强制播放可选或流的内容

Question

我希望我可以检查并将可选的内容转换为另一个类或拥有一个空对象，而不是编写自己的方法或类。

对于应用程序中的这个子问题，我希望将树节点实例的自定义用户对象转换为自定义用户对象，前提是树节点是默认可变树节点的实例。

private Optional<CustomUserObject> getCustomUserObject(TreeNode node) {
    Optional<DefaultMutableTreeNode> optDefaultMutableTreeNode = OptionalUtil.cast(Optional.ofNullable(node), DefaultMutableTreeNode.class);
    Optional<Object> optUserObject = optDefaultMutableTreeNode.map(DefaultMutableTreeNode::getUserObject); //
    return OptionalUtil.cast(optUserObject, CustomUserObject.class);
}


/**
 * Maps the given optional, if the containing element is an instance of the given class.
 * Returns empty if the containing object is not an instance of the given class.
 * 
 * @param orgOptional
 *        given optional
 * @param clazz
 *        given class.
 * @return the resulting {@link Optional}.
 */
public static <T, X> Optional<T> cast(Optional<X> orgOptional, Class<T> clazz) {
    return orgOptional //
        .filter(clazz::isInstance) // check instance
        .map(clazz::cast); // cast
}

/**
 * Maps the given stream, if the containing element is an instance of the given class.
 * Returns empty if the containing object is not an instance of the given class.
 * 
 * @param orgStream
 *        given optional
 * @param clazz
 *        given class.
 * @return the resulting {@link Optional}.
 */
public static <T, X> Stream<T> cast(Stream<X> orgStream, Class<T> clazz) {
    return orgStream //
        .filter(clazz::isInstance) // check instance
        .map(clazz::cast); // cast
}

我记得我需要经常以这种方式播放可选或流。它不流利。实际上，我希望java Optional或Stream有一个执行上述步骤的cast方法。我不想编写自己的fluent CustomOptional。我错过什么了吗？有没有更简单的方法？

Answer 1

我想出了这个可选的

/**
 * Creates a function that tries to cast to the given class.
 * If the given argument is not an instance of the given class <code>Optional.empty()</code> is returned.
 * This function can be used in an <code>optionalObject.flatMap(Functional.cast(SomeClass.class))</code> context.
 * 
 * @param clazz
 *        given Clazz
 * @return Returns an optional of the given class.
 * @param <X>
 *        generic input argument for the function
 * @param <T>
 *        type of the output of the function
 */
public static <X, T> Function<X, Optional<T>> cast(Class<? extends T> clazz) {
    return x -> {
        if (clazz.isInstance(x)) {
            return Optional.of(clazz.cast(x));
        }
        return Optional.empty();
    };
}

使用方法如下：

Optional<DefaultMutableTreeNode> optParent = Optional.of(node) // optional of original node
                    .map(ProfileTreeNode::getParent) // parent of this node.
                    .flatMap(FunctionalUtil.cast(DefaultMutableTreeNode.class)); // cast to DefaultMutableTreeNode
                optParent.ifPresent(y -> y.remove(node));

对于我写的流

/**
 * Creates a function that tries to cast to the given class.
 * If the given argument is not an instance of the given class an empty stream is returned.
 * This function can be used in an <code>stream.flatMap(Functional.castStream(SomeClass.class))</code> context.
 * 
 * @param clazz
 *        given Clazz
 * @return Returns an optional of the given class.
 * @param <X>
 *        generic input argument for the function
 * @param <T>
 *        type of the output of the function
 */
public static <X, T> Function<X, Stream<T>> castStream(Class<? extends T> clazz) {
    return x -> Stream.of(x) // stream of x.
        .filter(clazz::isInstance) // check if instance
        .map(clazz::cast);
}

Answer 2

通过使用返回函数的方法，您可以轻松地使其更加流畅。

对于可选的，这很容易，因为map（）可以通过返回null来充当过滤器。所以只需定义：

public static <U> Function<Object, U> filterAndCast(Class<? extends U> clazz) {
    return t -> clazz.isInstance(t) ? clazz.cast(t) : null;
}

并将其用作：

Optional<Number> number = Optional.of(42L);
System.out.println(number.map(filterAndCast(Integer.class)));
System.out.println(number.map(filterAndCast(Long.class)));

输出：

Optional.empty
Optional[42]

对于流，您可以通过使用返回空流的函数依赖flatMap（）或多或少地应用相同的技巧：

public static <U> Function<Object, Stream<U>> streamFilterAndCast(Class<? extends U> clazz) {
    return t -> clazz.isInstance(t) ? Stream.of(clazz.cast(t)) : Stream.empty();
    // or alternatively
    return t -> Stream.of(t).filter(clazz::isInstance).map(clazz::cast);
}

并将其用作：

Stream.of(42L, "Hello world", 1024, 3.14)
        .flatMap(streamFilterAndCast(Number.class))
        .forEach(System.out::println);

输出：

42
1024
3.14