Cannot access java.lang.String in script run in Rhino

I have problems accessing Java classes in JavaScript. Calling a code snippet

var String = Java.type("java.lang.String");

from Java via javax.script.ScriptEngine, yields follwing Error

Exception in thread "main" javax.script.ScriptException: sun.org.mozilla.javascript.internal.EcmaError: ReferenceError: "Java" is not defined. (path/to/string.js#1) in path/to/string.js at line number 1
    at com.sun.script.javascript.RhinoScriptEngine.eval(RhinoScriptEngine.java:156)
    at main.JsTest.main(JsTest.java:55)

Using non-Java classes in the script works fine, e.g. var value = a + b, where a and b are defined in a javax.script.ScriptContext.

This is the Java class that executes the script.

JsTest.java

public class JsTest
{
  public static void main(String[] args) throws Exception
  {
    ScriptEngineManager sem = new ScriptEngineManager();
    ScriptEngine se = sem.getEngineByExtension("js");

    String script = "path/to/string.js";
    File scriptFile = new File(script);
    FileReader fr = new FileReader(scriptFile);

    se.put(ScriptEngine.FILENAME, script);

    ScriptContext sc = new SimpleScriptContext();
    se.eval(fr, sc);
  }
}