COP3252 Advanced Java Programming

1
COP3252Advanced Java Programming

• 20-Mar-08
• Lecture Set 17
• Mouse Listeners, Keyboard Listeners, Applets,
  Progress Bars

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Mouse Event Handling

• Java includes two built-in interfaces for
  handling mouse events.
• MouseListener
• MouseMotionListener
• Part of the javax.swing.event package
• Each interface includes a set of methods
  available for working with various mouse events
  (work just like the ActionListener /
  ActionPerformed Interface/Method combo)

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Mouse Handling Interfaces and Methods

• Interface MouseListener
• Methods
• public void mousePressed(MouseEvent event)
• Called when a mouse button is pressed while the
  mouse cursor is on a graphical component
• public void mouseClicked (MouseEvent event)
• Called when a mouse button is pressed and
  realeased when the mouse remains stationary on a
  component. This event is always preceded by a
  call to mousePressed.
• public void mouseReleased (MouseEvent event)
• Called when a mouse button is released after
  being pressed. This event is always preceded by a
  call to mousePressed and one or more calls to
  mouseDragged.
• public void mouseEntered (MouseEvent event)
• Called when a mouse cursor enters the bounds of a
  component
• public void mouseExited (MouseEvent event)
• Called when a mouse cursor exits the bounds of a
  component.

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Mouse Handling Interfaces and Methods

• Interface MouseMotionListener
• Methods
• public void mouseDragged (MouseEvent event)
• Called when the mouse button is pressed while the
  mouse cursor is on a component and the mouse is
  moved while the mouse button remains pressed.
  This event is always preceded by a call to
  mousePressed. All drag events are sent to the
  component on which the user began to drag the
  mouse.
• public void mouseMoved (MouseEvent event)
• Called when the mouse is moved when the mouse
  cursor is on a component. All move events are
  sent to the component over which the mouse is
  currently positioned.

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Mouse Event Handling Example

• See MouseEvents.java

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Making a PaintPanel

• We can use the mouse event handlers to turn out
  JPanel into a PaintPanel
• See PaintPanelBasic.java
• Also PaintPanelRect.java for drawing rectangles
  in a ms paint way

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Handling Keyboard Events

• Just like with mouse events, we can tell our Java
  programs to handle events from the keyboard
• The listener for the keyboard is called
  KeyListener
• Key events are generated anytime a key is pressed
  and released.

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Implementing a KeyListener

• When implementing a KeyListener, we have to
  declare three different methods
• keyPressed
• Handles a key being pressed
• keyReleased
• Handles a key being released
• keyTyped
• Handles an aciton key being typed

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Action Keys

• The following are considered action keys
• Arrow keys
• Home
• End
• Page up
• Page down
• Function keys
• Num lick
• Print screen
• Scroll lock
• Caps lock
• Pause

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Key Events

• Key events are always generated in JAVA
• however, the component that handles the event
  must have the focus
• In other words, the frame where events are being
  handled must be the currently selected frame in
  your operating system.

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KeyEvent Class

• The key event class has several useful functions
  to allow us to pull information about the key
  event.
• int getKeyCode()
• Get the key code associated with this event. The
  key code identifies the particular key on the
  keyboard that the user pressed or released. The
  KeyEvent class defines many key code constants
  for commonly seen keys. For example, VK_A
  specifies the key labeled A, and VK_ESCAPE
  specifies the Escape key.
• String getKeyText(int) and String
  getKeyModifiersText(int)
• Return text descriptions of the event's key code
  and modifier keys, respectively.
• boolean isActionKey()
• Return true if the key firing the event is an
  action key. This information is valid only for
  key-pressed and key-released events.
• int getKeyLocation()
• Return the location of the key that fired this
  event. This provides a way to distinguish keys
  that occur more than once on a keyboard, such as
  the two shift keys, for example. The possible
  values are KEY_LOCATION_STANDARD,
  KEY_LOCATION_LEFT, KEY_LOCATION_RIGHT,
  KEY_LOCATION_NUMPAD, or KEY_LOCATION_UNKNOWN.

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Handling Keyboard events

• See KeyboardEvents.java

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Applets

• An applet is a software component that runs in
  the context of another program
• In JAVA, we create applets in order to allow our
  programs to be accessible from a webpage.

