CS242 Advanced Programming Concepts in Java

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CS242Advanced Programming Concepts in Java

• 9/27/07
• Painting a GUI

Prof. Searleman jets_at_clarkson.edu
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Announcements

• IBM Master the Mainframe Contest
• Registration is now open Contest starts Oct. 1st
  
• over 17,000 in prizes, including T-shirts,
  pre-paid debit cards, Nintendo Wii systems and
  trips to the mainframe lab in Poughkeepsie, NY.
• No mainframe or large systems experience is
  necessary.
• Contest homepage ibm.com/university/contest
• Clarkson Student Chapter of the ACM
• Why didnt you tell me this 8.923 years ago?
  Dr. Richard Sharp, Thursday, Oct.
  4th, 730 pm, Snell 212
• Java game challenge

a horrible death to die
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Outline

• Event-Driven Programming
• Event Object, Event Sources and Event Listeners
• Action Handling
• examples ButtonGUI ButtonPanel
• Identifying the Event Source
• getSource() and getActionCommand()
• Custom Painting
• PaintComponent()
• Graphics Context
• Toolkit
• HW3 due today
• Exam1 Thursday, Oct. 11, SC356, 700 pm

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Exam1 concepts

• Object-Oriented Programming
• polymorphism
• inheritance vs. composition
• Objects and Classes
• references, static, public/private, final,
  immutable
• casting
• Methods
• constructor, accessor, mutator
• this, this(), super, super()
• toString(), equals() hashCode(), compareTo()
  interactions
• Interfaces
• Arrays
• Exception Handling
• throwing, propagating, try/catch

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Recap Basic Swing code

• Import the pertinent packages
• Set up a top-level container.
• Display the container.
• Be thread-safe.
• javax.swing.SwingUtilities.invokeLater(
• new Runnable()
• public void run()
• / create and show the GUI /
• // end run
• // end Runnable
• ) // create and run a thread

Why thread-safe? Show example of non-responsive
GUI.
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Recap Action Handling

• A simple form of event handling just one action
  to perform when the event occurs
• Uses interface ActionListener
• ActionListener has only one method to implement
  actionPerformed()

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Example 2 ButtonGUI
Action to perform count show the number of
times the button was clicked
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exampleButtonGUI
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public class ButtonGUI implements ActionListener
private static String labelPrefix "Number
of button clicks " private int numClicks
0 final JLabel label new
JLabel(labelPrefix "0 ") public
Component createComponents() JButton
button new JButton(Click Me!")
button.addActionListener(this)
label.setLabelFor(button) JPanel pane
new JPanel(new GridLayout(0, 1))
pane.setBorder(BorderFactory.createEmptyBorder(30,
30, 10, 30)) pane.add(button)
pane.add(label) return pane
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// public class ButtonGUI // implements
ActionListener (cont) public void
actionPerformed(ActionEvent evt)
numClicks label.setText(labelPrefix
numClicks)
3. When the button is clicked, an ActionEvent
object is created and sent as arg evt in method
actionPerformed in listener ButtonGUI The
desired response is to increment the instance
variable numClicks and update the JLabel. (The
info in the event object is ignored)
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private static void createAndShowGUI() JFrame
frame new JFrame(ButtonClicks")
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOS
E) ButtonGUI interface new ButtonGUI()
Component contents interface.createCom
ponents() frame.getContentPane().add(contents,
BorderLayout.CENTER) frame.pack()
frame.setVisible(true) public static void
main(String args) javax.swing.SwingUtilities
.invokeLater(new Runnable() public void
run() createAndShowGUI() )
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Example 3 ButtonPanel
Action to perform paint the background red when
the red button is clicked or paint it blue when
the blue button is clicked
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exampleButtonPanel
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a ButtonPanel object will be the event listener
public class ButtonPanel extends JPanel
implements ActionListener private JButton
button1 private JButton button2 public
ButtonPanel() setLayout(new
BoxLayout(this, BoxLayout.X_AXIS))
setBorder(BorderFactory.createEmptyBorder(30,30
,30,30)) button1 new JButton("red")
button2 new JButton("blue")
event sources are two buttons, stored as instance
variables
the constructor sets the layout border for the
panel, and creates the 2 buttons
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// constructor for ButtonPanel(), cont
add(Box.createHorizontalGlue())
add(button1) add(Box.createHorizontalStrut(
15)) add(button2) add(Box.createHorizontal
Glue()) button1.addActionListener(this)
button2.addActionListener(this)
add the 2 buttons to the panel, with some glue
and struts between them
register the panel as a listener for both button1
and button2
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// ButtonPanel (cont) public void
actionPerformed(ActionEvent evt)

// Action to perform make the background // red
if the red button was clicked, or // blue if
the blue button was clicked
question how does the panel tell which button
was clicked?
answer that information is encapsulated in the
event object, evt
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Identifying the event source

• There are two methods
• getSource()
• getActionCommand()

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java.util.eventObject

• public Object getSource()
• Returns
• The object on which the Event actually occurred.

