Passing Parameters to Scripts - Scripting in Java: Integrating with Groovy and JavaScript - page 41

Return Value of the eval( ) Method
The eval() method of the ScriptEngine returns an Object , which is the last value in the
script. It returns null if there is no last value in the script. It is error-prone, and confusing
at the same time, to depend on the last value in a script. The following snippet of code
shows some examples of using the return value of the eval() method for Nashorn.
The comments in the code indicate the returned value from the eval() method:
Object result = null;
// Assigns 3 to result because the last expression 1 + 2 evaluates to 3
result = engine.eval("1 + 2;");
// Assigns 7 to result because the last expression 3 + 4 evaluates to 7
result = engine.eval("1 + 2; 3 + 4;");
// Assigns 6 to result because the last statement v = 6 evaluates to 6
result = engine.eval("1 + 2; 3 + 4; var v = 5; v = 6;");
// Assigns 7 to result. The last statement "var v = 5" is a
// declaration and it does not evaluate to a value. So, the last
// evaluated value is 3 + 4 (=7).
result = engine.eval("1 + 2; 3 + 4; var v = 5;");
// Assigns null to result because the print() function returns undefined
// that is translated to null in Java.
result = engine.eval("print(1 + 2)");
It is better not to depend on the returned value from the eval() method. You should
pass a Java object to the script as a parameter and let the script store the returned value
of the script in that object. After the eval() method is executed, you can query that Java
object for the returned value. Listing 3-5 contains the code for a Result class that wraps
an integer. You will pass an object of the Result class to the script that will store the
returned value in it. After the script finishes, you can read the integer value stored in the
Result object in your Java code. The Result needs to be declared public so it is accessible
to the script engine. The program in Listing 3-6 shows how to pass a Result object to
a script that populates the Result object with a value. The program contains a throws
clause in the main() method's declaration to keep the code short.