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Events

Events. Event types Catching different event types Getting information from components and events Distinguishing between events of the same type. The Java event model.

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Events

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  1. Events • Event types • Catching different event types • Getting information from components and events • Distinguishing between events of the same type SEM107, Kamin & Reddy

  2. The Java event model • An event is any occurrence an applets might want to respond to, e.g. user clicks mouse on button, user moves mouse, user enters text into text field. • Java event model is a method of allowing applets to respond to events. • The Java event model is based on classifying events into different types. SEM107, Kamin & Reddy

  3. Event types • We will consider four types of events, and explain how to write an applet to respond to each kind. An applet can also respond to more than one type of event. • Major event types: • Action events, e.g. button click • Item event, e.g. click check box • Mouse event, e.g. mouse button click • Mouse motion even, e.g. mouse moves in applet SEM107, Kamin & Reddy

  4. Action events • Declare applet: implements ActionListener • Register component:component.addActionListener(this); • Required methods:public void actionPerformed (ActionEvent e) Action events are: user clicks on button; user hits enter key in text field. E.g. following applet respond to button click by drawing a rectangle in a darker gray SEM107, Kamin & Reddy

  5. Mouse Motion Events • Declare applet:implements MouseMotionListener • Register component: no component to register; just write: addMouseMotionListener(this); • Required methods:public void mouseMoved (MouseEvent e) public void mouseDragged (MouseEvent e) SEM107, Kamin & Reddy

  6. Mouse event example public class MouseApplet extends Applet implements MouseListener { int red = 255, green = 255, blue = 255; public void init () { addMouseListener(this); } public void paint (Graphics g) { g.setColor(new Color(red,green,blue)); g.fillRect(10,40,100,50); } public void mouseClicked (MouseEvent e) { red = red-10; green = green-10; blue = blue-10; repaint(); } public void mousePressed (MouseEvent e) {} public void mouseReleased (MouseEvent e) {} public void mouseEntered (MouseEvent e) {} public void mouseExited (MouseEvent e) {} } SEM107, Kamin & Reddy

  7. Classes and objects • Every class has objects, or instances, created using new. Think of the class as an object-producing machine. Button new TextField new SEM107, Kamin & Reddy

  8. Example: Point3D Three-dimensional points consist of three values, the x, y, and z coordinates. x z y Point3D new x z y SEM107, Kamin & Reddy

  9. Instance variables containing objects (cont.) Client: Box3D box1 = new Box3D(new Point3D()); box1 c1 c2 x 1 z 1 y 1 x 0 z 0 y 0 SEM107, Kamin & Reddy

  10. Instance methods Methods defined without static keyword are instance methods. Unlike class methods, instance methods can refer to instance variables. class Point3D { double x, y, z; Point3D () { x=0; y=0; z=0; } void print () { System.out.print(x+”,”+y+”,”+z); } } SEM107, Kamin & Reddy

  11. Example: Appointment class (cont.) class Appointment { int time; int duration; String description; Appointment (int t, int d, String s) { time = t; duration = d; description = s; } } Client: dr_appt = new Appointment(9*60+30, 60, “Dr. No”); SEM107, Kamin & Reddy

  12. Instance methods calling instance methods • When an instance method im1 calls another instance method im2, it does not have to name a receiver. By default, the receiver of im2 will be the receiver of im1. boolean overlaps (Appointment appt) { return ((time <= appt.time && endingTime() > appt.time) || (appt.time <= time && appt.endingTime() > time)); } SEM107, Kamin & Reddy

  13. “this” • Methods can refer to the receiver as a whole - instead of just the receiver’s instance variables - by using variable this. • In every instance method in a class C, “this” is implicitly declared to be a variable of type C. SEM107, Kamin & Reddy

  14. “this” (cont.) • More important uses are when instance method needs to pass entire object to another method. E.g. rewrite overlaps: • In applets, “register” component by: b.addActionListener(this); boolean overlaps (Appointment appt) { if (time <= appt.time) return endingTime() > appt.time; else return appt.overlaps(this); } SEM107, Kamin & Reddy

  15. Private instance variables Often, you don’t want clients of a class to be able to access its instance variables. To prevent them from doing so, declare the variables private. class Appointment { private int time; private int duration; private String description; . . . } Client: Appointment dr = new Appointment(...); ... dr.time ... // compile error! SEM107, Kamin & Reddy

  16. Why private instance variables? An Appointment object represents an appointment at a particular time. There are many ways to represent appointments. Here are two alternatives: class Appointment { private int time; private int endtime; ... int endingTime () { return endtime; } . . . class Appointment { private int hour, minute; private int duration; String description; int endingTime () { return hour*60+minute+ duration; } . . . SEM107, Kamin & Reddy

  17. Iteration • Traditional method of repeating statements (not using recursion) loop control{ --------------- --------------- } repeat these statements zero or more times, controlled by loop control SEM107, Kamin & Reddy

  18. While loops contains variables that are changed in loop while (condition) statement repeat until condition becomes true Keep in mind that statement can be a compound statement. SEM107, Kamin & Reddy

  19. For loops init; while (cond) { S incr; }  for (init; cond; incr) S SEM107, Kamin & Reddy

  20. Do-while loops S; while (cond) S do S while (cond)  SEM107, Kamin & Reddy

  21. Aside: Rolling forever • Can have applet that simply rolls eyes forever. However, can’t do it like this:because it would tie up the browser. • Instead, need to relinquish control to the browser occasionally. public void paint(Graphics g) { int i; for (i=0; i<360; i = (i+1)%360) pair1.draw(g, 2*Math.PI*i/360.0); } SEM107, Kamin & Reddy

  22. Aside: Rolling forever (cont.) The new version of RollingEyesApplet: public class RollingEyesApplet6 extends Applet { RollingEyes5 pair1; int lastangle = 0; public void init () { pair1 = new RollingEyes5(200,100); } public void paint(Graphics g) { pair1.draw(g, 2*Math.PI*lastangle/360.0); lastangle = (lastangle+1) % 360; repaint(); } } SEM107, Kamin & Reddy

  23. Example: Slower rolling eyes (cont.) • public void paint(Graphics g) { • int i; • for (i=0; i<360; i = i+1) { • pair1.draw(g, 2*Math.PI*i/360.0); • try { • Thread.sleep(10); • } catch (InterruptedException t) {} • } • } The highlighted portion creates a pause in the middle of a computation, here adding a 10 millisecond delay to each iteration of the loop. SEM107, Kamin & Reddy

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