Threads

1
Threads
2
Thread independent flow of control
e.g. a server needs to communicate with
many customers gt each customer is served by
a separate thread
3
Threads toy example
increments x
increments y
calls repaint
MyApplet
4
Threads toy example
import java.applet.Applet import
java.awt. public class FirstApplet extends
Applet public int x,y public void init()
.... public void paint(Graphics
g) g.setColor(Color.white)
g.setFont(new Font("SansSerif",Font.BOLD,32))
g.drawString("x"x,10,40)
g.drawString("y"y,10,80)
g.drawString("x-y"(x-y),10,120)
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public void init() setBackground(Color.blac
k) new Thread(new Runnable () public
void run() while (true) x
).start() new Thread(new
Runnable () public void run() while
(true) y ).start()
new Thread(new Runnable () public void run()
while (true) try
Thread.currentThread().sleep(40)
catch (InterruptedException e)
repaint() ).start()

• create
• start

6
Runnable interface
run() - method
Thread(Runnable r)
a constructor of the Thread class
7
Anonymous classes
new Thread(new Runnable () public void
run() while (true) y
).start()
instance initializer
new ClassName() ? ?
local variables
8
Time slicing
time
Thread 1
Thread 2
Thread 3
Thread 1
Thread 2
Thread 3
Thread 2
Thread 3
9
Threads - priorities
new Thread(new Runnable () public void
run() try
Thread.currentThread().setPriority(Thread.MIN_PRIO
RITY) catch (IllegalArgumentExce
ption e) catch (SecurityException e)
while (true) x
).start()
MyApplet
10
Synchronization problems
Thread 2
Thread 1
object
Extremely difficult to debug...
11
Synchronization problems toy example
Thread 2
Thread 1
x,dx
MyApplet
while (true) x1000 dx-1 for
(i0ilt1000i) xdx
while (true) x-1000 dx1 for
(i0ilt1000i) xdx
12
Synchronization problems toy example
public void init() new Thread(new Runnable
() public void run() while (true)
x1000 dx-1 for
(i0ilt1000i) xdx
).start() new Thread(new Runnable ()
public void run() while (true)
x-1000 dx1 for
(i0ilt1000i) xdx
).start() .....
13
synchronized blocks of code
Thread 2
Thread 1
object
Do not touch the object.
14
synchronized blocks of code
public void init() new Thread(new Runnable
() public void run() while (true)
synchronized(lock) x1000
dx-1 for (i0ilt1000i)
xdx ).start()
new Thread(new Runnable () public void run()
while (true) synchronized(lock)
x-1000 dx1
for (i0ilt1000i) xdx
).start()
MyApplet
15
synchronized blocks of code
public void init() new Thread(new Runnable
() public void run()
synchronized(lock) while (true)
x1000 t1 dx-1 for
(i0ilt1000i) xdx
).start() new Thread(new Runnable ()
public void run() synchronized(lock)
while(true) x-1000 t2
dx1 for (i0ilt1000i)
xdx ).start()
MyApplet
16
synchronized methods
synchronized ? method(?) ?
? method(?) synchronized(this) ?
17
stop()
deprecated can leave objects it is
manipulating in inconsistent state
pleaseStop()
define your own
18
wait() and notify()
class Pipe LinkedList lnew LinkedList()
public synchronized void Push(Object o)
l.add(o) this.notify() public
synchronized Object Pop() while
(l.size()0) try this.wait()
catch (InterruptedException e)
return l.remove(0)
19
Sleeping threads
try Thread.currentThread().sleep(40)
catch (InterruptedException e)
20
Exceptions
21
Exceptions
method() some unexpected situation can occur
throw an exception
try method() catch (TypeOfTheException e)
// deal with the exception
22
Exceptions
Code dealing with normal execution
try catch (TypeOfTheException e)
Code dealing with exceptions
23
Exceptions are objects
class NoElectricityException extends Exception
NoElectricityException() super(No
electricity!) NoElectricityException(Strin
g message) super(message)
getMessage() method
24
Exceptions are objects
Class Chef ... Food makeRoastBeef(Kitchen
k) if (!k.lightOn())
k.turnLightOn() if (!k.lightOn())
throw new NoElectricityException() ...
...
25
Exceptions are objects
Food prepareDinner() try
fmakeRoastBeef(k) catch
(NoElectricityException e) ?
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Exceptions
Throwable
Error
Exception
RuntimeException
checked
unchecked
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Exceptions examples
InterruptedException ......... RuntimeException
ArithmeticException IndexOutOfBoundExceptio
n ArrayIndexOutOfBoundException
NullPointerException ........... Error
OutOfMemoryError ............
28
Throws
Food makeRoastBeef(Kitchen k) throws
NoElectricityException,
NoMeatException,... ...
checked exceptions
29
try/catch/finally
try catch (TypeOfTheException e) finally

30
EXERCISE
what will be the ouput for a) x1 b) x0
try System.out.println(1) if (x0)
throw(new MyException()) catch (MyException
e) System.out.println(3) finally
System.out.println(2)
31
what will be the ouput for a) MyTest(0)
b) MyTest(1) c) MyTest(2) d) MyTest(3)
void Test(int x) throws YourException try
System.out.println(1) if (x0)
throw(new MyException()) if (x1) throw(new
YourException()) if (x2) return
System.out.println(4) catch (MyException
e) System.out.println(3) finally
System.out.println(2) void MyTest(int
y) try System.out.prinln(6)
Test(y) System.out.println(7) catch
(YourException e) System.out.println(5)

EXERCISE
32
Applets cont.
33
Applet parameters
ltAPPLET CODEMyApplet.class" WIDTH"200"
HEIGHT"200"gt ltPARAM NAMEBALLS VALUE50gt
ltPARAM NAMEMINDIAM VALUE20gt ltPARAM
NAEMMAXDIAM VALUE50gt lt/APPLETgt
.... ballsInteger.parseInt(getParameter(BALLS"))
....
BouncingBall
BouncingBall
34
Why is it flickering?
repaint requests
repaint thread
update()
clears screen
calls paint()
35
Double buffering
repaint requests
repaint thread
calls paint()
update()
clears second-screen draws everything on the
second-screen copies second-screen to the screen
36
BouncingBall revisited
Image offscreenImage Graphics
offscreenGraphics offscreenImage
createImage(width,height) offscreenGraphics
offscreenImage.getGraphics()
offscreenGraphics.setColor(Color.black)
offscreenGraphics.fillRect(0,0,width,height)
g.drawImage(offscreenImage, 0, 0, this)
BouncingBall
BouncingBall