Java objects behave much like JS objects in Nashorn in that JavaBean properties can be accessed and methods invoked just like JS properties and functions. However, the "in" operator[1] is not supported by Java objects and always evaluates to false.

[1] http://es5.github.io/#x11.8.7

I stumbled over this when investigating a problem running mustache.js with Nashorn reported on nashorn-dev:

http://mail.openjdk.java.net/pipermail/nashorn-dev/2017-January/006810.html

Since we treat java properties and methods like JS properties and functions, it seems we should also support the in operator. This would include the following:

 - instance properties and methods on java objects
 - static properties and methods on java classes
 - map keys on instances of java.util.Map
 - indices for instances of java.util.List and arrays