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Applets

• The original Applet class was part of the AWT.
• A newer class called JApplet is included in
  Swing, but many people still use the older Applet
  class.
• The JApplet class makes turning an existing GUI
  program into an applet VERY easy

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JApplet

• Instead of extending JFrame, you want to extend
  JApplet in your program.
• JFrame and JApplet are very similar so, almost
  everything will work without modification
• There are a few exceptions

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Example Changing MouseEvents.java into an Applet

• The first step is to take the MouseEvents.java
  program and rename it to something like
  MouseEventsApplet.java
• Then --- Modify it so your public class extends
  JApplet instead of JFrame

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MouseEventsApplet.java

• If you try to compile the program at this point,
  you will get a few errors
• MouseEventsApplet.java64 cannot find symbol
• symbol method setDefaultCloseOperation(int)
• location class MouseEventsApplet
• setDefaultCloseOperation(JFrame.EXIT_ON_CL
  OSE)
• MouseEventsApplet.java70 incompatible types
• found MouseEventsApplet
• required javax.swing.JFrame
• JFrame frame new MouseEventsApplet()
• 2 errors
• Process completed.
• If the JFrame had a title, you would also get an
  error that looks like this
• EXAMPLE.java

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MouseEventsApplet.java

• The errors are as follows
• The JApplet class doest have a constructor that
  takes in a String for a title (because an applet
  has no title) so, you have to remove that.
• There is no window listener available with
  applets (because there is no close button)
  so, you will have to remove that line also.

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MouseEventsApplet.java

• Your program is no longer extending JFrame, it is
  extending JApplet
• So, we will create a standard JFrame and simply
  add our JApplet to it (to maintain application
  functionality).
• You will be modifying the main section of your
  program to create and customize the main JFrame.

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MouseEventsApplet.java

• Modify the public static void main (String
  args)
• function so that following appear
• //Create a JFrame here instead ....
• JFrame frame new JFrame()
• //Add the MouseEventsApplet to this frame
• frame.add(new MouseEventsApplet())

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MouseEventsApplet.java

• Now, the program should compile without any
  errors.
• Next, you will have to move the class files to
  your webserver. Make sure to set the appropriate
  file permissions on them.
• Finally, you will need to create a webpage to
  display the applet.

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Webpage for the MouseEventsApplet

• width500

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The Applet Webpage

• See http//www.cs.fsu.edu/cop3252/applet.html
• The applet should function just as the
  application did.
• The .class file can still be run as an
  application from a command line using the normal
  java run command.
• This program is now both and applet and an
  application.

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The Sandbox Security Model

• Applets are typically downloaded from non-trusted
  sources on the internet
• What if the applet is malicious?
• Java platform runs all applets in a sandbox to
  prevent applets from accessing local resources
  (such as files).

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The Applet life-cycle

• 5 methods are called by the Applet container from
  the time the applet is loaded into the browser to
  the time that the applet is terminated.
• init, start, paint, stop, destroy
• All but paint have empty bodies by default but
  we can declare these methods in our applet body
  to add functionality.

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public void init()

• Called once by the applet container when an
  applet is loaded for execution.
• Initializes the applet
• Typical actions
• Initialize fields
• Creating GUI components
• Loading sounds
• Loading images
• Creating threads

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public void start()

• Called by the applet container after the method
  init completes execution.
• If user browses to new website and returns, start
  is called again.
• Performs any tasks that must be completed when
  applet is loading for first time and must be
  performed every time the applets HTML page is
  revisited.
• Starting an animation, starting other threads of
  execution

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public void paint (Graphics g)

• Called by the applet after init and start.
• Also called when applet needs to be repainted.
• User covers applet.
• Typical actions
• Drawing with the Graphics object g

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public void stop()

• Called whenever the user leaves the applets web
  page
• Performs tasks that might be required to suspend
  applets execution (so that it does not continue
  processing when user is no longer viewing)
• Example stop animations or execution of threads

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public void destroy()

• Called when applet it removed from memory
• User exits browser session by closing browser
• Also can occur when browser clears memory
• Performs any tasks that are required to clean up
  the resources allocated to the applet.

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Progress Bars

• (Side note Your instructor just discovered this
  in the API today )
• http//java.sun.com/javase/6/docs/api/javax/swing/
  JProgressBar.html
• Example See Progress.java
• See Also
• http//java.sun.com/javase/6/docs/api/javax/swing/
  ProgressMonitorInputStream.html
• http//java.sun.com/javase/6/docs/api/javax/swing/
  ProgressMonitor.html
• http//java.sun.com/docs/books/tutorial/uiswing/co
  mponents/progress.html