Nub question concerning threads.

Question

I recently completed my first Comp Sci. class in Java and have decided to try to continue learning on my own. I am now trying to make a pong game, using a turorial. Unforunately my course did not cover threads in any way, shape, or form. I am able to get the beginning of the program to work (code posted below) and have no problems with it; however, I do have a few questions concerning what certain elements of the code do.
1. Can someone please give a definition of a Thread in "Dummy" speach? I've looked up definitions and I can't seem to really grasp what they are and what they are used for.
2. In my program there are two lines (32 & 79) where the thread priority is changed. Can someone explain what that means and how it affects my applet?
import java.applet.*;
import java.awt.*;
public class BallApplet extends Applet implements Runnable
     //Declare variables to change posistion
     int x_pos = 10;
     int y_pos = 100;
     int radius = 20;
     boolean rWall = false,
     //The second image of the ball
     private Image dbImage;
     private Graphics dbg;
     public void init() {}
     public void start()
          Thread th = new Thread(this);
          th.start();
     public void stop() {}
     public void destroy() {}
     public void run ()
          Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
          while(true)
               while (rWall == false )
                    if (x_pos == 700)
                         rWall = true;
                    else
                         x_pos= x_pos + 5;
                         // repaint the applet
                         repaint();
                         try
                              // Stop thread for 20 milliseconds
                             Thread.sleep (20);
                         catch (InterruptedException ex)
                             // do nothing
                         // set ThreadPriority to maximum value
                         Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
               while (rWall == true)
                    if (x_pos == 0)
                         rWall = false;
                         lWall = true;
                    x_pos = x_pos - 5;
                    repaint();
                    try
                         Thread.sleep(20);
                    catch (InterruptedException ex)
                    Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
     public void update(Graphics g)
          //initialize buffer
          if (dbImage == null)
               dbImage = createImage(this.getSize().width,this.getSize().height);
               dbg = dbImage.getGraphics();
          //clear screen in background
          dbg.setColor (getBackground());
          dbg.fillRect (0, 0,this.getSize().width,this.getSize().height);
          //draw elements in background
          dbg.setColor (getForeground());
          paint(dbg);
          //draw image on screen
          g.drawImage(dbImage,0,0,this);
     public void paint (Graphics g)
          g.setColor (Color.red);
          g.fillOval(x_pos - radius,y_pos - radius,2*radius,2*radius);
}Thanks for any help, it is greatly appreciated.

Answer 1

Questie,
Please control yourself. Please.??
As the program runs it is one thread. (You canthink
of each thread as an independent program if youlike.
This is not 100% accurate but close enough that
it
might help you understand)
It will only confuse the OP
You are a flat out wrong for a number of reasons.
1) You seemed to miss the part where I did say that a
doubling in peformance is fictious and is dependent
on a number of factors.As i was telling that would only confuse the OP.Its better to be 100% precise when you explain to an OP
2) You don't understand how a multi-process OS works
in the least.Assumptions as usual
3) You don't understand how processors work in the
least.Well,what was i saying?
2 and 3 are important because your point about
context switching is not valid. First of all because
this happens anyway because your program is not the
only thing running the processor. Second because of
the first point each thread gets a timeslice, you may
with multiple threads each get a timeslice and these
slices may (and often do) add up to more total time
then the first got by itself.When you consider a solution to a problem the average and the worst case has more weightage than the best case.You are taking the best case assumption here-"because ofthe first point each thread
gets a timeslice, you may with multiple threads each
get a timeslice and these slices may (and often do)
add up to more total time then the first got by itself".Now dont stop whining that you werent.Never take the best case always coz its the average and worst case which checks if the system festers to a condition beyond rectification
For example (these numbers are fictional but I am
trying to explain it so don't go off all half-cocked
again please)
Program with one thread. Each thread gets 100 cycles
per second.
Program with four threads. Each thread gets 50 cycles
per second.
The second program will finish in half the time as
the first. Because even though each thread is slower
as a whole the program is getting twice the cycles
of the processor as before. You are taking the best case again and moreover its the cpu scheduling algo which decides the time slice and not you sitting on the CPU.I have a vague feeling that you have an idea that you are sitting on the CPU deciding the Time Slice.
You need to understand something important here. Alot
of the time when you are doing things in a program,
and this doesn't just apply to IO here, you don't
have to use the processor for each step. There is
some shifting around of things to get them ready to
be executed by the processor. A more advanced
representation of this is to be found with
Hyperthreading. Hyperthreading processors (single
core) are of course single core but they act as if
they were multi-core (multi processes) because while
only one process is actually executing at one time
the others are pipelined in such a way to make sure
that the executing core of the processor has the
least amount of idle time possible.Digression
So that means on a basic processor each time you add
2 numbers it takes 5 steps. And the processor core is
idle 80% of the time.
From where did you get that 80%?Stop coming up with your own numberes.This is no number game
So for example while some data is being placed in the
outbox new data is also being collected and is at
step 2 already. In this model while we are still on a
single core the core is now idle only 60% of the time[b]From where did you get 60% now
So yes threading is very useful for IO and other
operations that may block your program but they have
more use than just that. Parallel execution can and
will have some performance advantages (if designed
properly) even on a single core processor without
hyperthreading. In a well written multithreaded
program when you add hyperthreading and multiple
cores or multiple processors to the mix then you
really have something.I never said it wont.Read properly what i have posted.I told dont take the best case always.You need to consider that too but the other two carries more weightage
Please don't feel you ever have to post to correct me
again thanks.^^
Get ride of this self conceited attitude.Even if you have it dont make it public.Its not that you are always right cotton.I am not saying that you are wrong.If you have read my first reply i mentioned that i knew that you gave that example just to make the OP understand.The problem with that is that if the OP is unable to fathom out what you mean he is going to misconstrue whatever you have said and that propogates