// Obtain an instance of JavaScript engine
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");

// Define a global variable
engine.put("textToPrint", "Data defined in Java.");

// Print the global variable
try {
    engine.eval("print(textToPrint);");
} catch (ScriptException ex) {
    ex.printStackTrace();
}

// Outcome:
// 'Data defined in Java.' printed on standard output

// Obtain an instance of JavaScript engine
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");

// Execute an hardcoded script
try {
    engine.eval("print('Hello Nashorn!');");
} catch (ScriptException ex) {
    // This is the generic Exception subclass for the Scripting API
    ex.printStackTrace();
}

// Outcome:
// 'Hello Nashorn!' printed on standard output

// Required imports
import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;
import java.io.FileReader;
import java.io.FileNotFoundException;

// Obtain an instance of the JavaScript engine
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");

// Load and execute a script from the file 'demo.js'
try {
    engine.eval(new FileReader("demo.js"));
} catch (FileNotFoundException ex) {
    ex.printStackTrace();
} catch (ScriptException ex) {
    // This is the generic Exception subclass for the Scripting API
    ex.printStackTrace();
}

// Outcome:
// 'Script from file!' printed on standard output

demo.js:

print('Script from file!');

// Obtain an instance of JavaScript engine
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");

// Setup a custom writer
StringWriter stringWriter = new StringWriter();
// Modify the engine context so that the custom writer is now the default
// output writer of the engine
engine.getContext().setWriter(stringWriter);

// Execute some script
try {
    engine.eval("print('Redirected text!');");
} catch (ScriptException ex) {
    ex.printStackTrace();
}

// Outcome:
// Nothing printed on standard output, but
// stringWriter.toString() contains 'Redirected text!'

// Obtain an instance of JavaScript engine
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");

//String to be evaluated
String str = "3+2*4+5";
//Value after doing Arithmetic operation with operator precedence will be 16

//Printing the value
try {
    System.out.println(engine.eval(str));
} catch (ScriptException ex) {
    ex.printStackTrace();
}

//Outcome:
//Value of the string after arithmetic evaluation is printed on standard output.
//In this case '16.0' will be printed on standard output.

It's possible to pass Java objects to Nashorn engine to be processed in Java code. At the same time, there are some JavaScript (and Nashorn) specific constructions, and it's not always clear how they work with java objects.

Below there is a table which describes behaviour of native Java objects inside JavaScript constructions.

Tested constructions:

1. Expression in if clause. In JS expression in if clause doesn't have to be boolean unlike Java. It's evaluated as false for so called falsy values (null, undefined, 0, empty strings etc)
2. for each statement Nashorn has a special kind of loop - for each - which can iterate over different JS and Java object.
3. Getting object size. In JS objects have a property length, which returns size of an array or a string.

Results:

Type	If	for each	.length
Java null	false	No iterations	Exception
Java empty string	false	No iterations	0
Java string	true	Iterates over string characters	Length of the string
Java Integer/Long	value != 0	No iterations	undefined
Java ArrayList	true	Iterates over elements	Length of the list
Java HashMap	true	Iterates over values	null
Java HashSet	true	Iterates over items	undefined

Recommendatons:

• It's advisable to use if (some_string) to check if a string is not null and not empty
• for each can be safely used to iterate over any collection, and it doesn't raise exceptions if the collection is not iterable, null or undefined
• Before getting length of an object it must be checked for null or undefined (the same is true for any attempt of calling a method or getting a property of Java object)

import java.io.FileReader;
import java.io.IOException;

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class InterfaceImplementationExample {
    public static interface Pet {
        public void eat();
    }

    public static void main(String[] args) throws IOException {
        // Obtain an instance of JavaScript engine
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        try {
            //evaluate a script
            /* pet.js */
            /*
                var Pet = Java.type("InterfaceImplementationExample.Pet");
                
                new Pet() {
                    eat: function() { print("eat"); }
                }            
            */
            
            Pet pet = (Pet) engine.eval(new FileReader("pet.js"));
            
            pet.eat();
        } catch (ScriptException ex) {
            ex.printStackTrace();
        }

        // Outcome:
        // 'eat' printed on standard output
    }
}

// Obtain an instance of JavaScript engine
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");

try {
    // Set value in the global name space of the engine
    engine.put("name","Nashorn");
    // Execute an hardcoded script
    engine.eval("var value='Hello '+name+'!';");
    // Get value
    String value=(String)engine.get("value");
    System.out.println(value);
} catch (ScriptException ex) {
    // This is the generic Exception subclass for the Scripting API
    ex.printStackTrace();
}

// Outcome:
// 'Hello Nashorn!' printed on standard output