Events in Java Swing

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Events in Java Swing

• Chris North
• cs3724 HCI

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Typical command line program

• Non-interactive
• Linear execution

program main() code code code code co
de code code code code code code cod
e
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Interactive command line program
program main() decl data storage initializa
tion code loop get command switch(comm
and) command1 code command2 c
ode

• User input commands
• Non-linear execution
• Unpredictable order
• Much idle time

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Typical GUI program
GUI program main() decl data
storage initialization code create
GUI register callbacks main event
loop Callback1() //button1 press code Cal
lback2() code

• User input commands
• Non-linear execution
• Unpredictable order
• Much idle time
• Event callback procs

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GUI Events
App1
App2
mouseclick
OK
OK
Cancel
Cancel
App2 code OKbtn_click() do
stuff OKbtn_mouseover() do more
stuff CancelBtn_click() do different
stuff
App1 event loop
WindowSystem event loop
App2 event loop
whichapp?
inputdevice
whichcallback?
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Java Swing program
Java program Class main() decl data
storage initialization code create GUI
objects register listeners listener1() do
stuff listener2() do stuff

• Event loop automaticin separate thread

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Example
Example draw program MyDrawClass main() Dat
aStruct drawn_shapes drawn_shapes.clear() c
reate Frame, Panel, buttons, register
listeners DrawPanel_listener_click() drawn_s
hapes.add(new shape) UndoButton_listener_click(
) drawn_shapes.deleteLast()
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Java Listeners

• Register with a component to receive events
• Give component a ref to your Listener object
• JButton1.addMouseListener(new myMouseListener)
• Receive events from component
• Component will call callback procs on your
  Listener object
• myMouseListener.mouseClicked(event)

click
JButton1
1. addMouseListener( )
2. mouseClicked( )
myMouse-Listener
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Listener API

• Listeners must inherit from Java Listener base
  classes
• ActionListener, KeyListener, MouseListener,
  MouseMotionListener, WindowListener,
• Abstract base classes xxxxListener
• Stubbed base classes xxxxAdapter
• MouseListener
• mouseClicked(), mouseEntered(), mouseExited(),
  mousePressed(), mouseReleased()

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Code

• button1 new JButton(press me)
• myListener new myListenClass
• button1.addMouseListener(myListener)
• // extending a class (subclassing)
• class myListenClass extends MouseAdapter
• public void mouseClicked(MouseEvent e)
• // button clicked, do stuff here
• // OR implementing an interface
• class myListenClass implements MouseListener
• public void mouseClicked(MouseEvent e)
• // button clicked, do stuff here

An abstract base class (methods, no code)
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Event objects

• mouseClicked(MouseEvent e)
• MouseEvent
• getX( ), getY( ), getClickCount( ), getSource( ),
  
• For each listener type
• Component.addxxxxListener( )
• xxxxListener abstract base class
• xxxxAdapter stubbed base class
• xxxxEvent

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Inheritance with Swing

• class myPanel extends JPanel
• public myPanel() //constructor
• // create buttons,
• public void paintComponent(Graphics g)
• super.paint(g) //call overriden method
• // paint stuff here
• myPanel creates JPanel via inheritance
• Override JPanel methods to add functionality

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Simplifying Implements
abstract base class

• class myPanel extends JPanel implements
  MouseListener
• public myPanel() //constructor
• button1 new JButton(press me)
• button1.addMouseListener(this)
• add(button1)
• public void mouseClicked(MouseEvent e)
• // button clicked, do stuff here

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Simplifying Anonymous classes

• class myPanel extends JPanel
• public myPanel()
• button1 new JButton(press me)
• button1.addMouseListener(
• new MouseAdapter()
• public void mouseClicked(MouseEvent e)
• // button clicked, do stuff here
• )
• add(button1)

Defining and instantiating a class on the fly
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In JBuilder

• Application
• JFrame, JPanel, JButton
• Layout managers
• Event listeners

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Homework 3

• Painter Applet