TCSS 305 Stepp

1
TCSS 305 (Stepp)

• Animation with Timers and Threads
• Horstmann Ch. 6 pp. 223-225
• Java Tutorial Timers, Threads.http//java.sun.co
  m/products/jfc/tsc/articles/timer/
• http//java.sun.com/docs/books/tutorial/uiswing/mi
  sc/timer.html
• http//java.sun.com/docs/books/tutorial/essential/
  threads/

2
Outline

• timed events
• Timer class
• threads
• multi-threaded programs
• Thread class
• Runnable interface

3
TimersWhy?

• execute an action multiple timesat given
  intervals
• create animations in our GUI programs
• add delays and pauses when required / desired
• a Timer is an example of a "callback" -- your
  code starts the timer, then later at a specified
  time, the timer activates, causing an event in
  your system

4
javax.swing.Timer

• public Timer(int ms_delay, ActionListener
  al)Causes al to fire an actionPerformed every
  ms_delay milliseconds.
• public void start()Causes timer to begin
  ticking the first tick happens after the initial
  delay, and the rest occur separated by the
  timer's delay.
• public void stop()Stops timer from ticking.
• public void restart()Restarts timer, causing it
  to wait its initial delay and then begin firing
  at its delay rate.

5
Timer more methods

• public void addActionListener(ActionListener
  a)Adds additional listeners to be fired as the
  timer ticks.
• public boolean isRunning()Returns true when
  timer is ticking (has been started).
• public void setDelay(int delay)Changes the delay
  between timer ticks.
• public void setInitialDelay(int delay)Sets a
  one-time delay to occur before ticking starts.
• public void setRepeats(boolean b)Set to false to
  make timer fire only once.

6
Timer example

• // This code could be used to move a shape to the
  right
• // across the screen.
• int DELAY 100
• ActionListener updater new ActionListener()
• public void actionPerformed(ActionEvent event)
  
• x
• myPanel.repaint()
• Timer tim new Timer(DELAY, updater)
• tim.start()

7
What is a Thread?

• a "lightweight process" -- a single sequential
  flow of execution within a program
• an isolated subtask inside a program
• a means to implement programs that seem to
  perform multiple tasks simultaneously (a.k.a.
  concurrency, multitasking)

8
A multithreaded program

• 2 or more threads program executes each thread
  sequentially, but interleaves themoverall
  program is concurrent

• 1 thread program executes sequentially

9
Threads in Java Runnable

• interface java.lang.Runnable
• public void run()
• Classes that implement Runnable can be wrapped up
  into Thread objects, which can then concurrently
  execute the Runnable's run method as a new thread

10
Threads in Java Thread

• class java.lang.Thread
• public Thread(Runnable toRun)creates a new
  thread to run the given object's code.
• public void start()executes this thread (if
  based on a Runnable, calls its run method)
• public void join()public void join(int ms)Waits
  the specified amount of time for the thread to
  finish.
• public boolean isAlive()Returns whether this
  Thread is currently running.

11
Thread more methods

• public void setPriority(int pri)Sets this
  thread's priority, which determines how much the
  thread tries to grab system resources, to the
  given amount. Examples are Thread.MAX_PRIORITY,
  Thread.MIN_PRIORITY, or Thread.NORM_PRIORITY.
• public static void sleep(long ms) throws
  InterruptedExceptionCauses the currently active
  thread to stop running and give up control of the
  system for at least the specified number of
  milliseconds, after which time the thread will
  resume running.

12
Thread example using Runnable

• class HelloRunnable implements Runnable
• public void run()
• while (true) // print every 500ms
• try
• Thread.sleep(500)
• catch (InterruptedException ie)
• System.out.println("Hello, world!")
• // elsewhere
• Thread ht2 new Thread(new HelloRunnable())
• ht2.start()

13
Places threads are used

• Graphical animation
• javax.swing.Timer is preferred for this
• event-handling loops
• largely handled for us by Java
• I/O
• loading a file in the background
• Networking
• example thread that waits for another machine to
  connect

14
Thread issues

• synchronization threads that are dependent on
  other threads may have to wait on each other
  (uses Java's monitors, synchronized, wait,
  notify...)
• starvation a thread that uses all the CPU /
  memory resources and leaves none for the other
  threads
• critical section problem what if 2 or more
  threads try to modify the same data
  simultaneously (example remove from an
  ArrayList) ?