Events and listeners

1
Events and listeners

• Definition An event is an object thrown in
  response to a user of programmatic action
• Definition A listener is a series of methods
  that executes in response to an events
• Definition An adapter is a class that has
  default listener methods
• Implementation
• Implement the listener interface or extend an
  adapter class
• Create listener methods appropriately
• Register the listener to a component or a
  listener manager
• Note An alternate implementation is to create an
  anonymous listener object passed as an argument
  when registering.

2
Actionlistener example

• Listen for button clicks
• public class MyPanel extends JPanel
  implements ActionListener
• Jbutton b
• public MyPanel() JButton b new
  JButton(hello)
• add(b)
• b.addActionListener(this)
• public void actionPerformed(ActionEvent e)
• if (e.getSource() instanceof JButton)
• JButton x (JButton)(e.getSource())
  System.out.println (its a
  x.getText() button)

Note A single listener can repond to events from
multiple objects
3
Key Listener

• Purpose
• Listen for user keyboard entries
• Limitation the component must have focus
• Implementation
• Create keyPressed, keyTyped, and keyReleased
  methods if using the implements KeyListener
  interface
• Create only the methods needed if using a
  KeyAdapter
• Register the listener with the component

4
Anonymous Keylistener example

• class KeyPanel extends JPanel
• private int x100, y100 String keyChar x'
• public KeyPanel()
• setFocusable(true) // Doesnt work for
  JApplets
• addKeyListener(new KeyAdapter() public
  void keyPressed(KeyEvent e) switch
  (e.getKeyCode()) case KeyEvent.VK_DOWN
  y10 break
• case KeyEvent.VK_UP y-10 break
• case KeyEvent.VK_LEFT x-10 break
• case KeyEvent.VK_RIGHT x10 break
• default keyChar e.getKeyChar()
  
• repaint() )
• public void paintComponent(Graphics g)
  super.paintComponent(g) g.drawString(keyChar,x,y)
  

5
KeyEventdispatcher

• Differences from KeyListener objects
• Dont need to worry about whether a component has
  focus
• Monitors keyboard events before they are sent to
  components with focus
• The listener method is dispatchKeyEvent, not
  keyPressed
• Register the Listener with the system
  KeyboardFocusManager
• Useful for applets, because applet JPanels wont
  get focus
• Implementation
• KeyboardFocusManager manager
  KeyboardFocusManager.getCurrentKeyboardFocusManage
  r()
• manager.addKeyEventDispatcher(new
  KeyEventDispatcher()
• public boolean dispatchKeyEvent(KeyEvent
  event)
• / code goes here
• return true )

6
Other listeners available

• Mouse Listener Events Mouse operations
• public void mousePressed(MouseEvent e)
• public void mouseReleased(MouseEvent e)
• public void mouseEntered(MouseEvent e)
• public void mouseExited(MouseEvent e)
• public void mouseClicked(MouseEvent e)
• Mouse Motion Listener Movements of the mouse
• public void mouseDragged(MouseEvent e)
• public void mouseMoved(MouseEvent e)
• Item Listener State of an object changed
• Public void itemStateChanged(ItemEvent e)
• Even more FocusListener, WindowListener, etc.

7
threads

• Definition A path of execution through a process
• Any Class can be a thread if it
• implements runnable
• provides a run method (public void run())
• A thread starts by calling start(), which in turn
  calls run()
• A thread ends when the run method exits
• Be careful
• Threads all run concurrently order of execution
  is not known
• One thread can step on variables set in another
  thread

8
The timer class

• public class MovingPanel extends JPanel
  implements ActionListener
• private String message Welcome to Java
• private int x0, y 20
• public MovingPanel() Timer timernew
  Timer(1000,this)
• timer.start()
• public void paintComponent(Graphics g)
• super.paintComponent(g)
• g.drawString(message, x, y)

• public void actionPerformed(ActionEvent e) if
  (xgtgetWidth() x -20
• x5
• if (ygtgetHeight() 20) y 20
• y5
• repaint()

9
Multidimensional Arrays
0 1 2 3
0 1 2 3 4
1 5 6 7 8
2 9 10 11 12
3 13 14 15 16

• int data new int44, count 1
• for (int row0 rowltdata.length row)for (int
  col0 colltdatarow.length col)
  datarowcol count
• OR
• int data 1,2,3,4, 5,6,7,8,
  9,10,11,12, 13,14,15,16
• for (int row0 rowltdata.length row)
• for (int col0 colltdatarow.length col)
  System.out.printf("4d", datarowcol)
• System.out.println()

10
Ragged Array
Definition Different rows have different numbers
of columnt

• count1
• int values new int4, cols 2, 4,
  1,3, count 1
• for (int row0 rowltvalues.length row)
• valuesrow new intcolsrow
• for (int col0 colltvaluesrow.length
  col)
• valuesrowcol count
• for (int row0 rowltvalues.length row)
• for (int col0 colltvaluesrow.length col)
• System.out.printf("4d",
  valuesrowcol) System.out.println()

11
Closest pair problem

• public double distance(Point first, Point second)
• double delta1 Math.pow(first.x second.x,
  2)
• double delta2 Math.pow(first.y second.y),
  2
• return Math.sqrt(delta1 delta2)
• Consider a two dimensional array, data
• Problem Find the pair of points closest together
  using the above method.

12
Application for the class project

• Moving sprites in four dimensions
• int directions 0,1, 1,0, 0,-1,
  -1,0
• Moving sprites in eight directions
• int directions 0,2, 1,1, 2,0,
  1,-1, 0,-2, -1, -1, 0,-2, -1,1
• Moving sprites in sixteen directions
• int directions 4, 0, 3, 1, 2,
  2, 1, 3, 0, 4, -1, 3, -2, 2, -3,
  1, -4, 0, -3,-1, -2,-2, -1,-3,
  0,-4, 1,-3, 2,-2, 3,-1
• Questions
• What is the relation between the directions and
  the one that is half away in the array?
• How would we extend this to twice the size?

13
Another possible game application

• Divide the panel into row and column cells.
• Define rules for what happens if predators or
  avatars intersect the various cells
• Examples of possible game rules
• Move them automatically to another place
• Have the game shift to another room
• Give the players points or create additional
  avatars
• Kill the avatar that hits that particular cells
• The rules for avatars may vary from that of
  predators

14
Higher dimensional array

• First dimension month
• Second dimension day
• Third dimension hour
• Fourth dimension Type of measurement0
  temperature, 1 humidity
• Symbolic constants public final static int T0,
  H1
• Questions
• To what does the following refer?int x3, y4,
  z5, k32, m50dataxyzT 32
• dataxyzH 50
• How do we instantiate such an array?