Jython and Swing

Posted on by Useless Admin

A simple demonstration of using Java Swing from the Jython interactive interpreter.

The following is an example of using the Jython interactive interpreter from the Windows 2000 command prompt. On this page, we demonstrate several Jython basics, including:

  • use of the jython interpreter from the MS Windows 2000 command prompt
  • collection of user input with javax.swing.JOptionPane.showInputDialog()
  • conversion of strings to integers, and integers to strings, plus simple addition
  • display of output with javax.swing.JOptionPane.showMessageDialog()

Please note that your web browser will probably wrap some longer lines of code.

In our example, Jython is installed in the \bin\ folder of a fairly typical Java installation. My comments are
interjected into the session, indicated by the traditional #.

C:\j2sdk1.4.0\bin>jython
Jython 2.1 on java1.4.0 (JIT: null)
Type "copyright", "credits" or "license" for more information.
>>> import javax.swing as sshwing
>>> firstNum = sshwing.JOptionPane.showInputDialog("Enter an integer: ")
>>> secondNum = sshwing.JOptionPane.showInputDialog("Enter an integer: ")

Each of the two previous lines of code caused a separate swing dialog box asking the user for an integer.

The user’s input is stored as a string, not dynamically determined to be an integer. This is easily demonstrated as follows:

>>> firstNum
'1'
>>> secondNum
'2'
>>> firstNum + secondNum
'12'

In the above lines of code, you see that the user input ‘1’ and ‘2’ in the input dialog boxes, which are strings and subject to concatenation.

Observe what happens if you try a traditional Java approach to converting the string to an integer:

>>> num1 = Integer.parseInt(firstNum)
Traceback (innermost last):
  File "<console>", line 1, in ?
NameError: Integer

If you want to use Integer.parseInt(), first use one of the following two import statements:

  1. from java.lang import Integer(which imports Integer from java.lang), or
  2. from java.lang import * (which imports all classes in java.lang, in case you need several of them).

A Python built-in function, however, does the trick:

>>> num1 = int(firstNum)
>>> num1
1
>>> num2 = int(secondNum)
>>> num2
2
>>> num1 + num2
3
>>> sum12 = num1 + num2
>>> sum12
3

Another Python built-in handily converts the sum on-the-fly to a string:

>>> sum12 = str(num1 + num2)
>>> sum12
'3'

Output of the sum in a swing message box is relatively painless. But in Jython, you use “None” in place of the more Java-esque “Null” in the following examples:

>>> sshwing.JOptionPane.showMessageDialog(None, "The sum of your integers is " + sum12)

The same output may be tweaked to show the box with a label “Sum” and with a more generic look:

 >>> sshwing.JOptionPane.showMessageDialog(None, "The sum of your integers is " + sum12, "Sum", sshwing.JOptionPane.PLAIN_MESSAGE)

A little tweak adds a “.” to the end of the Sum dialog statement.

 >>> sshwing.JOptionPane.showMessageDialog(None,"The sum is " + sum12 + ".", "Sum", sshwing.JOptionPane.PLAIN_MESSAGE)

Easy as Py! And, of course, using the interactive prompt is only one option. Save a .py source file and run it without a separate compile step.