COMP201 Java Programming Topic 7: Exceptions Reading: Chapter 11.
COMP201 Java Programming Part II: GUI Programming Topic 9: Event Handling Reading: Chapter 8.
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Transcript of COMP201 Java Programming Part II: GUI Programming Topic 9: Event Handling Reading: Chapter 8.
COMP201 Java Programming
Part II: GUI Programming
Topic 9: Event Handling
Reading: Chapter 8
COMP201 Topic 9 / Slide 2
Outline
Introduction Two examples Adapter Classes The AWT event hierarchy Individual events: KeyEvents, MouseEvents Separating GUI with Appl. Code
COMP201 Topic 9 / Slide 3
Event Handling Basics 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
(Programmer can create new event types.)
COMP201 Topic 9 / Slide 4
Event Handling BasicsThe Java AWT event delegation model Event source: an object that generates events
In order to capture interesting events and act accordingly, we need to create listeners A listener must be registered with an event source in order to listen
for events produced there. A listener must implement a listener interface, which decides the
response to an event. An event source can have multiple listeners and vice versa.
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.
COMP201 Topic 9 / Slide 5
Event Handling Basics
Event Listener
Any objects of class that implements an appropriate listener interfaceclass Listener implements ActionListener { … actionPerformed(Event evt) {…}}
Event Source
Has to register its own listeners
Source.addActionListener(Listener)
Event Object evt
COMP201 Topic 9 / Slide 6
Example 1 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.
COMP201 Topic 9 / Slide 7
Example 1
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();
}
}
COMP201 Topic 9 / Slide 8
Example 1 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(); }
} //Beeper1.java
COMP201 Topic 9 / Slide 9
Example 2 Background color changes when we click on the buttons (run program). Create three buttonsclass ButtonPanel extends JPanel { public ButtonPanel() { yellowButton = new JButton("Yellow"); blueButton = new JButton("Blue"); redButton = new JButton("Red");
add(yellowButton); add(blueButton); add(redButton); // create button actions ColorAction yellowAction = new ColorAction(Color.yellow); ColorAction blueAction = new ColorAction(Color.blue); ColorAction redAction = new ColorAction(Color.red);
COMP201 Topic 9 / Slide 10
Example 2
// associate actions with buttons
yellowButton.addActionListener(yellowAction); blueButton.addActionListener(blueAction); redButton.addActionListener(redAction); } private JButton yellowButton; private JButton blueButton; private JButton redButton;
} // ButtonTest.java:
COMP201 Topic 9 / Slide 11
Example 2 Make an action listener that sets the panel's
background color.private class ColorAction implements ActionListener{ public ColorAction(Color c) { backgroundColor = c; } public void actionPerformed(ActionEvent event) { setBackground(backgroundColor); repaint(); } private Color backgroundColor; }
COMP201 Topic 9 / Slide 12
Example 3 Make an action listener that sets the panel's
background color.private class ColorAction implements ActionListener{ public ColorAction(Color c) { backgroundColor = c; } public void actionPerformed(ActionEvent event) { setBackground(backgroundColor); repaint(); } private Color backgroundColor; }
COMP201 Topic 9 / Slide 13
Adapter Classes
Illustrate the concept of Adapter classes using WindowEvents With JDK1.3, we can terminate program when a window is closed
using setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Without it, program won’t terminate when window is closed. Try FirstTest.java:
COMP201 Topic 9 / Slide 14
FirstTest
public class FirstTest
{ public static void main(String[] args)
{ JFrame frame = new FirstFrame();
frame.show();
}
}
class FirstFrame extends JFrame
{ public FirstFrame()
{ setTitle("FirstFrame");
setSize(300, 200);
}
}
COMP201 Topic 9 / Slide 15
Let’s try to fix it. When user tries to close window, FirstFrame
object (window) generates a WindowEvent object.
Create a listener object that listens for WindowEvent and terminates the program when user closes the window.
The object must belong to a class (call it Teminator) that implements the WindowListener Interface.