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// ButtonPanel (cont) public void
actionPerformed(ActionEvent evt) Color
color getBackground() Object source
evt.getSource() if (source button1)
color Color.red else if (source
button2) color Color.blue
setBackground(color) repaint()
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private static void createAndShowGUI()
JFrame frame new JFrame("ColorSelector")
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOS
E) frame.getContentPane().setLayout(new
GridLayout(0,1)) ButtonPanel toolBar new
ButtonPanel() frame.getContentPane().add(toolBa
r) frame.pack() frame.setVisible(true)
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java.awt.event.ActionEvent

• public String getActionCommand()
• Returns the command string associated with this
  action. This string allows a "modal" component to
  specify one of several commands, depending on its
  state. For example, a single button might toggle
  between "show details" and "hide details". The
  source object and the event would be the same in
  each case, but the command string would identify
  the intended action.

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javax.swing.AbstractButton

• public void setActionCommand(String actionCommand)
  
• Sets the action command for this button.

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// Version 2 of the GUI (based on action
command) public class ButtonPanel extends JPanel
implements ActionListener // need to
change the constructor public ButtonPanel()
setLayout(new BoxLayout(this,
BoxLayout.X_AXIS))
setBorder(BorderFactory.createEmptyBorder(30,30,30
,30)) button1 new JButton("red")
button1.setActionCommand(redBackground)
button2 new JButton("blue")
button2.setActionCommand(blueBackground)
// the rest of the constructor stays the same
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// Version 2 need to change actionPerformed publ
ic void actionPerformed(ActionEvent evt)
Color color getBackground() String
cmd evt.getActionCommand() if
(cmd.equals(redBackground) color
Color.red else if (cmd.equals(blueBackgrou
nd) color Color.blue
setBackground(color) repaint()
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Things to remember

• event handlers can be instances of any class
• event handling code executes in a single thread
  (event-dispatching thread). This ensures that
  each event handler will finish executing before
  the next one starts. Painting code also occurs
  in the event-dispatching thread, so while the
  actionPerformed() is executing, the GUI is frozen
  (e.g. cant be repainted or respond to mouse
  clicks)
• event handlers MUST execute very quickly if only
  a few lines of code, can be implemented as an
  anonymous class

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Painting a GUI

• When a Swing GUI needs to (re)paint itself
• starts with the highest component works down
  the containment hierarchy
• Swing uses a repaint manager to repaint
  lightweight components
• Swing uses double-buffering by default
• a component should not be interrupted in the
  middle of (re)painting itself
• the repaint() method is thread-safe

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Painting is an Event

• like event-handling code, painting code executes
  on the event-dispatching thread
• while an event is being handled, no painting will
  occur, and
• if painting takes a long time, no events will be
  handled during that time

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Painting a JComponent

• each JComponent is responsible for painting three
  items (invoked in this order)
• 1. protected void paintComponent(Graphics g)
• - this is what to override to do custom painting
• - do NOT override the paint() method
• 2. protected void paintBorder(Graphics g)
• - do NOT override or invoke this method
• 3. protected void paintChildren(Graphics g)
• - do NOT override or invoke this method

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Painting a JComponent

• in other words, the JComponent first paints
  itself, then its border, then each of the
  children contained in it
• each component has a boolean property opaque
• - if false, then is transparent
• - if true, then the rectangular painting region
  is completely filled with its background color
  before it is rendered
• (i.e. it obscures anything under it)

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Example Painting ButtonPanel
GUI
Containment Hierarchy
Action to perform paint the background red when
the red button is clicked or paint it blue when
the blue button is clicked
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• When the GUI for ButtonPanel is painted, here's
  what happens
• The top-level container, JFrame, paints itself.
• The content pane first paints its background,
  which is a solid gray rectangle. It then tells
  the JPanel to paint itself. In most look and
  feels, the JPanel is opaque by default and the
  content pane's background rectangle doesn't
  actually appear in the finished GUI, being
  completely obscured by the JPanel.

