Nashorn explicitly allows [] operator on java lists and it also supports for…in
on them. ".length" is also supported since 8u20, see
<https://bugs.openjdk.java.net/browse/JDK-8039387>.
We indeed don't implicitly convert Lists into JS Array objects in Nashorn. The
reason is that JS Array has a Java array as backing storage; wrapping a large
list would incur a lot of copying. You can use
"Java.from(someObject.provideList())" to explicitly convert a Java List to a JS
array. We think it's better to provide an explicit conversion API than incur a
linear conversion cost whenever a List object passes into the JS context.
The Java.from created copy is shallow; if you have a List of Lists, the nested
List objects are not converted.
Typically you'll want to create a real Array if you want to use Array
functionality that points beyond [], .length, and for…in, e.g. JS Array
comprehension operations (arguably, we *could* implement even those so that
Array.prototype.forEach.call(javaList, ...) works as expected, but we aren't
there yet.)
Attila.
On Nov 27, 2014, at 10:41 AM, Tim Fox <timvo...@gmail.com> wrote:
> Hello again,
>
> I am a bit confused about how Lists passed from Java into JS are converted.
>
> I have a Java class as follows:
>
> public class SomeClass {
>
> public List<String> provideList() {
> List<String> list = new ArrayList<>();
> list.add("foo");
> list.add("bar");
> return list;
> }
> }
>
> I call this from JavaScript:
>
> var io = Packages.io;
> var someObject = new io.vertx.scratchpad.SomeClass();
>
> var arr = someObject.provideList();
>
> console.log(arr[0]); // prints: foo
> console.log(typeof arr.length); // undefined
> console.log(arr instanceof Array); // false
>
> I was under the impression that Nashorn automatically converted Java lists
> passed into JS into JS Arrays.
>
> The object arr returned in some ways resembles a JavaScript array - the
> operators [] and []= work on it, however it doesn't have the array property
> "length" and it's not an instanceof Array.
>
> Can anyone clarify to me what this object is? Any reason why Nashorn doesn't
> just wrap it as a real JS Array?
>
> Cheers