Adapter Classes
COMP201 Topic 9 / Slide 16
The WindowListener Interface has seven methods, one for each distinct event that can happen to a window
class Terminator implements WindowListener{ public void windowClosing(WindowEvent e)
{System.exit(0);} public void windowClosed(WindowEvent e) {}; public void windowIConified(WindowEvent e) {}; public void windowOpened(WindowEvent e) {}; public void windowDeiconified(WindowEvent e) {}; public void windowActivated(WindowEvent e) {}; public void windowDeactivated(WindowEvent e) {};}We are only interested in one event: user closing window.
Adapter Classes
COMP201 Topic 9 / Slide 17
Register listener with event source.
class FirstFrame extends JFrame{ public FirstFrame() { setTitle("FirstFrame"); setSize(300, 200);
addWindowListener( new Terminator() ); }} //CloseableW1.java
Adapter Classes
COMP201 Topic 9 / Slide 18
Keying in code for six do-nothing method is tedious.
Java provides companion adapter class for each AWT listener interface that has more than one method.
The adapter class implements all methods to do nothing.
We can extend the adapter by refining only methods that correspond to interesting events.
Adapter Classes
COMP201 Topic 9 / Slide 19
The adaptor for WindowListener interface is called WindowAdaptor.
Using the adapter, we can rewrite Terminator class as follows:class Terminator extends WindowAdapter{ public void windowClosing(WindowEvent e)
{ System.exit(0); }}
CloseableW2.java
Adapter Classes
COMP201 Topic 9 / Slide 20
So far, we haveclass FirstFrame extends JFrame{ public FirstFrame() { setTitle("FirstFrame"); setSize(300, 200);
addWindowListener( new Terminator() ); }}class Terminator extends WindowAdaptor{ public void windowClosing(WindowEvent e)
{ System.exit(0); }}
Making Terminator an anonymous inner class (replace its usage in FirstFrame with its definition.)
Adapter Classes
COMP201 Topic 9 / Slide 21
The current program:class FirstFrame extends JFrame{ public FirstFrame() { setTitle("FirstFrame"); setSize(300, 200);
addWindowListener( new WindowAdapter(){ public void windowClosing(WindowEvent e)
{ System.exit(0); }} );
}} //CloseableW3.java
This is exactly what we have in the textbook using JDK1.2.
Adapter Classes
COMP201 Topic 9 / Slide 22
The AWT Event Hierarchy
EventObject
AWTEvent
ComponentEvent
ItemEvent
ActionEvent
AdjustmentEvent
TextEvent
InputEvent
PaintEvent
ContainerEvent
FocusEvent
WindowEvent
KeyEvent
MouseEvent
java.util
java.awt.event
Programmer no need to worry about paint events
…….
COMP201 Topic 9 / Slide 23
11 listener interfaces in java.awt.event
ActionListener KeyListener*
AdjustmentListener MouseListener*
ComponentListener* MouseMotionListener*
ContainerListener* TextListener
FocusListener* WindowListener*
ItemListener
*: Listeners that have corresponding adapters
The AWT Event Hierarchy
COMP201 Topic 9 / Slide 24
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
COMP201 Topic 9 / Slide 25
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, frameComponentEvent 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
COMP201 Topic 9 / Slide 26
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
COMP201 Topic 9 / Slide 27
Individual Events/KeyEvent Only one component can have the focus at a time Only the component that has focus can receive
keystrokes 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.
FocusEvent is generated when a component gains focus or loses focus. Refer to textbook for how to handle focus events.
COMP201 Topic 9 / Slide 28
Methods of java.awt.Component void requestFocus(): Moves focus to this
component
boolean isFocusTraversible(): 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.
Individual Events/ KeyEvent
COMP201 Topic 9 / Slide 29
Java distinguish 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 objects are generated by component with focus when a key is pressed or released. Typing “a” generates three KeyEvent objects
keyPressed VK_A (Virtual key code) keyTyped “a” keyReleased VK_A (Virtual key code)
Individual Events/ KeyEvent
COMP201 Topic 9 / Slide 30
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?)
Methods of java.awt.event.KeyEvent Class:– getKeyCode() to get back virtual key code.– getKeyChar() to get back the key character– ….
