interface AsyncResult<T> {
    /* 获取异步结果，或者失败原因。倘若失败，那么结果为null */
    T getResult();
    Throwable cause();
    boolean succeeded();
    boolean failed();

    /* 对结果进行转换。如果成功，那么可以将成功的结果转换为另一种形式；否则可以得到一个替代的结果。*/
    <V> AsyncResult<V> map(Function<T, V> mapper);
    <V> AsyncResult<V> map(V value);
    <V> AsyncResult<V> mapEmpty();
    AsyncResult<T> otherwise(T value);
    AsyncResult<T> otherwise(Function<Throwable, T> mapper);
    AsyncResult<T> otherwiseEmpty();
}

interface Future<T>
    extends AsyncResult<T> {
    boolean isComplete();
    Future<T> onComplete(Handler<AsyncResult<T>> handler);
    Future<T> onSuccess(Handler<T> handler);
    Future<T> onFailure(Handler<Throwable> handler);
    <V> Future<V> compose(Function<T, Future<V>> mapper);
    <V> Future<V> compose(Function<T, Future<V>> successMapper, 
                      Function<Throwable, Future<V>> failureMapper);
    Future<T> recover(Function<Throwable, Future<T>> mapper);
    <V> Future<V> transform(Function<AsyncResult<T>, Future<V>> mapper);
    <V> Future<T> eventually(Function<Void, Future<V>> mapper);
    CompletionStage<T> toCompletionState();
}