Expressive Java & Fluent Interfaces

December 15, 2010

In this article I briefly explore Wealthfront’s extensive usage of expressive and fluent interfaces in Java such as Kawala, Guice, Mockito, and Guava. To the uninitiated, Java invokes parables of moronic XML, obtuse syntax, and Nile-like constructor calls. As a result, Java’s status as a living language is often questioned and it’s “uncoolness” is dogmatically spread. A common characteristic of programming languages and frameworks considered “cool” by the Silicon Valley hacker scene is expressiveness. I will show that modern Java frameworks such as Kawala, Guice, Mockito, and Guava corroborate Java’s adaptability. Java’s ability to give voice to newer, more expressive programming styles (despite political tomfoolery and slow progress on the, new, Java 7 specification) indicates that it will remain a leading choice for applications of any size.
Providing a definition of expressiveness is left as an exercise to the reader. Here, instead of a definition we’ll provide a few representative examples of non-Java expressive programming interfaces.
Node.js is a chief example of an emerging, “trendy” programming framework, with a non-mainstream, following. It capitalizes on Javascript’s conciseness to bring forth prose-like readability. The results are best exemplified by its Hello World example:

var http = require('http');
http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('=Hello Worldn');
}).listen(8124, "");

Similarly, RSpec is a well-respected (and trendy) behavior-driven development tool for Ruby that integrates aspects of Test-Driven Development (which we reverently practice here at Wealthfront). The fact that its “Get Started Now” example is amply self-descriptive and can be presented with no further commentary speaks to its expressiveness:

require 'bowling'
describe Bowling, "#score" do
it "returns 0 for all gutter game" do
bowling =
     20.times { bowling.hit(0) }
bowling.score.should == 0

In both cases, syntax lends itself to form a localized miniature domain-specific language to effectively express the concern at hand, and nothing further.

At Wealthfront, we believe that unit tests serve as self-correcting alive documentation that is guaranteed to evolve along with the code it accompanies. Describing the contract and testing the implementation at one pass is easy with Mockito‘s highly expressive interface. Here’s an example for the imaginary NasdaqMarket class. The expectation is that NasdaqMarket will disallow scheduling trades to be executed during non-trading hours:
import static org.mokcito.Mockito.*;
TradingSchedule schedule = mock(TradingSchedule.class); 
final DateTime weekend = [...]; 
NasdaqMarket m = NasdaqMarket.withTradingSchedule(schedule).build(); 
assertFalse(m.canScheduleTrade(weekend, SELL, 100, NASDAQ_WLTH)); 
Mockito along with a builder pattern for NasdaqMarket make this test extremely readable and concise. The contract defined in this test is unambiguous (as guaranteed by your friendly Java compiler): Given a DateTime that trading is disallowed, NasdaqMarket will refuse to schedule a trade. Furthermore, it’s guaranteed to propose to the Schedule class that said DateTime is allowed.
A similarly expressive Java framework is Guice. Guice allows the clear separation of behavior from dependency resolution. “A [Guice] module is a collection of bindings specified using fluent, English-like method calls” (Guice, Getting Started). Indeed, the binding specification interface speaks for itself:
bind(Integer.class).annotatedWith(named("login timeout seconds")).toInstance(10);

Requesting that a constructor receives its dependencies via dependency injection is as easy as specifying the @Inject annotation. Wealthfront’s entire backend application stack, processing immense amounts of market data daily, relies heavily on Guice for dependency resolution and configuration.

Guava allows the easy construction of type-safe ImmutableLists with a quick static call such as List s = ImmutableList.of(“A”, “B”, “C”). Guava includes a variety of functional methods to operate on its collections. For example, one could easily filter a list of integers based on parity:

List<integer> x = Iterables.filter(input, new Predicate<integer>() {
public boolean apply(Integer input) {return (input % 2) == 0;}});
Our very own Kawala, provides a variety of fluent and concise utility interfaces that should make your Java look like an avid reader of Vogue. Take a look at our previous blog post on the LessIOSecurityManager, to ensure awareness of hidden I/O operations in your application, and note its fluent annotation-based interface.
Martin Fowler’s discussion of Fluent Interfaces provides a few more examples and the discussion continued on Piers Cawley’s blog. Although the idea of fluent and expressive interfaces in Java has been around for more than half a decade, it’s only catching up now, after the “re-discovery” of beautifully designed interfaces in emerging frameworks. I believe it’s the duty of every Java coder, engineer, and architect, to embrace fluent and expressive interfaces in an effort to rid Java of the (now obsolete) rumors about obtuse interfaces and unintuitive design.