Individual Events/ KeyEvent
COMP201 Topic 9 / Slide 31
Example: Sketch.java Move with either cursor keys or “h”, “j”, “k”, “l” keys Move faster when SHIFT is pressed
public SketchPanel()
{
last = new Point2D.Double(100, 100);
lines = new ArrayList();
KeyHandler listener = new KeyHandler();
addKeyListener(listener);
}
public boolean isFocusTraversable() { return true; } …
}
Individual Events/ KeyEvent
We need this because normally a panel
cannot get keyboard focus
COMP201 Topic 9 / Slide 32
Pressing cursor keys:public void keyPressed(KeyEvent event)
{ int keyCode = event.getKeyCode(); // set distance int d; if (event.isShiftDown()) d = LARGE_INCREMENT; else d = SMALL_INCREMENT; // direction of move 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);
}
Individual Events/ KeyEvent
Note: Can also use getModifiers of InputEvent or getKeyModifiers of KeyEvent to find out whether SHIFT is down
COMP201 Topic 9 / Slide 33
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);}
Individual Events/ KeyEvent
Note: Can also use getModifiers of InputEvent or getKeyModifiers of KeyEvent to find out whether upper case or lower case
COMP201 Topic 9 / Slide 34
Individual Events/ 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.
MouseEvent objects generated by components where mouse cursor is located
COMP201 Topic 9 / Slide 35
Two listener interfaces MouseListener
–mousePressed(MouseEvent evt)–mouseReleased(MouseEvent evt)–mouseClicked(MouseEvent evt)
MouseMotionListener–mouseMoved(MouseEvent evt)–mouseDragged(MouseEvent evt)
Individual Events/ MouseEvent
COMP201 Topic 9 / Slide 36
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.
Individual Events/Mouse Events
COMP201 Topic 9 / Slide 37
Individual Events/ MouseEvent MouseTest.java -- place, move, and erase squares 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;
…
}
COMP201 Topic 9 / Slide 38
Individual Events/ MouseEvent MousePanel –Basic methods
class MousePanel extends JPanel
{…
// Finds the first square containing a 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){…}
…
}
COMP201 Topic 9 / Slide 39
Individual Events/ MouseEventMousePanel provides 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());
} …
}
COMP201 Topic 9 / Slide 40
Individual Events/ MouseEvent
MouseHandler is an inner class of MousePanelclass 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);
…
COMP201 Topic 9 / Slide 41
Individual Events/ MouseEventMouseMotionHandler 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();
}
}
COMP201 Topic 9 / Slide 42
Separating GUI & Appl Code
Suppose we want to change background color by doing one of the following: Click on one of the buttons Select a color from a menu Press a key :
ctrl B = blue, ctrl Y = Yellow, ctrl R = red
Separating GUI & Appl code: Deal with color change command in a uniformed way, no matter how the command is invoked. Make an ColorAction object for each command (appl.) Make the object listener to all events that trigger the
command (GUI)
COMP201 Topic 9 / Slide 43
Javax.swing.Action interface: An action is an object that encapsulates the following:
• A description of the commend (as a text string and an optional icon)
• Parameters that are necessary to carry out the command (such as the requested color in our example).
Action interface
COMP201 Topic 9 / Slide 44
• Javax.swing.Action interface extends the interface ActionListener– void actionPerformed(ActionEvent evt)– void setEnabled(boolean b)– boolean isEnabled()– void putValue(String key, Object value)– Object getValue(String key)– void addPropertyChangeListener(…)– void removePropertyChangeListener(…)
Separating GUI & Appl Code
COMP201 Topic 9 / Slide 45
• void putValue(String key, Object value)• Object getValue(String key)
– Store and retrieve name/value pair in the action object e.g., action’s name, action’s icon
– Some useful pre-defined keys: Action.NAME, Action.SMALL_ICON Action.putValue(Action.Name, “Blue”);
Action.putValue(Action.SMALL_ICON,
new ImageIcon(“Blue-ball.gif”));
• void addPropertyChangeListener(…)• void removePropertyChangeListener(…)
– Monitoring the properties of action object (e.g. if action name changes, menu item should also change).
