How do I use enum in switch statement?

By Wayan in Basic, Core API 0 Comment

This example show you how to use enumeration or enum type in a switch statement.

package org.kodejava.basic;

public enum RainbowColor {
    RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO, VIOLET
}

package org.kodejava.basic;

public class EnumSwitch {
    public static void main(String[] args) {
        RainbowColor color = RainbowColor.INDIGO;

        EnumSwitch es = new EnumSwitch();
        String colorCode = es.getColorCode(color);
        System.out.println("ColorCode = #" + colorCode);
    }

    public String getColorCode(RainbowColor color) {
        String colorCode = "";

        // We use the switch-case statement to get the hex color code of our
        // enum type rainbow colors. We can pass the enum type as expression
        // in the switch. In the case statement we only use the enum named
        // constant excluding its type name.
        switch (color) {
            // We use RED instead of RainbowColor.RED
            case RED -> colorCode = "FF0000";
            case ORANGE -> colorCode = "FFA500";
            case YELLOW -> colorCode = "FFFF00";
            case GREEN -> colorCode = "008000";
            case BLUE -> colorCode = "0000FF";
            case INDIGO -> colorCode = "4B0082";
            case VIOLET -> colorCode = "EE82EE";
        }
        return colorCode;
    }
}

How do I create enumerations type?

By Wayan in Basic, Core API 0 Comment

Enumeration is a list of named constants. Every most commonly used programming languages support this feature. But in Java it is officially supported since version 5.0. In Java programming language an enumeration defines a class type. Because an enumeration is a class it can have a constructors, methods, and instance variables.

To create an enumeration we use the enum keyword. For example below is a simple enumeration that hold a list of notebook producers:

package org.kodejava.basic;

public enum Producer {
    ACER, APPLE, DELL, FUJITSU, LENOVO, TOSHIBA
}

Below we use our enumeration in a simple program.

package org.kodejava.basic;

public class EnumDeclaration {
    public static void main(String[] args) {
        // Creates an enum variable declaration and assign the value to
        // Producer.APPLE.
        Producer producer = Producer.APPLE;

        if (producer == Producer.LENOVO) {
            System.out.println("Produced by Lenovo.");
        } else {
            System.out.println("Produced by others.");
        }
    }
}

The ACER, APPLE, DELL identifiers are called enumeration constants. Every named constants have an implicitly assigned public and static access modifiers. Although the enumeration is a class type, to create an enumeration variable we don’t use the new keyword. We create an enum just like creating a primitive type of data, as you can see in the example above.

How do I import Java package in script?

By Wayan in Scripting 0 Comment

Here you can see how to import a Java class so that you can use the class, creates an instance of it in the scripting environment. We want to print out the current date on the console. For this we need to import the Date class that’s packaged under the java.util package.

In the script we can import the java.util package using the following script let Date = Java.type("java.util.Date").

package org.kodejava.script;

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;
import java.util.function.Predicate;

public class ImportPackageExample {
    public static void main(String[] args) {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByExtension("js");
        Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
        bindings.put("polyglot.js.allowHostAccess", true);
        bindings.put("polyglot.js.allowHostClassLookup", (Predicate<String>) s -> true);

        try {
            engine.eval(getScript());
        } catch (ScriptException e) {
            e.printStackTrace();
        }
    }

    private static String getScript() {
        return """                               
                let Date = Java.type("java.util.Date");
                let today = new Date();
                print(`Today is ${today.toString()}`);
                print(`today is ${today.getClass().getName()}`);
                """;
    }
}

This program prints the following line:

Today is Sun Oct 10 15:15:13 CST 2021
today is java.util.Date

Maven Dependencies

<dependencies>
    <dependency>
        <groupId>org.graalvm.js</groupId>
        <artifactId>js</artifactId>
        <version>22.3.2</version>
    </dependency>
    <dependency>
        <groupId>org.graalvm.js</groupId>
        <artifactId>js-scriptengine</artifactId>
        <version>22.3.2</version>
    </dependency>
</dependencies>

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How do I modify Java object in script?

