1. Systems of Computation
CSC 171 FALL 2001
LECTURE 4

2. History The First Programmer
Ada Augusta King, Countess of Lovelace
translates Menabrea's pamphlet on the Analytical Engine, adding her own notes
the world's first programmer

3. A Whole System
Interface
Implementation of interface
Program

4. Interface // TempConverter.java // Compiled to TempConverter.class
public interface TempConverter {
public double F2C(double f);
public double C2F(double c); }

5. Implementation // MyConverter.java -> MyConverter.class
public class MyConverter
implements TempConverter {
double currenttemp;
public double F2C(double f) {return f*2 ;}
public double C2F(double c){return c/2 ;}
public double F2K(double f){return f ;} }

6. Program public class testConverter {
public static void main(String[] args){
MyConverter m1 = new MyConverter();
double t1 = 90;
double t2 = m1.F2C(t1);
System.out.println(t1 + " F == " + t2 + " C "); }

7. Applets Applications demonstrate event driven programming
We think of programs as “sequences”
Begin at the beginning
Continue to end
Then stop
But some computation “waits around to respond to the environment”

8. The lives of an applet public void init() public void start()
public void paint(Graphics g)
public void stop()
public void destroy()

9. public void init() Called only once Called when applet is loaded
Performs initialization
Instance variables
GUI components
Loading media
Creation of threads

10. public void start() Called after init() completes
Called every time th browser returns to the HTML page where the applet lives
Controls media
Starts animations
Starts threads

11. public void paint(Graphics g)
Called after init() completes and start() begins
Draws on the applet
On the Graphics object “g”
Graphics object “g” is autopassed to paint()
Called when applet is repainted
Window refresh

12. public void stop() Normally called when user leaves HTML page
Stops (suspends) animations and threads
Saves processing power

13. public void destroy() Called when the applet is removed from memory
When the browser exits
Releases allocated resorces

14. Decimal format Consider Output “t3 == 0.3333333333333333”
double t3 = 1.0/3.0;
System.out.println(“t3 == “ + t3);
Output
“t3 == ”

15. Decimal format import java.text.DecimalFormat; DecimalFormat twoDig =
new DecimalFormat(“0.00”);

16. Decimal Format Output “t3 == 0.33” double t3 = 1.0/3.0;
String s1 = twoDig.format(t3);
System.out.println(“t3 == “ + s1);
Output
“t3 == 0.33”