Introducing RJava

Posted on March 31, 2011 by Scott Leberknight

You’ve no doubt heard about JRuby, which lets you run Ruby code on the JVM. This is nice, but wouldn’t it be nicer if you could write Java code on a Ruby VM? This would let you take advantage of the power of Ruby 1.9’s new YARV (Yet Another Ruby VM) interpreter while letting you write code in a statically-typed language. Without further ado, I’d like to introduce RJava, which does just that!

RJava lets you write code in Java and run it on a Ruby VM! And you still get the full benefit of the Java compiler to ensure your code is 100% correct. Of course with Java you also get checked exceptions and proper interfaces and abstract classes to ensure compliance with your design. You no longer need to worry about whether an object responds to a random message, because the Java compiler will enforce that it does.

You get all this and more but on the power and flexibility of a Ruby VM. And because Java does not support closures, you are ensured that everything is properly designed since you’ll be able to define interfaces and then implement anonymous inner classes just like you’re used to doing! Even when JDK 8 arrives sometime in the future with lambdas, you can rest assured that they will be statically typed.

As a first example, let’s see how you could filter a collection in RJava to find only the even numbers from one to ten. In Ruby you’d probably write something like this:

evens = (1..10).find_all { |n| n % 2 == 0 }

With RJava, you’d write this:

List<Integer> evens = new ArrayList<Integer>();
for (int i = 1; i <= 10; i++) {
  if (i % 2 == 0) {

This example shows the benefits of declaring variables with specific types, how you can use interfaces (e.g. List in the example) when declaring variables, and shows how you also get the benefits of Java generics to ensure your collections are always type-safe. Without any doubt you know that “evens” is a List containing Integers and that “i” is an int, so you can sleep soundly knowing your code is correct. You can also see Java’s powerful “for” loop at work here, to easily traverse from 1 to 10, inclusive. Finally, you saw how to effectively use Java’s braces to organize code to clearly show blocks, and semi-colons ensure you always know where lines terminate.

I’ve just released RJava on GitHub, so go check it out. Please download RJava today and give it a try and let me know what you think!


(former )

Posted by Luca Bastos on April 01, 2011 at 09:00 AM EDT #

Excellent April 1st joke!

Posted by Dennis Khuy on April 01, 2011 at 10:33 AM EDT #

This one is a few years old and real:

Posted by Mwanji Ezana on April 04, 2011 at 04:53 AM EDT #

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