how to get the array output from a js function in java 8?

Question

I have the following method in the file test.js:

function avg(input, period) {
var output = []; 
if (input === undefined) {
      return output;
} 
var i,j=0;
 for (i = 0; i < input.length- period; i++) {
    var sum =0;
    for (j = 0; j < period; j++) {
        //print (i+j)
        sum =sum + input[i+j];
        }
    //print (sum + " -- " + sum/period)
    output[i]=sum/period;        
}

 return output;
 }

I want to pass an array from java to this function and to get the js output array in java. I used the following java code:

double[] srcC = new double[] { 1.141, 1.12, 1.331, 1.44, 1.751, 1.66, 1.971, 1.88, 1.191, 1.101 };

    try {
        ScriptEngineManager factory = new ScriptEngineManager();
        ScriptEngine engine = factory.getEngineByName("nashorn");

        String location = "test.js";
        engine.eval("load(\"" + location + "\");");
        Invocable invocable = (Invocable) engine;
        // double[] a = (double[]) invocable.invokeFunction("avg", srcC, 2);

        System.out.println("obj " + invocable.invokeFunction("avg", srcC, 2));
    } catch (Exception ex) {
        LOGGER.error(ex.getLocalizedMessage());
    }

I am able to see the output of the avg js function but I do not know how to get the js output arrays from the js avg function in java

Any support is appreciated.

Best Regards, Aurelian

Alnitak · Accepted Answer · 2014-09-03 10:51:35Z

7

The return type from Invocable.invokeFunction is implementation defined. The Nashorn scripting engine returns an instance of an object that implements jdk.nashorn.api.scripting.JSObject

This interface has a method Collection<Object> values(), so the only required changes are to cast the result of invokeFunction and then extract the collection of values:

 JSObject obj = (JSObject)invocable.invokeFunction("avg", srcC, 2);
 Collection result = obj.values();
 for (Object o : result) {
      System.out.println(o);
 }

output:

1.1305
1.2255
1.3855
1.5955
1.7054999999999998
1.8155000000000001
1.9255
1.5354999999999999

answered Sep 3, 2014 at 10:51

Alnitak

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1 Comment

aurelianr Over a year ago

I just saw your answer. Is better then mine. thanks alot

aurelianr · Accepted Answer · 2014-09-03 12:43:52Z

2

Thanks for your replies. The following code is better because uses the java 8 js engine instead of rhino from previous java versions:

    double[] output = {};
    try {
        ScriptEngineManager factory = new ScriptEngineManager();
        ScriptEngine engine = factory.getEngineByName("nashorn");

        String locationTemp = "D:/test.js";
        engine.eval("load(\"" + locationTemp + "\");");
        Invocable invocable = (Invocable) engine;

        ScriptObjectMirror obj = (ScriptObjectMirror) invocable.invokeFunction("avg",
        input, period);

        Collection<Object> values = obj.values();
        if (values.size() == 0) {
            return output;
        }
        output = new double[values.size()];
        int i = 0;
        for (Iterator<Object> iterator = values.iterator(); iterator.hasNext();) {
            Object value = iterator.next();
            if (value instanceof Double) {
                Double object = (Double) iterator.next();
                output[i] = object;
            }
        }

    } catch (NullPointerException | NoSuchMethodException | ScriptException ex) {
        log.error(ex.getLocalizedMessage());
    }

Thanks ninad for his quickly answer

Best Regards, Aurelian,

answered Sep 3, 2014 at 12:43

aurelianr

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2 Comments

Alnitak Over a year ago

You have Java 8 - no need to use an explicit interator, just use for (Object value: values) { ... } as I did in my answer. Also, you forgot to increment i ... ;-)

aurelianr Over a year ago

indeed... i observed that i forgot the increment. Thanks again for your answer.

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