 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.

 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 (“it’s a ” + x.getText() + “ button”); } } } Note: A single listener can repond to events from multiple objects

 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

class KeyPanel extends JPanel { private int x=100, y=100; String keyChar = ‘x'; public KeyPanel() { setFocusable(true); // Doesn’t work for JApplets addKeyListener(new KeyAdapter() { public void keyPressed(KeyEvent e) { switch (e.getKeyCode()) { case KeyEvent.VK_DOWN: y+=10; break; case KeyEvent.VK_UP: y-=10; break; case KeyEvent.VK_LEFT: x-=10; break; case KeyEvent.VK_RIGHT: x+=10; break; default: keyChar = “” + e.getKeyChar(); } repaint(); }}); } public void paintComponent(Graphics g) { super.paintComponent(g); g.drawString(keyChar,x,y);}}

 Differences from KeyListener objects  Don’t 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 won’t get focus  Implementation KeyboardFocusManager manager = KeyboardFocusManager.getCurrentKeyboardFocusManager(); manager.addKeyEventDispatcher(new KeyEventDispatcher() { public boolean dispatchKeyEvent(KeyEvent event) { /* code goes here; return true; } });

 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.

 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

public class MovingPanel extends JPanel implements ActionListener { private String message = “Welcome to Java” private int x=0, y = 20; public MovingPanel() {Timer timer=new Timer(1000,this); timer.start(); } public void paintComponent(Graphics g) { super.paintComponent(g); g.drawString(message, x, y); } public void actionPerformed(ActionEvent e) { if (x>getWidth() { x = -20; } x+=5; if (y>=getHeight() – 20) y = 20; y+=5; repaint(); }

int data[][] = new int[4][4], count = 1; for (int row=0; row<data.length; row++) for (int col=0; col<data[row].length; col++) data[row][col] = count++; OR int[][] data = {{1,2,3,4}, {5,6,7,8}, {9,10,11,12}, {13,14,15,16} }; for (int row=0; row<data.length; row++) { for (int col=0; col<data[row].length; col++) System.out.printf("%4d", data[row][col]); System.out.println(); } [[0][1][2][3] [0]1234 [1]5678 [2] [3]

count=1; int values[][] = new int[4][], cols[] = {2, 4, 1,3}, count = 1; for (int row=0; row<values.length; row++) { values[row] = new int[cols[row]]; for (int col=0; col<values[row].length; col++) values[row][col] = count++; } for (int row=0; row<values.length; row++) { for (int col=0; col<values[row].length; col++) System.out.printf("%4d", values[row][col]); System.out.println(); } Definition: Different rows have different numbers of columnt

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.

 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?

 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

 First dimension: month  Second dimension: day  Third dimension: hour  Fourth dimension : Type of measurement [0] = temperature, [1] = humidity  Symbolic constants: public final static int T=0, H=1;  Questions:  To what does the following refer? int x=3, y=4, z=5, k=32, m=50; data[x][y][z][T] = 32; data[x][y][z][H] = 50;  How do we instantiate such an array?