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Java interop errata

There are a few possibilities for improvement on this Scala Swing Example.

Removed unnecessary semi-colons
Inlined import
Removed = in main declaration (strongly advised on functions returning Unit and especially more for main)
Removed a few unnecessary parentheses

import javax.swing._
import java.awt.event.{ActionEvent, 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)
  }
 
  implicit def actionPerformedWrapper(func: (ActionEvent) => Unit) = new ActionListener {
    def actionPerformed(e:ActionEvent) = func(e)
  }
}

This entry was posted on Wednesday, August 13th, 2008 at 9:34 am and is filed under Programming. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

2 Responses to “Java interop errata”

Jeff Says:
August 13th, 2008 at 6:46 pm
Hi Tony

Could you explain what difference the equals sign makes in a function definition and why it is strongly advised to remove when returning Unit.

Thanks
Jeff
Tony Morris Says:
August 15th, 2008 at 8:31 am
Hi Jeff,
When you omit =, the method is guaranteed to be of the type Unit. This puts an additional type-check in place so that you don’t do something like call List.remove as the last statement expecting it to be a side-effect.

It becomes more important for main, since you have no type checks in place at the call site. You’ll just get a runtime error instead (no such method main).

Java interop errata

2 Responses to “Java interop errata”

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