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Here, Timer.schedule() expects a TimerTask as its argument, so Nashorn creates an instance of a TimerTask subclass and uses the passed function to implement its only abstract method, run(). In this usage though, you can't use non-default constructors; the type must be either an interface, or must have a protected or public no-arg constructor.

Java.extend function

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Java.extend function returns a type object for a subclass of the specified Java class (or implementation of the specified interface) that acts as a script-to-Java adapter for it. Note that you can also implement interfaces and subclass abstract classes using new operator on a type object for an interface or abstract class. However, to extend a non-abstract class, you will have to use this method. Example:

var ArrayList = Java.type("java.util.ArrayList")
var ArrayListExtender = Java.extend(ArrayList)
var printSizeInvokedArrayList = new ArrayListExtender() {
    size: function() { print("size invoked!"); }
}

var printAddInvokedArrayList = new ArrayListExtender() {
    add: function(x, y) {
      if(typeof(y) === "undefined") {
          print("add(e) invoked!");
      } else {
          print("add(i, e) invoked!");
      }
    }
}

Java.from function

Given a Java array or Collection, this function returns a JavaScript array with a shallow copy of its contents. Note that in most cases, you can use Java arrays and lists natively in Nashorn; in cases where for some reason you need to have an actual JavaScript native array (e.g. to work with the array comprehensions functions), you will want to use this method. Example:

var File = Java.type("java.io.File")
var listHomeDir = new File("~").listFiles()

// Java array to JavaScript array conversion by Java.from
var jsListHome = Java.from(listHomeDir)

var jpegModifiedDates = jsListHome
    .filter(function(val) { return val.getName().endsWith(".jpg") })
    .map(function(val) { return val.lastModified() })