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• 3. Next, the JPanel it paints its border.
  Finally, the panel asks its children to paint
  themselves.
• To paint themselves, each JButton paints its
  background rectangle, if necessary, then the text
  that the button contains, and then its border. If
  the button has the keyboard focus, meaning that
  any typing goes directly to the button for
  processing, the button does some
  look-and-feel-specific painting to make clear
  that it has the focus.

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note that JComponent optimizes its repainting
time if none of its children overlap (since
repaint manager does not have to compute the
hidden and visible areas for each child before
rendering them). 5. When a button is clicked,
the event is handled by changing the background
color of the panel and repainting it.
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Painting a transparent component

• When repaint is invoked on a JComponent such as a
  label that is both visible and non-opaque
• 1. Code inherited from JComponent causes the
  non-opaque component to look through its
  containment hierarchy to find the closest
  containing component that's completely opaque.
  For example, if the transparent component is a
  label in a transparent panel in a content pane,
  then the label's closest opaque container is the
  content pane.

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2. The opaque container paints itself. 3. The
children of the opaque container are asked to
paint themselves. Eventually, the transparent
component is painted. note transparent
components are expensive to render, so it can
help performance to make a component
opaque setOpaque(true)
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Graphics Context

• protected void paintComponent(Graphics g)
• the Graphics object g encapsulates graphics
  settings for drawing images text, and it
  provides some simple 2D methods for painting
• state current color, current font, current
  painting area
• methods (from Graphics2D) drawString(),
  drawRect(), fillRect(), drawImage(),

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Coordinate System
You can retrieve a Swing components current
position and size through the location (Point)
and size (Dimension) properties of JComponent
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JFrame frame new JFrame(test) frame.setBounds
(20,20,200,200) frame.setVisible(true) Rectangl
e r new Rectangle() r frame.getBounds(r) Sys
tem.out.println(X r.x()) System.out.println(
Y r.y()) System.out.println(Width
r.width()) System.out.println(Height
r.height())
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System.out.println(X frame.getX()) System.ou
t.println(Y frame.getY()) System.out.println
(Width frame.getWidth()) System.out.println(
Height frame.getHeight()) Point ulPoint
new Point() ulPoint component.getLocation(ulPoi
nt)
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Specifying sizes of components

• setSize() layout manager may reset this
• setPreferredSize() suggestion to the layout
  manager
• setMinimumSize() smallest size for the
  component when it is in a container
• setMaximumSize() - similar

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/ _at_version 1.20 25 Mar 1998 _at_author Cay
Horstmann / import java.awt. import
java.awt.event. import javax.swing. class
NotHelloWorldPanel extends JPanel public
void paintComponent(Graphics g)
super.paintComponent(g) g.drawString("Not a
Hello, World program", 75, 100)
When you do custom painting, the first line in
your paintComponent SHOULD ALWAYS BE
super.paintComponent(g)
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class NotHelloWorldFrame extends JFrame public
NotHelloWorldFrame() setTitle("NotHelloWorld
") setSize(300, 200)
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
Container contentPane getContentPane()
contentPane.add(new NotHelloWorldPanel())
public class NotHelloWorld public static
void main(String args) JFrame frame
new NotHelloWorldFrame() frame.show()

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Size Placement of a JComponent

• public abstract class Toolkit
• Abstract superclass of all implementations of the
  Abstract Window Toolkit (i.e. java.awt).
• Methods include
• Toolkit getDefaultToolkit()
• Dimension getScreenSize()
• int getScreenResolution()
• etc.
• example CenteredFrame

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/ _at_author Cay Horstmann / public CenteredFrame
extends JFrame public CenteredFrame()
setTitle("CenteredFrame")
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
Toolkit tk Toolkit.getDefaultToolkit()
Dimension d tk.getScreenSize() int
screenHeight d.height int screenWidth
d.width // make this window 1/4th the
area of the screen setSize(screenWidth / 2,
screenHeight / 2) // center the window on
the screen setLocation(screenWidth / 4,
screenHeight / 4) // add an icon to the
window title bar Image img
tk.getImage("icon.gif") setIconImage(img)