Using Java in Scripting Languages - Scripting in Java: Integrating with Groovy and JavaScript - page 190

JavaScript does not import all classes from the java.lang package automatically
because JavaScript classes with the same names, for example, String , Object , Numbe r,
and so on, will conflict with class names in the java.lang package. To use a class from
the java.lang package, you can import it or use the Packages or Java global object to use
its fully qualified name. You cannot import all classes from the java.lang package. The
following snippet of code generates an error because the String class name is already
defined in JavaScript:
// Load the compatibility module. It is needed in Nashorn, not in Rhino.
load("nashorn:mozilla_compat.js");
// Will cause a conflict with String object in Nashorn
importClass(java.lang.String);
If you want to use the java.lang.String class, you need to use its fully qualified
name. The following snippet of code uses the built-in JavaScript String class and the
java.lang.String class:
var javaStr = new java.lang.String("Hello"); // Java String class
var jsStr = new String("Hello"); // JavaScript String class
If a class name in the java.lang package does not conflict with a JavaScript top-
level class name, you can use the importClass() function to import the Java class. For
example, you can use the following snippet of code to use the java.lang.System class:
// Load the compatibility module. It is needed in Nashorn, not in Rhino.
load("nashorn:mozilla_compat.js");
importClass(java.lang.System);
var jsStr = new String("Hello");
System.out.println(jsStr);
In this snippet of code, jsStr is a JavaScript String that has been passed to the
System.out.println() Java method that accepts a java.lang.String type. JavaScript
takes care of the conversion from a JavaScript type to a Java type automatically in
such cases.