Advanced Flow of Control

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Advanced Flow of Control

• Joe Komar

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Advanced Flow of Control

• Exceptions -- to catch and process exceptions
  such as I/O errors
• Threads -- multiple paths through the same
  program at the same time

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Exceptions

• Exception -- thrown by program or runtime
  environment and can be caught and handled
• Error -- similar to an exception but represents
  an unrecoverable situation and should not be
  handled
• Java
• Exception and Error classes derived from the
  Throwable class
• Specific exceptions and errors derived from their
  classes

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Exception Execution Flow

• Distinguish Exception flow from Normal flow
• Debugging is made easier (pinpoint errors)
• Java API classes often throw exceptions, letting
  the programmer decide what to do with them
• Normal flow is typically 20 of the total code
• Some programs should never abnormally abort
• Deal with exceptions by
• choosing not to handle them (abnormal abort)
• handle the exception where it occurs
• handle the exception at another point

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Unhandled Exception
// Throws an Exception public class Zero
public static void main (String, args) int
numerator 10 int denominator
0 System.out.println (numerator /
denominator) //method main // class Zero
Java.lang.ArithmeticException / by zero
at Zero.main(Zero.java7)
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The try Statement

• The try statement identifies a block of
  statements that may throw an exception
• There can be a number of catch blocks following
  the try block that can process different types of
  errors

try statement-list catch (exception-class1
variable) statement-list catch
(exception-class2 variable) statement-list
catch .
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Adding Example
import java.io. public class Adding
public static void main (String args)
int num1 User_Reader.get_integer ("Enter a
number ") int num2 User_Reader.get_int
eger ("Enter another number ")
System.out.println ("The Sum is " (num1
num2)) // method main // class Adding
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Adding (contd)
class User_Reader public static int
get_integer (String prompt)
BufferedReader stdin new BufferedReader
(new InputStreamReader (System.in),1)
int number 0 boolean valid false
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Adding (contd)
while (! valid) System.out.print
(prompt) System.out.flush ()
try number
Integer.parseInt (stdin.readLine())
valid true catch
(NumberFormatException exception)
System.out.println ("Invalid input. Try
again.") catch (IOException
exception) System.out.println
("Input problem. Terminating.")
System.exit(0) // while
loop return number // method
get_integer // class User_Reader
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Adding Output
Normal Execution
Exception handling
Enter a number 8 Enter another number
joe Invalid input. Try again. Enter another
number 34 The Sum is 42
Enter a number 6 Enter another number 56 The
Sum is 62
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Exception Propagation

• Exceptions generated at called methods can be
  caught at the calling level
• Method .getMessage() will return the text of the
  exception message
• Method .printStackTrace() prints the stack trace
  showing where the exception occurred
• The throws clause must be used for checked
  exceptions that are not caught and handled

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The throw Statement

• The programmer can throw his/her own exception
• The throw statement causes transfer to the first
  applicable catch clause
• If the thrown variable is not caught, the program
  ends abnormally
• Can be used to catch application level errors and
  exceptions

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The finally Clause

• Executed no matter how the try block is exited
• if no exception is generated, the finally block
  is done after the try block
• if an exception occurs, the appropriate catch
  block is executed and then the finally block
• A finally block should be listed after all catch
  blocks
• Typically used to make sure that some section of
  code is always executed no matter what exceptions
  might occur

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Threads
Concurrent flow
Sequential flow
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Creating Threads

• extend the class Thread
• must then override the run() method of the Thread
  class
• implement the Runnable interface
• must implement the run() method
• must then pass this object as a parameter to the
  constructor of a new Thread object
• advantage of allowing the threaded object to
  belong to its own class heirarchy

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Simultaneous
public class Simultaneous public static
void main (Stringargs) Soda one new
Soda ("Coke") Soda two new Soda
("Pepsi") Soda three new Soda ("Seven
Up") one.start()
two.start() three.start() try
System.in.read() catch (Exception e)
// method main // class Simultaneous
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Simultaneous
class Soda extends Thread private String
name Soda (String str) name
str // constructor public void
run() for (int count 0 count lt 5
count) System.out.println (name)
// method run // class Soda
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Simultaneous Output
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Threads Issues

