Java.next #2: Java Interop

This is Part Two of a series of articles on Java.next. In Part Two, I will look at how Java.next languages interoperate with Java.

Java interop is trivial in all of the Java.next languages. We have Java itself to thank for this--the Java Virtual Machine Specification makes it easy for other languages to reflect against and call Java code.

A Swing example

As a first example, consider calling into the Java Swing API to create an application [1] that has

a frame
button
a button handler that responds with a model dialog

For starters, here is the application in plain old Java:

// Java
import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JOptionPane;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class Swing {
  public static void main(String[] args) {
    JFrame frame = new JFrame("Hello Swing");
    JButton button = new JButton("Click Me");

    button.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent event) {
        JOptionPane.showMessageDialog(null,
            String.format("<html>Hello from <b>Java</b><br/>" +
                          "Button %s pressed", event.getActionCommand()));
      }
    });
    frame.getContentPane().add(button);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
  }
}

Below, I will present the same Swing application, ported to the Java.next languages. Please take note of two things about these examples:

For this post, I am presenting the languages in order of increasing syntactic distance from Java. This makes sense for porting a simple example from the well-known to the increasingly unfamiliar.
The ports below are not best practice in the Java.next languages. They are deliberately simplistic, so that I can focus on Java interop. In later installments of this series I will show more idiomatic Java.next code.

Groovy Swing example

Groovy is the Java.next language that looks most like Java. Here is the same example in Groovy:

// Groovy
import javax.swing.JFrame
import javax.swing.JButton
import javax.swing.JOptionPane
import java.awt.event.ActionListener

frame = new JFrame("Hello Swing")
button = new JButton("Click Me")

button.addActionListener({
  JOptionPane.showMessageDialog(null, """<html>Hello from <b>Groovy</b>
Button ${it.actionCommand} pressed""")
} as ActionListener) 
frame.contentPane.add button

frame.defaultCloseOperation = JFrame.EXIT_ON_CLOSE
frame.pack()
frame.visible = true

If you compare this to the Java example, it is almost the same code, minus a bunch of unnecessary ceremony. The Groovy version lets us omit:

semicolons
type declarations
most parentheses
get and set for property access

The most important benefit, however, comes in the action listener. The Groovy version sports

a multiline string (delimited by """)
string interpolation of it.actionCommand (inside ${})
no need to write an anonymous inner class, simply pass an anonymous function

Since this post is about Java interop I will state the obvious: From Groovy, Java interop is entirely trivial.

Scala Swing example

Next, let's look at the Scala version:

// Scala (almost right, see below)
import javax.swing._; 
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

object HelloWorld extends JFrame("Hello Swing") {
  def showButtonMessage(msg: String)  =       
    JOptionPane.showMessageDialog(null, String.format("""<html>Hello from <b>Scala</b>.
Button %s pressed""", Array(msg)));

  def main(args: Array[String]) =
  {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
    val button = new JButton("Click Me")
    button.addActionListener((e:ActionEvent) => 
      showButtonMessage(e.getActionCommand.toString))
    getContentPane.add(button)
    pack;
    setVisible(true);
  }
}

The Scala version offers many of the same advantages over Java that the Groovy version provided:

fewer type declarations than Java
fewer semicolons
fewer parentheses

We also see a few items unique to Scala:

In Scala, the import wildcard is _, not *. In Scala, * is a valid identifier. (Scala's punctuation-friendly identifiers will be a big advantage later when I am writing DSLs.)
Since we only need one, we declare an object instead of a class.
Our object extends JFrame, and Scala lets us call the JFrame constructor inline, instead of having to declare a separate constructor.

Again, the most important differences are in the action listener. Like Groovy, Scala lets us skip the anonymous inner class ritual, and simply pass a function:

button.addActionListener((e:ActionEvent) => 
  showButtonMessage(e.getActionCommand.toString))

That looks great, except I cheated a little. Scala's implementation of strong typing won't automatically coerce a function into an ActionListener, so the above code won't compile out of the box. Fortunately, Scala's implicit conversions let us have our cake and eat it too: strong typing plus much of the syntactic convenience of a looser type system. All we have to do is tell Scala the the conversion is legal:

// Yes, we can
implicit def actionPerformedWrapper(func: (ActionEvent) => Unit) = 
  new ActionListener { def actionPerformed(e:ActionEvent) = func(e) }

With this one-time setup in place, we can now pass a function where an ActionListener is expected.

There seem to be several projects to wrap Swing in more idiomatic Scala. Using one of these libraries you should be able to get a syntax cleaner than the sample code here. See ScalaGUI for one example.

From Scala, Java interop is trivial.

JRuby Swing example

Let's see how JRuby fares:

include Java
import javax.swing.JFrame
import javax.swing.JButton
import javax.swing.JOptionPane
import java.awt.event.ActionListener

button = JButton.new "Click Me"
button.add_action_listener(ActionListener.impl { |m,evt|
  JOptionPane.showMessageDialog(nil, <<-END)
<html>Hello from <b>JRuby</b>. 
Button '#{evt.getActionCommand()}' clicked.
END
})

frame = JFrame.new "Hello Swing"
frame.content_pane.add button
frame.default_close_operation = JFrame::EXIT_ON_CLOSE
frame.pack
frame.visible = true

If you compare this to the earlier Groovy example, you will see almost exactly the same feature set:

fewer type declarations
fewer semicolons
fewer parentheses
simplified property access (no get or set)
a multiline string (delimited by END)
string interpolation of evt.getActionCommand (the stuff inside #{})

The action listener callback is simplified, but in a slightly different way. For implementing a Java interface, JRuby provides the impl method, which implements an interface and passes all methods to a block:

button.add_action_listener(ActionListener.impl { |method,evt|
  # do stuff
END

This is more verbose than the Groovy technique, because it is more general: the unused method parameter allows for interfaces with more than one method.

