Event Handling

Swing is event-driven: the user clicks, types, or resizes something, and the toolkit fires an event object. You register a listener (a Java interface implementation) and your Python method runs when the event happens. Jython makes this a one-line registration with a Python callable.

ActionListener - Buttons and Menus

from javax.swing import JFrame, JButton, JOptionPane

frame = JFrame("Events")
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)

def on_click(event):
    JOptionPane.showMessageDialog(frame, "Button clicked!")

btn = JButton("Click me")
btn.addActionListener(on_click)
frame.add(btn)
frame.setSize(280, 100)
frame.setVisible(True)

A plain Python function works as a listener as long as it accepts the event argument - Jython adapts it to the Java interface.

Listeners as Classes

class QuitListener(java.awt.event.ActionListener):
    def actionPerformed(self, e):
        import sys
        sys.exit(0)

# or with a lambda for a one-off:
btn2.addActionListener(lambda e: print("second button"))

Common Event Types

Text Field Events

from javax.swing import JFrame, JTextField, JLabel

frame = JFrame("Echo")
field = JTextField(16)
label = JLabel("type something")

def on_key(event):
    label.setText("typed: " + field.getText())

field.addKeyListener({
    "keyPressed": on_key,
    "keyReleased": lambda e: None,
    "keyTyped": lambda e: None,
})
frame.add(field)
frame.add(label)
frame.setSize(320, 100)
frame.setVisible(True)

Jython allows passing a dict of method names to addKeyListener for the multiple methods of that interface.

Reading the Event Object

def on_key(e):
    print("key code:", e.getKeyCode())
    print("key char:", e.getKeyChar())
    print("modifiers:", e.getModifiersEx())

Pitfalls

• Update the UI only from the EDT; if your handler is invoked elsewhere, wrap updates in SwingUtilities.invokeLater.
• Do long work in a worker thread and post the result to the EDT - the EDT must stay responsive.
• Lambdas and functions are the easiest listener style; use classes when the handler needs state.

Summary

Event handling in Jython Swing is: register a Python callable (function, lambda, or class) with addXxxListener, and implement the interface method (actionPerformed, keyPressed, ...) taking the event object. Read state from the event's getters, keep the EDT responsive by moving long work to worker threads, and prefer the dict style for multi-method listeners.