Java

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• Java
• How to Program, 2e
• Deitel Deitel
• Chapter 13
• Multithreading
• Slides by Sylvia Sorkin, Community College of
  Baltimore County - Essex Campus

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Chapter 13 Topics

• Class Thread and Thread methods
• Thread States Life Cycle of a Thread
• Thread Synchronization
• Runnable Interface

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How do you eat breakfast?

• Do you eat all your cereal, followed by all your
  toast, followed by all your coffee?
• SINGLE-THREADING
• Or, do you eat a few spoonfuls of cereal and then
  a bite of toast and then a sip of coffee, and so
  on? MULTI-THREADING

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Concurrent programming

• When several tasks are done at one time we say
  they are done concurrently.
• In Java, concurrent programming is handled by
  threads.
• This is called multitasking, and requires the use
  of a separate thread for each task.

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Time slicing

• Even though the CPU executes one statement at a
  time, it can run multiple threads concurrently by
  rapidly alternating among them. How it does this
  is called scheduling.
• Time slicing is a scheduling technique used in
  Win32 implementations of Java.
• Each thread is given a limited amount of time to
  execute, and when that time expires the thread
  must wait while other threads get their chances
  to use their time in round-robin fashion.

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Inheritance Hierarchy
superclass
Object
Thread
subclass
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java.lang.Thread class
public class Thread extends Object implements
Runnable // Default Constructor public
Thread( ) // Constructor with Runnable
object supplied public Thread( Runnable target
) // Constructor with String supplied
public Thread( String threadName )
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java.lang.Thread class
// Class Methods // Argument ms specifies
how long the currrently // executing thread
should sleep in milliseconds. // Meanwhile
other threads can execute. public static
native void sleep( long ms ) throws
InterruptedException // A call to yield
gives other threads of equal priority // a
chance to execute on non-timesliced systems.
public static native void yield( )
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java.lang.Thread class
// Instance Methods public final String
getName( ) public final int getPriority( )
public void run( ) public final void setName(
) public final void setPriority( ) public
synchronized native void start( ) public final
void stop( )
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Instance Method start
public synchronized native void start( )
// Is used to launch a threads execution. //
Calls method run and immediately returns to its
caller. // The caller executes concurrently
with launched thread.
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Life Cycle of a Thread
notify() notifyAll()
start()
I/O done
Ready
Done sleeping
yield()
Dispatch
sleep()
Sleeping
Running
I/O requested
wait()
Blocked
Waiting
stop()
Dead
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Thread State Java Description

• Ready The thread is ready to run and
  waiting
• for the CPU
• Running The thread is executing on
  the CPU
• Waiting The thread is waiting for some
  event to happen
• Sleeping The thread has been told to
  sleep
• for some time
• Blocked The thread is waiting for I/O to
  finish
• Dead The thread is terminated

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2 Ways to Create a Java Thread
USEFUL WHEN YOU ALREADY HAVE A CLASS THAT YOU
WANT TO CREATE A THREAD FROM
Define a subclass of Thread Have the
class implement the Runnable
interface. Override the default method run( )
You must define method run( )
Pass a runnable object of your class
to a new Thread instance
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Creating a Thread

• One way to create a thread in Java is to define a
  subclass of Thread and overriding the default
  run() method.
• Another way to create a thread in Java is to pass
  a Runnable object to a new Thread instance. The
  objects run() method is invoked automatically as
  soon as the threads start() method is called.

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Morellis Fly subclass definition
import java.awt. // Import GUI
components public class Fly extends CyberPet
implements Runnable // We want the
fly to buzz around in rectangular area //
just outside the spider's web. private
static final int XMIN 225 private static
final int XMAX 300 private static final
int YMIN 245 private static final int YMAX
305 private static final int SIDE 5
// Size of fly private static final int
MAX_RANGE 15 // Max and min change of
location private static final int MIN_DELTA
-10 private CyberPetApplet applet //
Simulation interface private Point location
// Coordinates of fly in applet
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Morellis Fly subclass definition
public Fly (CyberPetApplet app)
applet app location new Point(XMAX,
YMAX) // Starting location state
FLYING public Point getLocation()
return location
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public synchronized void buzzaround()
state FLYING Graphics g
applet.getGraphics() // Erase current
image g.setColor(Color.white)
g.fillRect(location.x, location.y, SIDE,
SIDE) // Calculate new location int
dx (int)(MIN_DELTA Math.random() MAX_RANGE)
int dy (int)(MIN_DELTA Math.random()
MAX_RANGE) if (location.x dx gt XMIN)
location.x dx else location.x
XMIN if (location.y dy gt YMIN)
location.y dy else
location.y YMIN if (location.x dx lt
XMAX) location.x dx else
location.x XMAX if (location.y dy
lt YMAX) location.y dy else
location.y YMAX
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// Draw new image at new
location g.setColor(Color.red)
g.fillRect(location.x, location.y, SIDE, SIDE)
// buzzaround() public synchronized void
die() state DEAD public void run()
while (state ! DEAD)
buzzaround() delay(125) //while
// run() // Fly
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ACKNOWLEDGMENT