这是什么Java构造`(InputStream Is)>；()>；{}`

我不理解我在这里的文档中看到的这个Java构造.

public static <T,U> HttpResponse.BodySubscriber mapping(HttpResponse.BodySubscriber upstream, Function<? super T,? extends U> mapper)个

我认为我理解但似乎无法复制的部分是这个例子:

public static <W> HttpResponse.BodySubscriber<Supplier<W>> asJSON(Class<W> targetType) {
    HttpResponse.BodySubscriber<InputStream> upstream = HttpResponse.BodySubscribers.ofInputStream();
    HttpResponse.BodySubscriber<Supplier<W>> downstream = HttpResponse.BodySubscribers.mapping(
            upstream,
            (InputStream is) -> () -> {
                try (InputStream stream = is) {
                    ObjectMapper objectMapper = new ObjectMapper();
                    return objectMapper.readValue(stream, targetType);
                } catch (IOException e) {
                    throw new UncheckedIOException(e);
                }
            }
        );

    return downstream;
}

具体地说，(InputStream is) -> () -> {部分.当我读到它的时候，我想它应该是Supplier，提供它所做的事情的Function.然而，当我自己try 编写这样的东西时，它不起作用.

public static void main(String[] args) throws IOException {
    String r = myMethod((String s)->()->{return s + "ddd";});
}

public static String myMethod(Function<String, String> func) {
    return func.apply("Hello");
}

给我的编译错误是Target type of a lambda conversion must be an interface.但是，Function是一个接口.

我有什么不明白的？