What parts of a Java application should be written in Scala?

Question

I'm writing a Java application using Struts 2, but now I'd like to make it a hybrid Java & Scala project instead. I don't have much experience with Scala, but I learned Haskell years ago at college -- I really liked the functional programmed paradigm, but of course in class we were only given problems that were supremely suited to a functional solution! In the real world, I think some code is better suited to an imperative style, and I want to continue using Java for that (I know Scala supports imperative syntax, but I'm not ready to go in the direction of a pure Scala project just yet).

In a hybrid project, how does one decide what to code in Java and what to code in Scala?

Answer 1

Ask these questions of your project: "What operations need side-effects?" and "What functionality is already covered well by Java libraries?" Then implement the rest in Scala.

However I would warn that hybrid projects are by their very nature more difficult than stand alone projects as you need to use multiple languages/environments. So given you claim not much experience with Scala I'd recommend playing with some toy projects first, perhaps a subset of your full goal. That will also give you a feel for where the divide should occur.

Answer 2

Two things:

1. 99% of Java code can be expressed in Scala
2. You can write projects that support mixed Java+Scala compilation. Your Scala code can call your Java code and your Java code can call your Scala code. (If you want to do the latter, I suggest defining the interface in Java and then just implementing it in Scala. Otherwise, calling Scala code from Java can get a little ugly.)

So the answer is: whatever parts you want. Your Scala code does not need to be purely functional. Your Scala code can call Java libraries. So pretty much any parts you could write in Java you could also write in Scala.

Now, some more practical considerations. When first trying Scala, some people pick relatively isolated, non-mission-critical parts of their program to write in Scala. Unit tests are a good candidate if you like that approach.

If you're familiar with Java and have learned Haskell in the past, I suggest treating Scala as a "better Java". Fundamentally, Scala compiles to JVM bytecode that is very, very similar to what Java outputs. The only difference is that Scala is more "productive": it produces more bytecode per line of code than Java does. Scala has a lot of things in common with Haskell (first-class functions, for-comprehensions are like Haskell's do-notation, limited type inference), but it's also very different (it's not lazy by default, it's not pure). So you can use some of your insights from Haskell to inspire your Scala style, but "under the hood" it's all Java bytecode.

Answer 3

In the spirit of your question, I recommend you write in Scala any code that involves heavy manipulation of collections, or that handle XML.

Scala's collection library is the foremost functional feature of Scala, and you'll experience great LoC reduction through its usage. Yes, there are Java alternatives, such as Google's collection library, but you asked what you should write in Scala. :-)

Scala also has native handling of XML. You might well find the transition difficult, if you try to take DOM code and make it work on Scala. But if you, instead, try to approach the problem and the Scala perspective and write it from scratch for Scala, you'll have gains.

I'd advise using Actors as well, but I'm not sure how well you can integrate that with Struts 2 code on Java. But if you have concurrent code, give Actors in Scala a thought.

Answer 4

It might sound silly, but why not write your entire project in Scala? It's a wonderful language that is far more expressive than Java while maintaining binary-compatible access to existing Java libraries.