java events

1. Topic 9: Event Handling
Reading: Chapter 8
Advanced Programming Techniques

2. Objective and Outline
• Objective:
– Show how to write GUI programs that react to user
actions.
• Outline:
– Java AWT event delegation model
• An example
– The AWT event hierarchy
– Individual events
• KeyEvents
• MouseEvents
– Special classes and interfaces for event handling
• Adapter classes
• The Action interface

3. Java AWT event delegation model
Java automatically generates event objects when
Mouse or button clicked
Menu, checkbox, or text selected
Keyboard typed
Scrollbar adjusted
…..
It is up to the programmer to decide whether to do
anything or what to do when an event happens

4. • Event source:
– An object that generates events
• Listener:
– Receives events and decides what to do
Java AWT event delegation model
Event Listener Event Source
Event Object evt

5. Java AWT event delegation model
• Notes
– A listener must be registered with an event source
in order to listen for events produced there.
• An event source can have multiple listeners and vice
versa
– A listener class must implement a listener
interface, which decides the response to an event.

6. Java AWT event delegation model
Any objects of class that implements
an appropriate listener interface
class Listener implements
ActionListener {
…
actionPerformed(Event evt) {…}
}
Has to register its
own listeners
Source.addActionL
istener(Listener)
Event Listener Event Source
Event Object evt

7. Java AWT event delegation model
• The event handling process:
– When an event occurs, event source sends event
objects to all registered listeners.
– Listeners use information encapsulated in the
event objects to determine what to do.
Event Listener Event Source
Event Object evt

8. Example
• A button that beeps when clicked (Beeper.java)
– Create a GUI with one button
class BeeperFrame extends JFrame
{
public BeeperFrame()
{
setSize( 300, 200);
button = new JButton("Click Me");
getContentPane().add(button,
BorderLayout.CENTER);
}
private JButton button;
}
Javax.swing.JButton
has five constructors
Java.awt.Container
void add(Component)
Adding button directly onto
contentPane. Bad programming style.

9. Example
• When a button is clicked, an ActionEvent object is produced
(java.awt.event.ActionEvent).
• Listener for an ActionEvent: an object of a class that implements the
ActionListener Interface
• class ClickListener implements ActionListener
{
public void actionPerformed(ActionEvent
e) {
Toolkit.getDefaultToolkit().beep();
}
}

10. Example
• Create a listener object and register it with the button:
class BeeperFrame extends JFrame
{ public BeeperFrame()
{ …
button = new JButton("Click Me");
button.addActionListener(new
ClickListener());
}
}
• The driver class:
public class Beeper
{ public static void main(String[] args)
{ BeeperFrame frame = new BeeperFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}
} //Check out Beeper1.java (inner class)

11. Outline
• Outline:
– Java AWT event delegation model
• Two examples
– The AWT event hierarchy
– Individual events
• KeyEvents
• MouseEvents
– Special classes and interfaces for event handling
• Adapter classes
• The Action interface

12. The AWT Event Hierarchy
EventObject
AWTEvent
Component
Event
Item
Event
Action
Event
Adjustment
Event
Text
Event
Input
Event
Paint
Event
Container
Event
Focus
Event
Window
Event
Key
Event
Mouse
Event
java.util
java.awt.event
Programmer no need
to worry about paint
events
…….
Swing has more event types
Programmer can create new event types

13. 11 listener interfaces in java.awt.event
ActionListener KeyListener
AdjustmentListener MouseListener
ComponentListener MouseMotionListener
ContainerListener TextListener
FocusListener WindowListener
ItemListener
The AWT Event Hierarchy

14. Semantic Events: What user does
ActionEvent (button click, menu selection)
AdjustmentEvent (adjusting scroll bar)
ItemEvent (selection from checkbox or list items)
TextEvent (the contents of a text field or text area were
changed)
Additional events (like MenuEvent, ChangeEvent) can be
found in javax.swing.event. They are associated with
Swing components and easy to handle. Will discuss some in the
next topic.
The AWT Event Hierarchy

15. Low-Level Events: facilitate semantic events
ComponentEvent (component resized, moved, shown, or hidden)
 (A component is a user interface elements such as button, text
field, scrollbar, panel, frame)
 ComponentEvent is the ancestor of all low-level event
classes
FocusEvent (component get focus)
ContainerEvent (a component has been added or removed.
Programmers no need to worry about it)
(A container is a screen area or component that can contain
components. E.g. window, panel)
The AWT Event Hierarchy

16. Low-Level Events: facilitates semantic events
KeyEvent (key pressed or released)
MouseEvent (mouse button pressed, released, moved, or
dragged)
WindowEvent (window activated, deactivated, iconified,
deiconified, or closed)
Will discuss KeyEvent and MouseEvent
The AWT Event Hierarchy

17. Last Time
• AWT event delegation model
– Event source:
• What are inside an event class?
• How to find out the types of events that a component
generates?
– Listener:
• What are inside an listener class?

