Java to JavaScript (java2js)

java2js can translate simple Java programs to JavaScript and runs them using a JavaScript based JDK.

I made significant improvements to the java-to-javascript transpiler by @wyattades. I also did a modern JS implementation of the fundamental classes in the JDK (Java Development Kit), inspired by java2javascript by @BobHanson, @zhourenjian, @abego, and others.

I created this project so intro level computer science students could write Java code but still use my QuintOS retro game engine library, which is web based.

Java Classes Included in the JS JDK

java.io
File	InputStream	PrintStream

java.lang
Boolean	Byte	Character
Double	Exception	Float
Integer	Long	Short
String	StringBuilder	System
Thread	Throwable

java.security
MessageDigest

java.time
Instant

java.util
AbstractCollection	AbstractList	AbstractMap
AbstractSet	ArrayList	Arrays
Collections	Formatter	HashMap
HashSet	IllegalFormatException	Itr
LinkedList	Random	Scanner
Set	Stack

API

await jdk.init(root)

• root (optional) path to the parent folder of the JS jdk folder, by default it is ./ you could also link to this package online 'https://unpkg.com/java2js'

This function imports much of the standard Java lang classes into the global scope. You must use it before translating or running files.

await jdk.translate(javaFile)

• javaFile is a string with contents of a Java file

returns the Java file translated into JavaScript

jdk.load(translatedJSFile)

• translatedJSFile is the translated JS class to run

Loads the JS class file but doesn't run the main method.

jdk.run(jvmArgs)

Runs the main method with optional JVM arguments.

Features

• imports
• static classes
• static methods
• array literals
• arrays with initial size
• two dimensional arrays
• lambda arrow functions
• triple quotes

Known limitations

• not very good error reporting

• casting to int (truncation) requires parenthesis around the number being cast int x = (int) (Math.random() * 100);

• no support for method overloading, though a workaround might be possible by making a function with the og name route to each of the variations of the overloaded function

• no support for private/public methods yet, though this could be done since they are included in modern JavaScript classes

• no three dimensional arrays

• no third level static classes

• no method can be named .length() because it will be replaced with .length for getting the length of Strings in JS

Contribute

I've only done a partial implementation of the Java 17 JDK in JavaScript, so if you're interested in adding more please go for it and submit a pull request!