Hi Michael,
> > Am 11.05.2016 um 21:35 schrieb Uwe Schindler <uschind...@apache.org>:
> > With Java 9 this gets a bit worse: There is no "easy way" with the
> MethodHanldes class to generate a MethodHandles.countedLoop() on all
> elements of an array:
> >
> > public static MethodHandle countedLoop(MethodHandle iterations,
> MethodHandle init, MethodHandle body) [new in Java 9]
>
> this isn't a remedy when you need the index in each iteration, but you can
> generate a loop over all elements of an array using iteratedLoop(), as
> illustrated by this test from LoopCombinatorTest:
>
> @Test
> public static void testIterateSum() throws Throwable {
> // Integer[] a = new Integer[]{1,2,3,4,5,6}; int sum = 0; for (int e : a)
> { sum
> += e; } return sum; => 21
> MethodHandle loop =
> MethodHandles.iteratedLoop(Iterate.MH_sumIterator, Iterate.MH_sumInit,
> Iterate.MH_sumStep);
> assertEquals(Iterate.MT_sum, loop.type());
> assertEquals(21, loop.invoke(new Integer[]{1, 2, 3, 4, 5, 6}));
> }
>
> ... where MH_sumIterator is a handle to this method:
>
> static Iterator<Integer> sumIterator(Integer[] a) {
> return Arrays.asList(a).iterator();
> }
Of course this works, too.
My proposal or question here was more about the API inconsistency in general.
You can do a lot here and there, you can implement effective or non-effective
loops with MethodHandles, but still the following holds true:
java.lang.invoke.MethodHandles is missing an accessor for "array.length", which
is not understandable. The "countedLoop" here was just an example use case (if
that one is great or not does not matter), it was just posted to actually show
the API and how it *could* be used.
The real example from our script engine was not related to loops, it was an
actual invokedynamic callsite implementation where it was not possible to
figure out how to get a method handle to the very special arraylength byte
code, aka "array.length javac sugar". Everything else to handle arrays is there
at one single place, but the array length is missing and you get very annoyed.
If you are not so familiar to bytecode and the mechanics behind, you have a
very hard time to find out how to do this: the intuitive solutions does not
work: "array.length" -> aha it’s a field, so let's try
Lookup.findGetter(long[].class, "length", int.class) -> does not work!
What is so hard to accept the proposal to have MethodHandles.arrayLengthGetter,
implemented in the most effective way (as a methodhandle that solely calls the
special bytecode "arraylength")? In fact thais is like a 10 liner to implement
+ Javadocs. No need to delay a release, there are still changes in JDK 9 going
on that are far more risky than this.
Best,
Uwe
> Best,
>
> Michael
>
> --
>
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