18. Creating a Responsive GUI
• Set up GUI
– Layout various components on the contenPane of top-level container
• Decide which events to handle
– Begin with design objectives: Functionalities of GUI
– For each component, find out the events that it generates
• Check API of component class and look for methods addXXXXListener
– implies that the component produces events of type XXXEvent
• Handle events
– Write listener class
• Check API of listener interface and decide which methods to override
• Check API of event class so as to get information about events
– Create listener object
– Register listener object with event source

19. Objective and Outline
• Objective:
– Show how to write GUI programs that react to user
actions.
• Outline:
– Java AWT event delegation model
• An example
– The AWT event hierarchy
– Individual events
• KeyEvents
• MouseEvents
– Special classes and interfaces for event handling
• Adapter classes
• The Action interface

20. • Focus
– A GUI consists of many components.
– Which component received keystrokes?
• The component that has focus
• Focus is gained or lost in response to user actions:
– A component gains focus if the user clicks the mouse
inside it.
– Or TAB/SHIT-TAB key can be used to traverses
components which can receive input focus
KeyEvent

21. • Methods of java.awt.Component for
managing focus
– void requestFocus(): Moves focus to this
component
– boolean isFocusable(): Tells whether a
component can be reached by using TAB or SHIFT-
TAB
– void transferFocus(): Transfer focus to the next
component in the traversal order.
KeyEvent

22. KeyEvent
FocusEvent is generated when a component gains
focus or loses focus. Refer to textbook for how to
handle focus events.

23. Java distinguishes between characters and virtual key code
– “A” and “a” have the same code VK_A
– There is no VK_a !!
– More examples: VK_COMMA, VK_PERIOD, VK_OPEN_BRACKET,
VK_SHIFT ...
KeyEvent

24. • KeyEvent objects are generated by the component with focus when a key
is pressed or released.
– Typing “a” generates three KeyEvent objects
• 2 Lower-level events
– keyPressed VK_A (Virtual key code)
– keyReleased VK_A (Virtual key code)
• One higher-level event: keyTyped “a”
– Only for keys that generate character input
• Methods of java.awt.event.KeyEvent Class:
– getKeyCode() to get back virtual key code.
– getKeyChar() to get back the key character
– ….
KeyEvent

25. Methods in the KeyListener Interface:
keyPressed(KeyEvent e)
(what to do when VK_A pressed?)
keyReleased(KeyEvent e)
(what to do when VK_A released?)
keyTyped(KeyEvent e)
(what to do when “a” is typed?)
e.getKeyCode() seems to give 0 within keyTyped
Will illustrate with example
KeyEvent

26. Example: Sketch.java
Move with either cursor keys or “h”, “j”, “k”, “l” keys
Move faster when SHIFT is pressed
KeyEvent

27. Setup:
public SketchPanel extends JPanel
{ public void paintComponent(Graphics g)
{ super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
// draw all lines
for (int i = 0; i < lines.size(); i++)
g2.draw((Line2D)lines.get(i));
}
public void add(int dx, int dy) ..
// add line segement last (x, y) –-- (x+dx, y+dy)
// calls repaint()
private ArrayList lines
private Point2D last; // last point
KeyEvent

28. Getting focus and register key listener
public class SketchPanel
{
public SketchPanel()
{ …
KeyHandler listener = new KeyHandler();
addKeyListener(listener);
}
public boolean isFocusable() { return true; }
…
}
..}
KeyEvent
We need this because by default a panel
cannot get keyboard focus

