1. GUI Programming III: Events24
GUI Programming III: Events

2. Header with column labels
Previously
Hierarchy
Overview Swing Components
Swing Components
Would you like to save the data?
Question!
Each cell displays a data item
Each column displays one type of data
Header with column labels
Application title

3. Overview Delegation Event Model Event Object Event Listener
Event Control Flow
Registration
Event Listener Interfaces
Listener Methods
Process
Action Event Example

4. Delegation Event Model
Who started the action is not who execute it
The information is transmitted using an event object
The model consisted of the following actors
User that start the action
GUI component generate an object, e.g. JButton
Listener
Is notified of the request; receive an event object
Execute request; contains the business logic, e.g. Save the data

5. Event Object
Created when an event occurs, e.g. user interacts with a GUI component
Composed of all necessary information about the
When the event has occurred
Type of event that has occurred, e.g. keyboard events, mouse event, ...
Source of the event, e.g. JButton
Of type Event class from java.awt package

6. Event Listener
Need to implement the corresponding event listener interface, e.g. ActionListener
Need to register with the source object
Wait until an valid event is fired
Notified of an event of the type of interest
The corresponding method in the implemented interface is called with the generated event
Processes the received event

7. Event Control Flow Event Source Event Listener Register Fire event
Process event

8. Registration
Call following method on the source object
void add<Type>Listener(<Type>Listener listenerObj)
<Type> refers to the type of event source
Can be Key, Mouse, Focus, Component, Action and others
One event source can register several listeners
Registered listener can unregister by calling the following method on the source object
void remove<Type>Listener(<Type>Listener listenerObj)

9. Event Classes EventObject class is part of the java.util package
Subclasses follow the naming convention:
<Type>Event
Base one already defined in package java.awt.event

10. Event Classes
ComponentEvent - Created when the source is moved, resized, made visible or hidden
ActionEvent - Created when the source is moved, resized, made visible or hidden
ItemEvent - Created when the source is pressed, double clicked or selected, i.e. A button
KeyEvent - Created when an item is selected or deselected, e.g. in a list or check box
MouseEvent - Created when the mouse is pressed, released, clicked (press and release), on tope or leaving the top of the source
WindowEvent - Created when the source (a window) is open, close, activated, deactivated, iconified, ...
Just some of the events

11. Event Listener Interfaces
ActionListener must be implement to receive action events (ActionEvent)
MouseListener must be implement to receive mouse events (MouseEvent)
MouseMotionListener must be implement to receive mouse motion events that include dragging
WindowListener must be implement to receive window events (WindowEvent)
Defined in package java.awt.event

12. Listener Methods ActionListener void actionPerformed(ActionEvent evt)
Invoked when an action occurs
MouseMotionListener
void mouseDragged(MouseEvent evt)
Invoked when a mouse button is pressed on a component and then dragged
void mouseMoved(MouseEvent evt)
Invoked when the mouse cursor has been moved onto a component but no buttons have been pushed

13. Listener Methods MuoseListener void mouseClicked(MouseEvent evt)
Invoked when the mouse button has been clicked (pressed and released) on a component
void mouseEntered(MouseEvent evt)
Invoked when the mouse enters a component
void mouseExited(MouseEvent evt)
Invoked when the mouse exits a component
continue ...

14. Listener Methods MuoseListener (continue)
void mousePressed(MouseEvent evt)
Invoked when a mouse button has been pressed on a component
 void mouseReleased(MouseEvent evt)
Invoked when a mouse button has been released on a component.
WindowListener
 void windowClose(WindowEvent evt)
Invoked when a window has been closed as the result of calling dispose on the window. Many more methods.

15. Process
Create Event Listener class; class which implements the appropriate listener interface
Implement all methods of the appropriate listener interface
Describe in each method how you would like the event to be handled
May give empty implementations for methods you don't need

16. Process public class MyActionListener implements ActionListener {
/**
* Processes the action event from the "OK" JButton.
evt the fired action event. */
public void actionPerformed(ActionEvent evt) {
String strAction = evt.getActionCommand();
if (strAction.equals("OK")) {
// TODO: add code for when button is pressed }
} // actionPerformed()
} // end class MyActionListener

17. Process Register the listener object with the event source
The object is an instantiation of the listener class from previous step
Use the add<Type>Listener method of the event source
MyActionListener oListener = new MyActionListener();
JButton jb = new JButton("OK");
jb.setActionCommand("OK");
jb.addActionListener(oListener); // register the listener
...

18. super("Pressing Buttons"); // Default Layout Manager is FlowLayout
/**
* GUI application with two button than when clicked print a message into
* the standard out device, i.e. console. */
public class ButtonsGUI extends JFrame implements ActionListener {
* Builds the GUI for the application.
public ButtonsGUI() {
super("Pressing Buttons");
JPanel contentPane= new JPanel(); // Default Layout Manager is FlowLayout
JButton jb1 = new JButton("Press me");
JButton jb2 = new JButton("Or press me");
jb1.setActionCommand("Press me");
jb1.addActionListener(this);
contentPane.add(jb1);
jb2.setActionCommand("Or press me");
jb2.addActionListener(this);
contentPane.add(jb2);
setContentPane(contentPane);
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
setVisible(true);
} // Constructor ()
* Processes clicking any of the buttons.
evt the event on the button.
public void actionPerformed(ActionEvent evt) {
String strAction = evt.getActionCommand();
if (strAction.equals("Press me")) {
System.out.println("'Press me' button was pressed");
} else if (strAction.equals("Or press me")) {
System.out.println("'Or press me' button was pressed"); }
} // actionPerformed()
} // end class ButtonsGUI
Action Event Example
public ButtonsGUI() {
super("Pressing Buttons");
// Default Layout Manager is FlowLayout
JPanel contentPane= new JPanel();
// Build buttons
JButton jb1 = new JButton("Press me");
JButton jb2 = new JButton("Or press me");
jb1.setActionCommand("Press me");
jb1.addActionListener(this);
contentPane.add(jb1);
jb2.setActionCommand("Or press me");
jb2.addActionListener(this);
contentPane.add(jb2);
setContentPane(contentPane);
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
setVisible(true);
} // Constructor ()
public void actionPerformed(ActionEvent evt) {
String strAction = evt.getActionCommand();
if (strAction.equals("Press me")) {
System.out.println("'Press me' button was pressed");
} else if (strAction.equals("Or press me")) {
System.out.println("'Or press me' button was pressed"); }
} // actionPerformed()
Amadeo Ascó, Adam Moore

19. Action Event Example
After pressing both buttons; ‘Press me’ first and ‘Or press me’ the output is as bellow
'Press me' button was pressed
'Or press me' button was pressed