFuture objects as arguments and returns a new CompletableFuture that completes when all of the input CompletableFuture objects have completed.

The following code sample shows how to use the allOf method to wait for multiple asynchronous operations to complete:

<code class="java">CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> "Hello");
CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> "World");
CompletableFuture<Void> allOf = CompletableFuture.allOf(future1, future2);

allOf.join();

System.out.println(future1.get()); // Prints "Hello"
System.out.println(future2.get()); // Prints "World"</code>

How does CompletableFuture's cancellation mechanism work?

CompletableFuture provides a cancel method that can be used to cancel the asynchronous operation represented by the CompletableFuture. The cancel method takes a boolean argument that indicates whether or not the cancellation should be interrupting.

If the cancel method is called with the interrupting flag set to true, the asynchronous operation will be interrupted if it is still running. If the asynchronous operation has already completed, the cancel method will have no effect.

If the cancel method is called with the interrupting flag set to false, the asynchronous operation will be cancelled if it has not yet completed. If the asynchronous operation has already completed, the cancel method will have no effect.

The following code sample shows how to use the cancel method to cancel an asynchronous operation:

<code class="java">CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    try {
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    return "Hello World";
});

future.cancel(true); // Interrupt the asynchronous operation

if (future.isCancelled()) {
    System.out.println("The asynchronous operation was cancelled.");
}</code>

How to handle exceptions and return values with CompletableFuture?

CompletableFuture provides methods for handling both exceptions and return values. The thenApply and thenAccept methods can be used to handle return values, while the exceptionally and handle methods can be used to handle exceptions.

The thenApply method takes a function as an argument and returns a new CompletableFuture that will be completed with the result of applying the function to the result of the original CompletableFuture. The following code sample shows how to use the thenApply method to handle a return value:

<code class="java">CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello World");

CompletableFuture<Integer> future2 = future.thenApply(s -> s.length());

future2.join();

System.out.println(future2.get()); // Prints 11</code>

The thenAccept method takes a consumer as an argument and returns a new CompletableFuture that will be completed when the consumer has been applied to the result of the original CompletableFuture. The following code sample shows how to use the thenAccept method to handle a return value:

<code class="java">CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello World");

CompletableFuture<Void> future2 = future.thenAccept(s -> System.out.println(s));

future2.join(); // Prints "Hello World"</code>

The exceptionally method takes a function as an argument and returns a new CompletableFuture that will be completed with the result of applying the function to the exception that caused the original CompletableFuture to complete exceptionally. The following code sample shows how to use the exceptionally method to handle an exception:

<code class="java">CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    throw new RuntimeException("Error!");
});

CompletableFuture<String> future2 = future.exceptionally(e -> "Error occurred: " + e.getMessage());

future2.join();

System.out.println(future2.get()); // Prints "Error occurred: java.lang.RuntimeException: Error!"</code>

The handle method takes a bi-function as an argument and returns a new CompletableFuture that will be completed with the result of applying the bi-function to the result of the original CompletableFuture and the exception that caused the original CompletableFuture to complete exceptionally (if any). The following code sample shows how to use the handle

<code class="java">CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    if (Math.random() > 0.5) {
        return "Success";
    } else {
        throw new RuntimeException("Error!");
    }
});

CompletableFuture<String> future2 = future.handle((result, exception) -> {
    if (exception != null) {
        return "Error occurred: " + exception.getMessage();
    } else {
        return result;
    }
});

future2.join();

System.out.println(future2.get()); // Prints "Success" or "Error occurred: java.lang.RuntimeException: Error!"</code>

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