29. Processing cursor keys:
class KeyHandler implements KeyListener
{
public void keyPressed(KeyEvent event)
{ // set distance: whether shift is down
int d;
if (event.isShiftDown()) d = LARGE_INCREMENT;
else d = SMALL_INCREMENT;
// direction of move: what key is typed
int keyCode = event.getKeyCode();
if (keyCode == KeyEvent.VK_LEFT) add(-d, 0);
else if (keyCode == KeyEvent.VK_RIGHT) add(d, 0);
else if (keyCode == KeyEvent.VK_UP) add(0, -d);
else if (keyCode == KeyEvent.VK_DOWN) add(0, d);
}
KeyEvent
Note: Can also use getModifiers of InputEvent or
getKeyModifiers of KeyEvent to find out whether SHIFT is down

30. Typing keys “h”. “j”, “k”, “l”:
public void keyTyped(KeyEvent event)
{ char keyChar = event.getKeyChar();
int d; // distance of move
if (Character.isUpperCase(keyChar))
{ d = LARGE_INCREMENT;
keyChar = Character.toLowerCase(keyChar);
}
else d = SMALL_INCREMENT;
// direction of move
if (keyChar == 'h') add(-d, 0);
else if (keyChar == 'l') add(d, 0);
else if (keyChar == 'k') add(0, -d);
else if (keyChar == 'j') add(0, d);
}
KeyEvent
Note: Can also use getModifiers of InputEvent or getKeyModifiers
of KeyEvent to find out whether upper case or lower case

31. Things to Try
• Delete isFocusable()
• getKeyCode within keyTyped
• Modify example so that event source and
listener are the same object
• Make the listener class in Sketch.java an
anonymous inner class.

32. Objective and Outline
• Objective:
– Show how to write GUI programs that react to
user actions.
• Outline:
– Java AWT event delegation model
• An example
– The AWT event hierarchy
– Individual events
• KeyEvents
• MouseEvents

33. MouseEvent
• Just want to know whether a button or menu
is clicked?
– No need to use mouse events explicitly.
– Instead use ActionEvents generated by
button or menu.
• For drawing with mouse, mouse events are
important.

34. • MouseEvent objects generated by components where mouse cursor is
located
• Getting information about MouseEvents
– int getX(), int getY() to get the x , y coordinates of mouse pointer.
– Point getPoint() to get coordinates of mouse pointer as a Point object
– int getClickCount() to get number of consecutive clicks.
– int getModifiers() to find out, among other things, which mouse button is
clicked.
MouseEvent

35. • Two listener interfaces
– MouseListener
• mousePressed(MouseEvent evt)
– Invoked when a mouse button has been pressed on a component.
• mouseReleased(MouseEvent evt)
– Invoked when a mouse button has been released on a component.
• mouseClicked(MouseEvent evt)
– Invoked when the mouse button has been clicked (pressed and released) on
a component.
• mouseEntered(MouseEvent e)
Invoked when the mouse enters a component.
• mouseExited(MouseEvent e)
Invoked when the mouse exits a component.
MouseEvent

36. • MouseMotionListener
– mouseMoved(MouseEvent evt)
• API: Invoked when the mouse cursor has been moved
onto a component but no buttons have been pushed.
• Actual: invoked frequently while mouse is moving
– mouseDragged(MouseEvent evt)
• API: Invoked when a mouse button is pressed on a
component and then dragged.
• Actual: invoked frequently while mouse is being
MouseEvent

37. MouseEvent
MouseTest.java -- place, move, and erase squares
Setup:
class MousePanel extends JPanel
{ public void paintComponent(Graphics g)
{ super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
// draw all squares
for (int i = 0; i < squares.size(); i++)
g2.draw((Rectangle2D)squares.get(i));
}
private static final int SIDELENGTH = 10;
private ArrayList squares;
private Rectangle2D current;
…
}

38. MouseEvent
MousePanel –Basic methods
class MousePanel extends JPanel
{…
// Finds the first square containing a point.
// Returns null when no square contains the point
public Rectangle2D find(Point2D p){…}
// Adds a square to collection
public void add(Point2D p){…}
// Removes a square to collection
public void remove(Point2D p){…}
…

39. MouseEvent
MousePanel generates MouseEvents, which will be handled by
MouseHandler and MouseMotionHandler
class MousePanel extends JPanel
{ …
public MousePanel()
{ squares = new ArrayList();
current = null;
addMouseListener(new MouseHandler());
addMouseMotionListener(
new MouseMotionHandler());
} …
}