Separating GUI & Appl Code
COMP201 Topic 9 / Slide 46
• 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)
Separating GUI & Appl Code
COMP201 Topic 9 / Slide 47
Use of class AbstractAction in the background color change example
Define ColorAction class as an inner class of ActionPanel:.class ColorAction extends AbstractAction
{ public ColorAction(String name, Icon icon,
Color c)
{ putValue(Action.NAME, name);
putValue(Action.SMALL_ICON, icon);
putValue("Color", c);
}
//actionPerformed
}
Separating GUI & Appl Code
New property with value c
COMP201 Topic 9 / Slide 48
class ColorAction extends AbstractionAction
{ // constructor here
public void actionPerformed(ActionEvent evt)
{ Color c = (Color)getValue("Color");
setBackground(c);
repaint();
}
} Apply setBackGround(c) & repaint() to this.
Separating GUI & Appl Code
Create three Action objects:
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);
COMP201 Topic 9 / Slide 50
Associate those actions with buttons by invoke JButton constructor which takes an Action object
Add those buttons to this JPanel
add(new JButton(yellowAction));
add(new JButton(blueAction));
add(new JButton(redAction));
Separating GUI & Appl Code
COMP201 Topic 9 / Slide 51
Keystrokes are delivered to the component that has focus.
Each of the three buttons and the panel can have focus. So we need to deal with keystrokes in each of the three buttons and the panel
It is implemented in two steps
1. Use one of the three InputMaps of JComponent (a superclass of JPanel) to associate keystrokes with some arbitrary objects.
2. A second map, implemented by ActionMap class maps those objects to actions.
Separating GUI & Appl Code
COMP201 Topic 9 / Slide 52
Every Jcomponent has three InputMaps that map KeyStroke objects to actions:
1. WHEN_FOCUSED
When this component has keyboard focus
2. WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
When this component contains the component that has keyboard focus
3. WHEN_IN_FOCUSED_WINDOW
When this component is contained in the same window as the component that has keyboard focus
Separating GUI & Appl Code
COMP201 Topic 9 / Slide 53
// Obtain an input map with getInputMap method
InputMap imap = panel.getInputMap(JComponent.WHEN_FOCUSED);
//In the map associate "ctrl Y” to an action key “panel.yellow”
imap.put(KeyStroke.getKeyStroke("ctrl Y"), "panel.yellow");
Every Jcomponent also has one ActionMap to associate some objects to Actions.
// To obtain the action map:
ActionMap amap = panel.getActionMap().
// link the action key to the real action
amap.put("panel.yellow", yellowAction);
Separating GUI & Appl Code
// associate the ctrl_Y, ctrl_B, and ctrl_R with action keys InputMap imap = getInputMap( // since 1.3
JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
imap.put(KeyStroke.getKeyStroke("ctrl Y"), "panel.yellow");
imap.put(KeyStroke.getKeyStroke("ctrl B"), "panel.blue");
imap.put(KeyStroke.getKeyStroke("ctrl R"), "panel.red");
// associate action keys with actions
ActionMap amap = getActionMap();
amap.put("panel.yellow", yellowAction);
amap.put("panel.blue", blueAction);
amap.put("panel.red", redAction);
The action will be triggered by keystroke if the component pointed at by mouse and the the component that has focus are in the same window.
In our example, keystrokes can trigger actions if mouse cursor inside window
COMP201 Topic 9 / Slide 55
Steps to carry out the same action in response to a button, a menu item, or a keystroke:
1. Make a class that extends the AbstractAction.
2. Make an object of that class.
3. Construct a button or menu item from that object. The constructor will read the label text and icon from the action object.
4. For action that can be triggered by keystrokes
1. Locate the top-level component of the window, such as a panel that contains all other components.
2. Get the when_ancestor_of_focused_component input map of the top-level 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.
3. Get the action map of the top-leve component. Add the pair (action key, action object) into the map.
Separating GUI & Appl Code