COMP201 Java Programming Topic 9: Event Handling Reading: Chapter 8.

COMP201 Java Programming

Topic 9: Event Handling

Reading: Chapter 8

COMP201 Topic 9 / Slide 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


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


Event Listener Event Source

Event Object evt


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.




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 Object evt


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 Object evt


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.


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


– 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.event

ActionListener KeyListener

AdjustmentListener MouseListener

ComponentListener MouseMotionListener

ContainerListener TextListener

FocusListener WindowListener

ItemListener



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.



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)



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



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


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.


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());

} …

}


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


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 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){}

}


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

A side note: Adapter classes


MouseEvent

Using adapter class, we now have:


{ private class MouseHandler extends MouseAdapter

{ public void mousePressed(MouseEvent event)

{ // add a new square if the cursor isn't inside a square


if (current == null)

add(event.getPoint()); }

public void mouseClicked(MouseEvent event)

{ // remove the current square if double clicked


if (current != null && event.getClickCount() >= 2)

remove(current);

…


MouseEventMouseMotionHandler is also an inner class of MousePanel


{ 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();

}

}


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


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(); }}



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



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


Next How to write action classes? How to associate action objects with event sources?



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)

– …



• 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)



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



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);



Next How to write action classes? How to associate action objects with event sources?



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));

…

}



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 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?



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



JComponent has method

public final ActionMap getActionMap()

that returns the ActionMap used to determine what Action to fire for particular KeyStroke binding



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);

…..

}



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.



4. For action that can be triggered by keystrokes1. 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.


COMP201 Java Programming Topic 9: Event Handling Reading: Chapter 8.

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Transcript of COMP201 Java Programming Topic 9: Event Handling Reading: Chapter 8.