40. MouseEvent
MouseHandler is an inner class of MousePanel
class MousePanel extends Jpanel
{ private class MouseHandler implements MouseListener
{ public void mousePressed(MouseEvent event)
{ // add a new square if the cursor isn't inside a square
current = find(event.getPoint());
if (current == null)
add(event.getPoint()); }
public void mouseClicked(MouseEvent event)
{ // remove the current square if double clicked
current = find(event.getPoint());
if (current != null && event.getClickCount() >= 2)
remove(current);
…

41. A side note: Adapter classes
class MouseHandler implements MouseListener
{ public void mousePressed(MouseEvent event) { ..}
public void mouseClicked(MouseEvent event) {..}
}
• not needed
– mouseReleased(MouseEvent evt)
– mouseEntered(MouseEvent e)
– mouseExited(MouseEvent e)
• But we have to implement it anyway
class MouseHandler implements MouseListener
{ public void mousePressed(MouseEvent event) { ..}
public void mouseClicked(MouseEvent event) {..}
public void mouse Released(MouseEvent event) {}
// does nothing. No code
Public void mouseEntered(MouseEvent e){}
Public void mouseExited(MouseEvent e){}
}

42. A side note: Adapter classes
• Adaptor classes are introduced to save us from the trouble of providing
empty methods
– Example:
• MouseAdapter is a class that consists of default implementation (do nothing) of
all the three methods in MouseListener
• Hence, the following is fine
class MouseHandler extends MouseAdapter
{ public void mousePressed(MouseEvent event) { ..}
public void mouseClicked(MouseEvent event) {..}
}
– No compiler error even though mouseReleased, mouseEntered,
and mouseExited are not provided.
– MouseHandler objects are MousListeners.
• Most listener interfaces have adapter classes

43. MouseEvent
Using adapter class, we now have:
class MousePanel extends Jpanel
{ private class MouseHandler extends MouseAdapter
{ public void mousePressed(MouseEvent event)
{ // add a new square if the cursor isn't inside a square
current = find(event.getPoint());
if (current == null)
add(event.getPoint()); }
public void mouseClicked(MouseEvent event)
{ // remove the current square if double clicked
current = find(event.getPoint());
if (current != null && event.getClickCount() >= 2)
remove(current);

44. MouseEvent
MouseMotionHandler is also an inner class of MousePanel
class MousePanel extends Jpanel
{ private class MouseMotionHandler
implements MouseMotionListener
{
public void mouseMoved(MouseEvent event){…}
public void mouseDragged(MouseEvent event)
{ if (current != null)
{ int x = event.getX(); int y = event.getY();
// drag the rectangle to center it at (x, y)
current.setFrame(x - SIDELENGTH/2,
y - SIDELENGTH/2, SIDELENGTH, SIDELENGTH);
repaint();
}
}

45. Objective and Outline
• Objective:
– Show how to write GUI programs that react to
user actions.
• Outline:
– Java AWT event delegation model
• An example
– The AWT event hierarchy
– Individual events
• KeyEvents
• MouseEvents

46. The Action Interface
• Run ActionTest.java
– One can change background color by doing one
of the following:
1. Click on one of the buttons
2. Press a key :
ctrl B = blue, ctrl Y = Yellow,
ctrl R = red

47. The Action Interface
How to write the program?
Do set up first
class ActionPanel extends JPanel
{
public ActionPanel()
{
// add buttons for these actions
add(new JButton(“Red”));
add(new JButton(“Blue”));
add(new JButton(“Yellow”));
}
public void actionPerformed(ActionEvent event)
{
setBackground(…);
repaint();
}
}

48. The Action Interface
• How to handle events?
– Need a class to listen for ActionEvent from button
Class Listener1 implements ActionListener
{ public void actionPerformed (ActionEvent e)
{ // action codes: change background colour
}
}
– Also need a class to listen for KeyEvent from
keystroke
Class Listener2 implements KeyListener
{ public void keyPressed (ActionEvent e)
{ // action codes: change background colour
}
}
Bad solution: “action codes” appear in two different places

49. The Action Interface
• Better solution
– Place “action codes” in one class, called action
class
– Associate objects of the action class to different
event sources
• This mechanism is provided in Swing, beyond the AWT
event model.
Event Soure 2
Event Source 1
Action Codes
Action Class Object

