Scala syntax bug?

Question

I have encountered with something I think is a bug/no-syntax-sense in scala syntax but I prefer to post it here after reporting it as I might be wrong.

The controller class:

package some

import java.awt.event.ActionListener
import java.awt.event.ActionEvent

class ItemController extends ActionListener {

  val NEW_ITEM = "new item"
  val UPDATE_ITEM = "update item"

  private val newItem = new ItemNew

  override def actionPerformed(e: ActionEvent) = {

  }

  def create(view: String) = {

    view match {
      case NEW_ITEM => {
        newItem eraseForm //Here eclipse says "Unit does not take parameters".
        newItem setVisible true
      }

      case UPDATE_ITEM =>
    }

  }

}

The view class

  package some

  import scala.collection.mutable.HashMap
  import javax.swing.JPanel
  import java.awt.GridLayout
  import javax.swing.JFrame
  import scala.collection.mutable.LinkedHashMap
  import javax.swing.JTextField
  import java.awt.BorderLayout
  import java.awt.FlowLayout
  import javax.swing.JLabel

  class ItemNew extends JFrame {

    private val formFields = LinkedHashMap[String, JTextField]()

    init

    private def init(): Unit = {
      // defining form fields
      formFields += ("name" -> new JTextField(20),
        "surname" -> new JTextField(20),
        "age" -> new JTextField(3))

      setLayout(new BorderLayout)

      val formPanel = new JPanel
      formPanel setLayout (new GridLayout(formFields.size, 1))

      generateForm(formPanel)
      getContentPane add formPanel

      pack
      setVisible(true)

      def generateForm(formPanel: JPanel) = {

        var leftPanel: JPanel = null
        var rightPanel: JPanel = null
        var gridPanel: JPanel = null

        formFields foreach (entry => {

          leftPanel = new JPanel(new FlowLayout)
          leftPanel.getLayout().asInstanceOf[FlowLayout] setAlignment FlowLayout.LEFT

          rightPanel = new JPanel(new FlowLayout)
          rightPanel.getLayout.asInstanceOf[FlowLayout] setAlignment FlowLayout.LEFT

          gridPanel = new JPanel(new GridLayout(1, 2))

          leftPanel add new JLabel(entry._1 + ':')
          rightPanel add entry._2

          gridPanel add leftPanel
          gridPanel add rightPanel

          formPanel add gridPanel

        })
      }
    }

    def getFormContent: HashMap[String, String] = {

      val formData = HashMap[String, String]()

      formFields foreach (entry => {
        formData += (entry._1 -> entry._2.getText)
      })

      return formData
    }

    def eraseForm : Unit = {
      formFields foreach (entry => {
        entry._2 setText ""
      })
    }

    private def addListeners = {

    }

  }

See the comment in the ItemController class. If I leave a blank line between the statement where the comment is and the newItem setVisible true then the code gives no error.

Thanks in advance.

score 5 · Accepted Answer · edited May 23 '17 at 12:13

newItem eraseForm
newItem setVisible true

is parsed as

newItem.eraseForm(
newItem).setVisible(true);

but what you want to have is:

newItem.eraseForm;
newItem.setVisible(true);

Leaving out dots and parentheses means that the Scala compiler applies "operator notation". This means that your expression always needs to be of the form obj meth param. It is possible to have obj meth; but as you can see this form needs to be stopped by a semicolon. You can either insert it by yourself or let the compiler infer it by adding an empty line, an expression that starts with a keyword in the following line or with closing braces and parentheses.

Since 2.10 you will also get a warning if you write obj meth because such postfix operators can be harmful as you can see with your question.

See these two question and their answers for more information on the "operator notation" rule:

Scala syntax bug?

1 Answers1