Topic 9: Event Handling

Reading: Chapter 8

Advanced Programming Techniques

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

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

• 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

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.

Java AWT event delegation model

Any objects of class that implements an appropriate listener interfaceclass Listener implements ActionListener { … actionPerformed(Event evt) {…}}

Has to register its own listeners

Source.addActionListener(Listener)

Event Listener Event Source

Event Object evt

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

Example • A button that beeps when clicked (Beeper.java)

– Create a GUI with one buttonclass 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.

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(); }}

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)

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

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

…….

Swing has more event types

Programmer can create new event types

11 listener interfaces in java.awt.eventActionListener KeyListener

AdjustmentListener MouseListener

ComponentListener MouseMotionListener

ContainerListener TextListener

FocusListener WindowListener

ItemListener

The AWT Event Hierarchy

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

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

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

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?

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

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

• 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

• 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

KeyEvent

FocusEvent is generated when a component gains

focus or loses focus. Refer to textbook for how to handle focus events.

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

• 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

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

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

KeyEvent

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

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

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

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

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.

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

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

• 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

• 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

• 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 dragged

MouseEvent

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; …}

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){…} …}

MouseEventMousePanel 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()); } …}

MouseEventMouseHandler 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);

…

A side note: Adapter classesclass 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 anywayclass 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){}

}

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

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);

…

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(); } }

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

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

The Action InterfaceHow 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(); }}

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 keystrokeClass Listener2 implements KeyListener{ public void keyPressed (ActionEvent e) { // action codes: change background colour }}

Bad solution: “action codes” appear in two different places

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

The Action Interface

• Next– How to write action classes?– How to associate action objects with event

sources?

The Action Interface• The interface Javax.swing.Action introduced for specifying actions

– Extends ActionListener• Has method: place for placing action codesvoid 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)

• …

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)

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

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);

The Action Interface

• Next– How to write action classes?– How to associate action objects with event

sources?

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));

…}

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 solutionInputMap imap = new InputMap();imap.put(KeyStroke.getKeyStroke("ctrl Y"), "panel.yellow");

ActionMap amap = new ActionMap(); amap.put("panel.yellow", yellowAction);

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?

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_FOCUSEDWhen 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

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

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);…..

}

The Action Interface

Summary: To make action objects for multiple event sources1. 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.

The Action Interface4. 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.

3. Get the action map of the component. Add the pair (action key, action object) into the map.