Java to JavaScript (java2js)

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

Try out the demo.html and ide.js, a barebones implementation.

Java classes included in the java2js 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

Features

The java2js transpiler supports:

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

API

await jdk.init(root)

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

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

await jdk.transpile(javaFile)

Translates a Java class and loads it.

• javaFile is a String with a Java class in it

returns a String with the JavaScript transpilation of the Java class

jdk.run(jvmArgs)

Runs the main method with optional JVM arguments.

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

Why did you make this?

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

Credits

This project builds upon the java-to-javascript transpiler by @wyattades. I also did a modern JS implementation of the fundamental classes in the JDK (Java Development Kit), which I got the inspiration to do from java2javascript by @BobHanson, @zhourenjian, @abego, and others.

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!