@UnstableApi
    @SuppressWarnings("unchecked")
    public <U> FutureTransformingRequestPreparation<U> as(
            ResponseAs<? super T, ? extends U> responseAs) {
        requireNonNull(responseAs, "responseAs");
        this.responseAs = (FutureResponseAs<T>) this.responseAs.map(responseAs::as);
        return (FutureTransformingRequestPreparation<U>) this;
    }
Sets a {@link ResponseAs} that converts the {@code T} type object into another.
core/src/main/java/com/linecorp/armeria/client/FutureTransformingRequestPreparation.java