Objectives. OO Exceptions (ch( ch.. 10) and a little Applets thrown in. Applets. Understand Applet model Understand Exceptions

Transcription

1 Objectives OO Exceptions (ch( ch.. 10) and a little Applets thrown in CS201 Spring 2005 Week 7 Understand Applet model Understand Exceptions throw-catch block rethrowing Development Methods 2 Applets import java.awt.*;.*; import java.applet.*;.*; Superclass No need to implement main() public class Minimal extends Applet { Development Methods 3 Development Methods 4 1

2 Model in mind Controller keeps track of your applet and its graphics monitoring events, like mouse clicks, window motion, etc. if there is a need to repaint your screen, it calls the paint method on your applet object if there is a mouse motion (mouse click, mouse in, mouse out) calls the appropriate methods on your applet object Development Methods 5 Model (Good enough) method controller() { registers applet and its graphics objects loop { if need to repaint for all registered objects Graphic g = object.graphic object.paint(g); if mouse Down for all registered objects object.mousedown(x,y); if mousemove etc. // forever or until stop Development Methods 6 Applet responsibilities init() read external images, set up the graphics, buttons, text, etc. paint(graphic g) modify the Graphic object mouseclick(event e, int x, int y) you can ignore the Event object if you want, x and y are obvious Problem load an image into an applet mouse over the image determine the color of the pixel under the mouse? Development Methods 7 Development Methods 8 2

3 public void init() public void repaint(graphics g) public class test extends Applet { Image b; public void init() { b= getimage(getdocumentbase(), (), "rocketship.gif"); { g.drawimage(b,0,0,this); Development Methods 9 Development Methods 10 mouse events? public boolean mousedown(event f,int x,int y) { System.out.println("pixel x = " + x + "pixel y = " + y); repaint(); Development Methods 11 Development Methods 12 3

4 Exceptions why? Logic errors array bounds problems Users don't know correct input Devices are not always predictable files, e.g., move Without Exceptions need to set globals pass extra variables hard to understand logic for handling exceptions mixed into problem solving code Development Methods 13 Development Methods 14 Exception objects to the rescue! Exception object describes problem Exception handler invoked to analyze exception object and take action 3 things to understand representation definition of the handler control flow Development Methods 15 Development Methods 16 4

5 Exception representation Derives from Throwable objects Throwable() Throwable (String message) String getmessage() void printstacktrace() Exception representation Unchecked exceptions system errors or runtime errors low memory - you won't handle these Checked exceptions things you or some other class throw Catch or Throw policy networking Streams, etc. - pain Development Methods 17 Development Methods 18 Creating an exception new myexception(" oops") throwing and catching if a problem throw new MyException("oops); Development Methods 19 Development Methods 20 5

6 throwing and catching public class myclass {... public int mymethod() throws MyException { if a problem throw new MyException("oops); else return 1; catching int b; MyClass mc = new MyClass(); b = mc.mymethod(); Development Methods 21 Development Methods 22 catching int b; MyClass mc = new MyClass(); try { b = mc.mymethod(); catch (Exception e) { e.printstacktrace(); Development Methods 23 catching int b; MyClass mc = new MyClass(); try { b = mc.mymethod(); catch (Exception e) { throw e; Development Methods 24 6

7 return to applet Convert the image to a pixel map in init()... int a[] = new int[myheight*mylength mylength]; r = new PixelGrabber(b,0,0,myLength,myHeight,a,0,myLength); try { r.grabpixels(); catch (InterruptedException( e) { System.out.println("problem in grabpixels");... Development Methods 25 get pixel in mouse event // get the pixel where the mouse is int pixelcolor = a[y*mylength + x]; System.out.println("pixelcolor = " + pixelcolor); Development Methods 26 Development Methods 27 Development Methods 28 7

8 For week after break Read Chapter 4 Lab on Monday of return uses this code Development Methods 29 Development Methods 30 Objectives Enhance your understanding of program design methodology Give insights into following stages: problem requirements problem specification program design program implementation and testing Objectives Enhance understanding of specialization inheritance Enhance understanding of specification inheritance Enhance understanding of role specification inheritance plays in event- based programming Development Methods 31 Development Methods 32 8

9 Stages Problem requirements problem specifications software design identifying classes implementation testing Requirements Heating simulation thermostat furnace living area environment (outside the room) Development Methods 33 Development Methods 34 Problem requirements Room initial temp, time T=0 Determine room temp at T+ heat from furnace and loss from environment Operations turn on if temp too low and furnace off turn off if temp too high and furnace on do nothing Rinse and repeat Development Methods 35 Problem Specifications Input/Output document exactly what output produced for EVERY possible input Development Methods 36 9

10 Inputs Started from the command line java HeatingSimulation parameters... in = initial temperature (72.0) out = outside environment (50) set = thermo setting (72) cap = capacity (8500 BTU/hour) eff = efficiency (.95) size = size in sq. ft. (250) freq = number of ticks between output (5) length = seconds to run (7200) Development Methods 37 Execution java HeatingSimulation size=1000 in=65 Development Methods 38 Problem spec on p. 156 model 4 components room of size x, init temp thermo setting furnace cap and eff environment clock how far to advance clock determine new temp using formulas check thermostat adjust furnace accordingly produce a line of output id the classes state behavior diagram Software Design Development Methods 39 Development Methods 40 10

11 Problem spec on p. 156 model 4 components room of size x, init temp thermo setting furnace cap and eff environment clock how far to advance clock determine new temp using formulas check thermostat adjust furnace accordingly produce a line of output Problem spec on p. 156 model 4 components room of size x, init temp thermo setting furnace cap and eff environment clock how far to advance clock determine new temp using formulas check thermostat adjust furnace accordingly produce a line of output Development Methods 41 Development Methods 42 Problem spec on p. 156 See book for classes and behaviors model 4 components room of size x, init temp thermo setting furnace cap and eff environment clock how far to advance clock determine new temp using formulas check thermostat adjust furnace accordingly produce a line of output Development Methods 43 Development Methods 44 11

12 Use of inheritance Use of interfaces Specialization lots of furnaces gas, electric, etc could just include lots of special case code for furnace pilot light on, etc. let's abstract and localize superclass Furnace shared state and common behavior add new behaviors override existing ones Development Methods 45 How do we know when to update? One scenario Room checks clock thermostat checks clock Loop { clock.update(); room.update(); thermostat.update(); until time up Development Methods 46 Use of interfaces Better way clock sends messages all we have to do is make sure interested parties have a Listener method Use Interfaces ClockListener public interface ClockListener { void preevent(double timeinterval); void event(); Development Methods 47 PreEvent() and event() Commonly used technique preevent does a calculation based on state of simulation room check on/off of furnace, check outdoor temp, solve heat loss and gain formulas DONT update state event() update state Development Methods 48 12

13 Example Scissors, paper, rock // method to check 1. check my status (scissor, paper or rock) 2. check other status (scissor, paper or rock) 3. calculate who wins 4. print out winner 5. change to new status Development Methods 49 Ordering problem Rock guy goes first I Win Now I'll be paper Development Methods 50 Ordering problem Paper gal goes I win, I'll become rock PreEvent I Win Next I'll be paper Development Methods 51 I Lose Next I'll be scissors Development Methods 52 13

14 event Interfaces Makes it easy to add new class they just need to implement ClockListener interface To set up the simulation create a clock create room, furnace, etc. clock.addlistener(room) clock.addlistener(furnace) Development Methods 53 Development Methods 54 Development Methods 55 Development Methods 56 14

15 Next time Read 4.5,4.6, 4.7 In particular, while reading 4.5, think about unit tests you might write Development Methods 57 15