Does this mean I am stuck with a verbose, general approach that is overkill for my needs? Absolutely not. In Java.next, you bend languages to your will (not the other way around). So, if you want JRuby to support a more Groovy-like syntax, roll it yourself! (See the Profligacy project for an example.) Ruby's flexibility has encouraged a lot of experimentation with alternate syntaxes for Java interop. See JRUBY-903 for some of the history.

In the JRuby example I used Ruby conventions for method names, even on Java objects:

# Ruby
frame.content_pane

Java programmers expect camel case. As a convenience, JRuby supports both naming conventions:

# Groovy, Scala, or JRuby
frame.contentPane

From JRuby, Java interop is trivial.

Clojure Swing example

Here is the Clojure version:

; Clojure 
(import '(javax.swing JFrame JButton JOptionPane))
(import '(java.awt.event ActionListener))

(let [frame (new JFrame "Hello Swing")
      button (new JButton "Click Me")]
  (.addActionListener 
   button
   (proxy [ActionListener] []
     (actionPerformed 
      [evt]
      (.showMessageDialog 
       JOptionPane 
       nil, 
       (str "<html>Hello from <b>Clojure</b>.<br/> Button " (.getActionCommand evt) " clicked.")))))

  (.. frame (getContentPane) (add button))
  (doto frame
    (setDefaultCloseOperation (. JFrame EXIT_ON_CLOSE))
    (pack)
    (setVisible true)))

Because Clojure is a Lisp, the syntax is radically different from the others. This deserves hours of discussion, or none. Since my focus here is on Java interop, I am going to save The Great Parenthesis Debate for a later entry in this series. For now, let us suspend judgment on syntax, and focus exclusively on the Java interop.

Importing Java classes is easy. import takes a list. The first element of the list is a package, and the remaining elements are classes to add to the current namespace. Note that this allows the import of multiple classes in a single line.

(import '(javax.swing JFrame JButton JOptionPane))

Creating a Java instance is easy. Use the (new class &args) form.

(new JFrame "Hello Swing")

There are multiple ways to call methods on a Java class. If you want to call a single method, you can use the (.methodName obj &args) form. obj can be a class or an instance.

(.showMessageDialog JOptionPane nil "A message")

Sometimes you want to chain multiple calls together. Where in Java you would say x.y().z(), in Clojure you can use the (.. x (y) (z)) form.

(.. frame (getContentPane) (add button))

The last three method calls in our example are all on the same frame object. With Clojure's doto form, you can perform multiple operations on an object without having to repeat the object each time.

(doto frame
  (setDefaultCloseOperation (. JFrame EXIT_ON_CLOSE))
  (pack)
  (setVisible true)))

As with the other examples, the action listener is the most interesting part. In Clojure, proxy will dynamically create a Java instance [2], allowing you to implement interfaces and methods as needed.

(proxy [ActionListener] []
  (actionPerformed [evt] {do stuff here...}))

As with JRuby, this solution is more general, and requires more syntax, than the Groovy approach. Also as with JRuby, you can easily roll your own syntax.

From Clojure, Java interop is trivial.

Try this at home

The interop story in Java.next is almost boring: It Just Works. So to spice things up a little, here is an exercise in rolling your own constructs, inspired by the examples above. Consider Clojure's import, which can import multiple Java classes in a single line of code.

(import '(javax.swing JFrame JButton JOptionPane))

Why can't this be even more general? Try your hand at writing a custom import function in one of the Java.next languages. Some useful features might be

import all the classes in a JAR
import all the classes in the intersection of a package and a JAR
import only interfaces
import all classes macthing some criteria
import all classes except those matching some criteria

Let me know what you come up with, and I will link to it here.

Conclusion

In the examples above, I have demonstrated how all of the Java.next libraries can trivially interoperate with Java. Each of examples called the Swing library with fewer lines of code than the Java version. More importantly, the Java.next versions capture the essence of the program with less ceremony.

Seamless interoperation with Java should not be the primary yardstick when measuring Java.next languages, because they all get it right. There are complexities and corner cases beyond what I have shown here, in all of the Java.next languages. But I consider the Java interop problem to be basically solved.

In these first two articles, I have stayed fairly close to Java style while demonstrating Java.next language features. With that groundwork in place, it is time to start using idiomatic Java.next. In the next installment of the Java.next series, we will look at how the Java.next languages support Domain-Specific Languages.

Notes

This series is taken from the JVM Language Shootout talk. Check the schedule for a talk near you.
Suggestions for improving the code samples above are most welcome.
Thanks to Jason Rudolph, Glenn Vanderburg, and Greg Vaughn for reading an earlier draft of this article.

Footnotes

I took the Swing application example from the JRuby samples, and ported it to the other Java.next languages.
Clojure's proxy creates classes as necessary behind the scenes. In Java.next, the dichotomy of class and object is not constantly center stage.

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