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How
does the interpreter decide which statement to execute next?
Consider the Java application BallWorld (version 1 from unit 2, with comments removed to make listing shorter):
import
java.awt.*;
public class
BallWorld extends Frame
{
//////////
variables ///////////
private
Ball myBall;
/////////////
methods //////////
static
public void main (String [] args)
{
BallWorld world = new BallWorld ();
world.show ();
}
private BallWorld ()
{
setSize(600, 400);
setTitle("Ball World");
myBall = new Ball(100, 100, 50, 4, 5);
}
public void paint(Graphics g)
{
myBall.paint( g );
pause( 2000 );
System.exit(0);
}
private void pause(int numMilliseconds)
{
try{ Thread.sleep( numMilliseconds ); } catch
(InterruptedException e){}
}
} // class
BallWorld
There are some important questions we
need to know the answers to:
-
How does the Java run-time interpreter
decide which statement to execute first?
-
After executing a statement, how does
the interpreter decide which statement to execute next?
-
How does the interpreter know when to
stop executing statements ?i.e. when does a program terminate ?
Also, most non-trivial Java programs are
composed of 2 or more classes. Version 1 of the unit 2 BallWorld application
consisted of 2 classes: BallWorld and Ball. So we have another question
to answer:
-
How does the interpreter decide from which
class to choose a statement to execute?
All of these questions relate to the control
of statement execution by the Java run-time interpreter. The answer to
some of these questions is determined by the design of the Java programming
language, and the answer to others is that we can design programs to choose
statements according to the rules we wish the program to follow.
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