50. The Action Interface
• Next
– How to write action classes?
– How to associate action objects with event
sources?

51. The Action Interface
• The interface Javax.swing.Action introduced for specifying actions
– Extends ActionListener
• Has method: place for placing action codes
void actionPerformed(ActionEvent evt)
– Other methods for specifying features of the action:
• setEnabled(boolean b), boolean
isEnabled()
• void putValue(String key, Object value)
• Object getValue(String key)
– (key value pairs can be used to store parameters necessary for
carrying out the action, i.e. colour)
• …

52. The Action Interface
• Class AbstractAction is a default implementation of
Action
– The only abstract method is
actionPerformed()
• Instead of implementing Action, you can simply extending
AbstractAction (similar to adapter class)

53. The Action Interface
• Action class for our example:
.class ColorAction extends AbstractAction
{ public ColorAction(String name, Icon icon, Color c)
{ putValue(Action.NAME, name);
putValue(Action.SMALL_ICON, icon);
putValue("Color", c);
}
public void actionPerformed(ActionEvent evt)
{ Color c = (Color)getValue("Color");
setBackground(c);
repaint();
}
}
Action.NAME and Action.SMALL_ICON are used in display on buttons

54. The Action Interface
• Make objects of the ColorAction Class
Action blueAction =
new ColorAction("Blue",
new ImageIcon("blue-ball.gif"),
Color.blue);
Action yellowAction =
new ColorAction("Yellow",
new ImageIcon("yellow-ball.gif"),
Color.yellow);
Action redAction =
new ColorAction("Red",
new ImageIcon("red-ball.gif"),
Color.red);

55. The Action Interface
• Next
– How to write action classes?
– How to associate action objects with event
sources?

56. The Action Interface
• How do we associate the action objects with
event sources?
– The answer is easy for one event source: buttons
• Associate those actions with buttons by invoke JButton
constructor which takes an Action object
• Add those buttons to this JPanel
Class ActionPanel extends JPanel
{
add(new JButton(yellowAction));
add(new JButton(blueAction));
add(new JButton(redAction));
…
}

57. The Action Interface
• How do we associate the action objects with
keystrokes?
– InputMap provides a binding between an input event
(currently only KeyStrokes are used) and an Object
– ActionMap provides mappings from Objects to Actions
– So, we have this solution
InputMap imap = new InputMap();
imap.put(KeyStroke.getKeyStroke("ctrl Y"), "panel.yellow");
ActionMap amap = new ActionMap();
amap.put("panel.yellow", yellowAction);

58. The Action Interface
• The solution on the previous slide does not
work
– We did not specify where the maps should take
effect
• We want them to take effect within ActionPanel and
nowhere else.
– How to create maps what take effect within
ActionPanel?

59. The Action Interface
• JComponent has method
public final InputMap getInputMap(int condition)
that returns the map used under each of the following conditions
1. JComponent.WHEN_FOCUSED
When this component has keyboard focus
2. JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
When this component contains the component that has keyboard focus
3. JComponent.WHEN_IN_FOCUSED_WINDOW
When this component is contained in the same window as the component
that has keyboard focus

60. The Action Interface
• JComponent has method
public final ActionMap
getActionMap()
that returns the ActionMap used to
determine what Action to fire for particular
KeyStroke binding

61. The Action Interface
• The correct solution:
Class ActionPanel extends JPanel
{
InputMap imap = getInputMap( // since 1.3
JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
imap.put(KeyStroke.getKeyStroke("ctrl Y"),
"panel.yellow");
ActionMap amap = getActionMap();
amap.put("panel.yellow", yellowAction);
…..
}

62. The Action Interface
Summary: To make action objects for multiple
event sources
1. Make a class that extends the AbstractAction.
2. Make an object of that class.
3. Construct a button or menu item with that object.
l The constructor will read the label text and icon from the
action object.

63. The Action Interface
4. For action that can be triggered by keystrokes
1. Locate an appropriate component , such as a panel, within which
you want the keystrokes to invoke actions.
2. Get the when_ancestor_of_focused_component input
map of the component.
– Make a KeyStroke object for the desired keystroke.
– Make an action key object, such as a string that describes
your action.
– Add the pair(keystroke, action key ) into the input map.
1. Get the action map of the component. Add the pair (action key,
action object) into the map.