Why do I have to convert a List to an Array to use forEach?

medini_1024169437

Well, actually I have asked myself that questin a few times, simply because the java.util.List interface offers a forEach function with a similar signature as JS. Any way it does not work in medini JS and never did.

Now with 2024 R2 we have upgraded the Rhino engine version and I had the hope that this has improved so I had a closer look at it and would like to share my findings.

So basically this is how we did it in the past, we simply converted the List to an Array. Now with the new Rhino we can use the "Arrow Function": 

var scope = finder.project;
var list = scope.nestedPackages;

function log(p) {
	console.log(p.name);
}

// works now with arrow function
list.toArray().forEach(p => log(p));

Works nicely now lets try to apply this to the List directly:

var scope = finder.project;
var list = scope.nestedPackages;

function log(p) {
	console.log(p.name);
}

// still does not work for Collections
list.forEach(p => log(p));

This results in "Cannot convert [...] ArrowFunction to [...] Consumer". So we probabaly hit a problem with the new => style. Hm, next I tried to go back to classic inline function:

var scope = finder.project;
var list = scope.nestedPackages;

function log(p) {
	console.log(p.name);
}

// even this does not work
list.forEach(log);

Not working  nut gives a different error: "Cannot convert function to interface [...] Consumer since it contains methods with different names. Aha, that gave the final hint: java.util.Consumer has an "andThen" fucntino since a while with a default implementation. Rhino doe snot like that. Anyway, with this little wrapper it finally worked:

var scope = finder.project;
var list = scope.nestedPackages;

function log(p) {
	console.log(p.name);
}

// the trick
function toConsumer(f) {
	return {
		accept: f,
		andThen: function(after) {
			throw "not implemented";
		}
	};
}

// works!
list.forEach(toConsumer(log));

So bottom line is that List.forEach is still not the same as Array.forEach but with a little trick we come closer.

Happy Scripting

Jan

medini_1063548292

 

Hi jan,

I will add my tiny piece of my mind;

So I have been also exploring this topic 2 years ago. I have actually made a conclusion but didn't share it in here because I though that the problem was on my side.

But to extended your idea.

It is not only about forEach. but there are also other useful cases where some of the methods accept argument that is supposed be to implementation of Interface

My conclusion was that when it does not accepts the plain Function object It signifies that it actually has more methods coming from the Interface object.

So it actually requires an Object implementing given interface, but not all methods are needed of course good habit is to check the actual Interface classes to understand the behavior and requirements of individual methods.

So that is also a reason why your example worked with the Object implementing Consumer Class.

Example

 =>

Consumer

    void accept(x);

    Consumer andThen(x);

used in List.forEach

 ->

//Predicate https://docs.oracle.com/javase/8/docs/api/java/util/function/Predicate.html

    boolean test(x);

    Predicate negate();

    Predicate or(Predicate);

    Predicate and(Predicate);

used in List.removeIf(Predicate)

or some well-now methods of the Stream:

List.stream().filter(Predicate).anyMatch(Predicate).mapToInt(ToIntFunction).sum();

  

//count the number of contained elements in all project Packages except of FMEA named package
finder.findByType(Metamodel.projectmodel.PJPackage).asList()
 .stream()
 .filter({
 test:function(obj){return obj.name != 'FMEA';}
 })
 .mapToInt({
 applyAsInt:function (a){return a.containedElements.size()}
 })
 .sum();

//collect filetred project redirect packages except of FMEA named
var streamResults = finder.scope.nestedPackages.stream()
 .filter({
 test:function(obj){return obj.name != 'FMEA';}
 })
 .collect( java.util.stream.Collectors.toList());

  

So the rule of thumb if your method will have issue with the plain Function object as an argument try to check and implement the Interface required for the given argument.

Happy Consuming,

Marian