• Simultaneous access to the same data
• Use the synchronized modifier on a method
• Only one thread can then use that method at one
  time
• Actually locks all synchronized methods for that
  object
• Synchronization without the modifier

synchronized (expression) statement (usually a
block)
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Additional Thread Methods

• suspend() -- temporarily halts execution of a
  thread
• resume() -- resumes a suspended threads
  execution where it left off
• sleep (long milliseconds) -- suspends the thread
  for the specified number of milliseconds

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Bouncing Ball 2
import java.awt. import java.applet. import
java.awt.event. public class Bouncing_Ball2
extends Applet // private final int PAUSE
100000 private final int SIZE 300
private Ball2 ball new Ball2 (150,10,250,200)
private Graphics page private
Control_Panel controls public void init()
setVisible (true) setSize
(SIZE,SIZE) page getGraphics()
page.setXORMode (getBackground()) //
method init
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Bouncing Ball 2
public void start() controls new
Control_Panel (Thread.currentThread())
controls.start() ball.pause()
while (ball.moving()) ball.bounce
(page) // method start // class
Bouncing_Ball2
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Bouncing Ball 2
class Control_Panel extends Thread private
Button suspend new Button ("suspend")
private Button resume new Button ("resume")
private Frame frame new Frame ("Bouncing Ball
Control Panel") private Thread
applet_thread Control_Panel (Thread
applet_thread) this.applet_thread
applet_thread // constructor
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Bouncing Ball 2
public void run () Resume_Action
resume_action new Resume_Action
(applet_thread) Suspend_Action
suspend_action new Suspend_Action
(applet_thread) suspend.addActionListener
(suspend_action) resume.addActionListene
r (resume_action) frame.setLayout (new
FlowLayout()) frame.add (suspend)
frame.add (resume) frame.pack()
frame.setLocation (250, 250)
frame.setVisible (true) // method run //
class Control_Panel
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Bouncing Ball 2
class Suspend_Action implements ActionListener
Thread applet_thread Suspend_Action
(Thread applet_thread)
this.applet_thread applet_thread //
constructor public void actionPerformed
(ActionEvent action) applet_thread.suspen
d() // method actionPerformed // class
Suspend_Action
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Bouncing Ball 2
class Resume_Action implements ActionListener
Thread applet_thread Resume_Action (Thread
applet_thread) this.applet_thread
applet_thread // constructor public
void actionPerformed (ActionEvent action)
applet_thread.resume() // method
actionPerformed // class Resume_Action
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Bouncing Ball 2
class Ball2 private final int MOVE 2
private final float DISTANCE 0.97f private
final int SIZE 20 private final int PAUSE
5 private int x private int
start_y private int end_y private int
length private boolean moving_up true
Ball2 (int new_x, int new_start_y, int
new_end_y, int new_length) x new_x
start_y new_start_y end_y
new_end_y length new_length //
constructor
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Bouncing Ball 2
public void pause() try
Thread.currentThread().sleep(PAUSE)
catch (InterruptedException exception)
System.out.println ("have an exception")
// method pause
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Bouncing Ball 2
void move() if (moving_up)
end_y end_y - MOVE else
end_y end_y MOVE // method move
void draw_ball (Graphics page)
page.drawOval (x-(SIZE/2), end_y, SIZE, SIZE)
page.drawLine (x, start_y, x, end_y)
// method draw_ball
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Bouncing Ball 2
public boolean moving () return
length ! 0 // method moving public
void bounce (Graphics page) for (int
count 1 count lt length count MOVE)
draw_ball (page) pause()
draw_ball(page) move()
moving_up !moving_up
length (int) (DISTANCE length) //
method bounce //class Ball2
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Bouncing Ball2
Bouncing Ball 2 in Action