By Wayan in Scripting 0 Comment

This example show how to modify Java object from scripting environment. Below we manipulate a collection of string data. To pass data into the scripting engine we use a key-value binding to the script engine.

package org.kodejava.script;

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;
import java.util.List;
import java.util.ArrayList;

public class ModifiedJavaObjectFromScript {
    public static void main(String[] args) {
        // Creating an array of five colors
        List<String> colors = new ArrayList<>();
        colors.add("White");
        colors.add("Black");
        colors.add("Red");
        colors.add("Green");
        colors.add("Blue");

        // Obtain a ScriptEngine instance.
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByExtension("js");
        Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
        bindings.put("polyglot.js.allowHostAccess", true);

        // Place the colors list into the engine using colorList key.
        // After passing the list into the engine our script will be
        // able to read it.
        engine.put("colorList", colors);

        try {
            engine.eval(getScript());

            // Redisplay the modified version of colors.
            for (String color : colors) {
                System.out.println("In Java: " + color);
            }
        } catch (ScriptException e) {
            e.printStackTrace();
        }
    }

    private static String getScript() {
        // Below is our script to read the values of Java List that
        // contains string of colors. We also add some other colors
        // to the list object in the script environment.
        return """
                let index;
                let colors = colorList.toArray();

                for (index in colors) {
                    print(`In script: ${colors[index]}`);
                }

                colorList.add("Yellow");
                colorList.add("Purple");
                colorList.add("Orange");
                """;
    }
}

The output of the code snippet above is:

In script: White
In script: Black
In script: Red
In script: Green
In script: Blue
In Java: White
In Java: Black
In Java: Red
In Java: Green
In Java: Blue
In Java: Yellow
In Java: Purple
In Java: Orange

Maven Dependencies

<dependencies>
    <dependency>
        <groupId>org.graalvm.js</groupId>
        <artifactId>js</artifactId>
        <version>22.3.2</version>
    </dependency>
    <dependency>
        <groupId>org.graalvm.js</groupId>
        <artifactId>js-scriptengine</artifactId>
        <version>22.3.2</version>
    </dependency>
</dependencies>

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How do I access Java object from a script?

By Wayan in Scripting 2 Comments

This example demonstrate how you can access Java object from a script. We start by creating the ScriptEngine object. Then we obtain the Bindings object from the engine and set the polyglot.js.allowHostAccess options to true. We do this using the Bindings‘s put(String name, Object value) method.

After that we can put Java object into the script engine by calling the ScriptEngine‘s put(String key, Object value) method. This value can later be read or access by our script. For example, we pass an array of string and a date object for our script to display.

package org.kodejava.script;

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;
import java.util.Date;
import java.util.function.Predicate;

public class AccessJavaObjectFromScript {
    public static void main(String[] args) {
        // Creating an array of five colors
        String[] colors = {"White", "Black", "Red", "Green", "Blue"};
        Date now = new Date();

        // Below is our script to read the values of Java array that
        // contains string of colors and also read the date object.
        String script = """
                let index;
                let colors = colorArray;

                for (index in colors) {
                    print(colors[index]);
                }

                print('----------');
                print('Today is ' + date.toString());
                """;

        // Obtain a ScriptEngine instance.
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByExtension("js");
        Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
        bindings.put("polyglot.js.allowHostAccess", true);
        bindings.put("polyglot.js.allowHostClassLookup", (Predicate<String>) s -> true);

        // Place the colors array into the engine using colorArray key. And
        // the date object using the date key.
        engine.put("colorArray", colors);
        engine.put("date", now);

        try {
            engine.eval(script);
            engine.eval("print(date instanceof Java.type('java.util.Date'))");
        } catch (ScriptException e) {
            e.printStackTrace();
        }
    }
}

Our code will print as follows:

White
Black
Red
Green
Blue
----------
Today is Sun Oct 10 09:32:06 CST 2021
true

Maven Dependencies

<dependencies>
    <dependency>
        <groupId>org.graalvm.js</groupId>
        <artifactId>js</artifactId>
        <version>22.3.2</version>
    </dependency>
    <dependency>
        <groupId>org.graalvm.js</groupId>
        <artifactId>js-scriptengine</artifactId>
        <version>22.3.2</version>
    </dependency>
</dependencies>

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