Java to JavaScript

Convert Java Classes to ES6 Classes.

Translates: classes, methods, variables, statics, and more!

(Originally created for converting Processing to p5.js)

🔗 Live Editor

Example

Input Java:

class MyClass {
  int x = 42;
  static String y = "Life";
  
  MyClass(String secret) {
    final String result = MyClass.y + secret + x;
    if (result != null) {
      purpose();
    }
  }
}

Output JavaScript:

class MyClass {
  constructor(secret) {
    this.x = 42;

    const result = MyClass.y + secret + this.x;
    if (result !== null) {
      purpose();
    }
  }
}
MyClass.y = 'Life';

More Examples...

Install

NPM

$ npm install java-to-javascript

Script tag

<script src="https://unpkg.com/java-to-javascript@latest/build/java-to-javascript.min.js"></script>

Note: You can replace latest with the library's version you want to use, e.g. v0.1.10. The library will be exposed on the global window scope as javaToJavascript.

Module API (For Node and the browser)

javaToJavascript(javaString, options?, progress?)

Returns: string - - Converted JavaScript

// Node module:
var javaToJavascript = require('java-to-javascript');
// Browser script:
var javaToJavascript = window.javaToJavascript;

var jsString = javaToJavascript( /* params */ );

Param	Type	Description
javaString	string	Java file contents
[options]	object
[options.globalVars]	object	Object keys are added to the globalScope object. If the value is a string, the variable is renamed to that string
[options.globalScope]	string	If specified, variables in globalVars are appended to globalScope object
[options.p5]	boolean	Sets globalScope to 'p5', adds p5 variable mappings to globalVars, and allows for global methods and variables
[options.ugly]	boolean	Don't beautify JavaScript code
[progress]	function	Callback on progress of conversion. Args are progress value (0.0 to 1.0), and a message string

Command Line API

Usage: java-to-javascript [options] <input_file>

Options:

  -V, --version         output the version number
  -o, --output <file>   Specifies the output filename. (Default is the input filename with a .js extension)
  -s, --scope <scope>   If specified, variables in `globals` are appended to `scope` object
  -g, --globals <file>  JSON or JavaScript file containing a global variable mapping. See README
  --p5                  Sets `scope` to "p5", adds p5 variable mappings to `globals`, and allows for global methods and variables
  --ugly                Don't beautify JavaScript code
  -h, --help            output usage information

How It Works

• Parse Java code to create an AST (abstract syntax tree)
• Replace global variables that the user specifies
• Generate JavaScript code from the AST
• Beautify JavaScript

Polyfills

I've included some Java Class (partial) polyfills in polyfills.js that help in the conversion of Java to JS.

Included Polyfills: List (alias ArrayList), Map (alias HashMap)

BUGS!

• Local variables or method parameters with the same name as a variable in their class will incorrectly be assigned to the this object.

  Example

  Input Java:

  class Thing {
    int x, y;
    myMethod(int x) {
      int y = 20;
      this.x = x + y;
    }
  }

  Output JavaScript:

  class Thing {
    myMethod(x) {
      let y = 20;
      this.x = this.x + this.y;
    }
  }

• Unnecessary or deeply nested parentheses can cause very long or infinite process hanging.

  Example

  /* I don't know the exact cause, but here's what I have found: */
  ((testFunc())); // Normal
  (((testFunc()))); // Long parse time
  ((((testFunc())))); // Infinitely hangs

• I convert chars to numbers e.g. 'W' -> 87, which can cause discrepences in your code

Unsupported

• Nested classes aka anonymous classes
• Synchronized methods and statements
• Interfaces and abstract classes
• Enums
• Packages and imports

DISCLAIMER

This is a work in progress! Not all Java features are supported, and some are too difficult to translate to JS, so make sure to doublecheck the resulting code (Also, it assumes you are passing in valid Java).

Please report bugs to Github Issues!

CREDITS

• Java lexical parser: Github user mazko (https://github.com/mazko/jsjavaparser/blob/master/src/Java.1.7.pegjs)
• Lexical Parser -> JS: peg.js