Let's look at the RunLight program from HW4 for a simple example. Whiteboard Picture of Event Handling in RunLight.

What are Events?

• An event can be a mouse click, keyboard input, or other changes in environment.
• A well-design program will use the Delegation Event Model (more in a bit)
• Each event uses a different class.
  • The java.util.EventObject class is the parent class for all event classes.
  • The java.awt.AWTEvent class is the parent class for all AWT events.

• There are 4 basics parts to event handling:
  1. An object that generates events (in the case of a GUI, a Component) keeps a list of listener objects.
  2. Each listener object needs to be added to the Component's "list of listeners". This is done by registering the listener to listen for a particular type of event.
  3. When the event is generated, the listener is notified with a call to a specific method in the listener, and the event is passed in as a argument.
  4. The method in the listener object calls a method belonging to the UI so that the UI implements the behavior that is tied to the event. Note: this method is called a Callback.

• Your event handlers will either implement the Listener or extend the Adaptor.
• Your event handlers will either be inner classes of the GUI, or be pass a reference to the GUI on construction (is this weak or strong association?).
• In general, if designed properly, the listener class will have little to no application logic written in the methods.

Event Types

Action Events:

For Text Components:

• An event of class ActionEvent is generated when the user edits the text in a text field and presses Enter (this indicates that they have finished editing.)
• The actionPerformed(ActionEvent event) method is called when the Enter key is pressed.
• Implement the ActionListener interface to handle an ActionEvent.
• Since there is only one method in the interface, there is no ActionAdapter class.
• Use the addActionListener() method to add the listener object to the text component.

For Buttons:

• An event of class ActionEvent is generated when the user clicks on the button.
• The actionPerformed(ActionEvent event) method is called when the button is clicked.
• Implement the ActionListener interface to handle an ActionEvent.
• Since there is only one method in the interface, there is no ActionAdapter class.
• Use the addActionListener() method to add the listener object to the button component.

Text Events:

• An event of class TextEvent is generated when the user changes the text in a text component.
• The textValueChanged(TextEvent event) method is called when the user changes the text.
• Implement the TextListener interface to handle a TextEvent.
• Since there is only one method in the interface, there is no TextAdaptor class.
• Use the addTextListener() method to add the listener object to the text component. (This registers the listener.)

Item Events:

• An event of class ItemEvent is generated when the state of an item, such as a radio button or checkbox, is changed.
• The itemStateChanged(ItemEvent event) method is called when the state is changed.
• Implement the ItemListener interface to handle a ItemEvent.
• Since there is only one method in the interface, there is no ItemAdaptor class.
• Use the addItemListener() method to add the listener object to the component. (This registers the listener.)
• Your handler should check if the state changed to selected or unselected:

  if (e.getStateChange() == ItemEvent.SELECTED)
  or
  if (e.getStateChange() == ItemEvent.DESELECTED)

More examples here: downloads/swing