How to synchronised a variable in a class such that the variable will remain each time a thread is run

[hellokitty25]

Hi all

Suppose I have a main method that do the following:
Ball [] ball = new Ball[5];
private int numOfball = 10;

for (int i = 0; i < ball.length i++)
{
ball[i] = new Ball(i,numOfball);
ball[i].start();

numOfball= ball.getNum();

}

Inside the Ball class I have the following:
public class Ball extends Thread
{
private static int numOfball;
private int id;



public Ball(int id,int numOfball)
{
this.id = id;
this.numOfball = numOfball;
}


public synchronized int getNum()
{
System.out.println("Number left:" + numOfball);
if(numOfball>0)
{
System.out.println("Ball" + id + " throw);
Thread.sleep(id);
numOfball--;
System.out.println("Ball " + id + " return." );

}else
{
System.out.println( "There are no more ball!" );
numOfball = 10;
}

return numOfball;
}

public void run()
{
getNum();
}

}
Each time I call the ball class in my main method, the numOfball seem to be the same for a few times.May I know how to make the numOfball in the Ball class such that it will be synchronized and not repeated?

Thank you.

[Norm]

how to make the numOfball in the Ball class such that it will be synchronized and not repeated?

Not sure what "synchronized" means here.
Can you post some output from the program that shows what you are talking about? Add some comments to explain what the problem is.

Please edit your post and wrap your code with code tags:
[code=java]
<YOUR CODE HERE>
[/code]
to get highlighting and preserve formatting.

[Chris.Brown.SPE]

The problem is that you're setting the value of numOfBall in your constructor for the Ball class. This is going to reset your static variable each time. You really shouldn't set the value of static variables in a constructor anyways. What is this program supposed to be doing? If you give us a description it may help to understand what's going on.

[hellokitty25]

Hi all

What I am trying to do is this:

The output should be:
Ball0 throw
Ball6 throw
Ball7 throw
Ball8 throw
Ball0 return
Ball6 return....
When numOfball reach 0 after each Ball throw,it shd display the message "There are no more ball!"

However my output is
Ball0 throws
Ball4 throws
Ball8 throws
Ball4 return
and so on
the numOfballs after each throw is not what I want.
For example after Ball0 throws, the numOfballs is reduced by 1 to 9.But when comes to Ball4 throws the number still remain as 10 in which it should be 9 .

[Norm]

Please post a small complete program that compiles, executes and shows the problem.
Be sure to wrap your code with code tags:
[code=java]
<YOUR CODE HERE>
[/code]
to get highlighting and preserve formatting.

[Chris.Brown.SPE]

This is definitely because you are setting the numOfballs in your constructor for Ball. You are resetting it to 10 with every object. It should be set once at the beginning of the program.

[hellokitty25]

public class TestBall
{
 
    public static void main(String[] args)
	{
	     Ball [] ball = new Ball[5];
 int numOfball = 2;
 
for (int i = 0; i < ball.length; i++)
 {
 ball[i] = new Ball(i,numOfball);
 ball[i].start();
 
numOfball= ball[i].getNum();
 
 }
  }
 
}

import java.util.*;
 
public class Ball extends Thread
 { 
private static int numOfball;
 private int id;
 
 
 
public Ball(int id,int numOfball)
 {
 this.id = id;
 this.numOfball = numOfball;
 }
 
 
public synchronized int getNum()
 {
 System.out.println("Number left:" + numOfball);
 if(numOfball>0)
 {
 System.out.println("Ball" + id + " throw");
 try{
 Thread.sleep(id);
 }catch (InterruptedException e){}
 
 numOfball--;
 System.out.println("Ball " + id + " return." );
 
 }else
 {
 System.out.println( "There are no more ball!" );
 numOfball = 2;
 }
 
return numOfball;
 }
 
public void run()
 {
 getNum();
 }
 
}

Output is:

Number left:2
Ball0 throw
Ball 0 return.
Number left:1
Ball0 throw
Number left:1
Ball1 throw
Ball 0 return.
Ball 1 return.Number left:-1
There are no more ball!
Number left:2
Ball1 throw
Number left:2
Ball3 throw
Number left:2
Ball2 throw
Ball 2 return.
Ball 1 return.
Ball 3 return.
Number left:-1
There are no more ball!
Number left:-1
Ball4 throw
Ball 4 return.
Number left:1
Ball4 throw
Ball 4 return.


There should be only one ball0 throw not a repeat of ball0 throw. the same go for the rest.Also Number left:1 should be only one time after reduced by one each time a thread is run

[Norm]

Why does each constructor set the value of the static variable? Each one is changing the value that the last one set.
There is only one copy of a static variable shared among all the class objects.

BTW The formatting of the posted code is very poor which makes the code hard to read and understand.

[hellokitty25]

Hi in the main program there is a return value which will change the value
numOfball= ball[i].getNum();

[Chris.Brown.SPE]

You are setting your static variable in the constructor. This is wrong unless you have some sort of justification for it. Can you confirm you understand this error?

[hellokitty25]

If I do not set it,is there any other ways to get the numOfball in the main program?

[Norm]

ways to get the numOfball in the main program

Can you describe what values the numOfball variable is supposed to hold? Should each instance of the Ball class have its own value? Making it static means that all instances share the ONE variable.

If you wanted a method to have access to a static variable, create a static getVariable() method that returns its value.

[Chris.Brown.SPE]

Make a static setter for numOfball

Example:

	public static void main(String[] args) {
 
		Book.setTitle("Test");
	}
 
 
	private static String title;
 
	public static void setTitle(String newTitle){
	       title = newTitle;
	}

[hellokitty25]

Hi all instances should shared one value.That is the numOfball will be shared by the instances each time a thread is started.

[Norm]

Then why does the constructor set the value of numOfball? Each call of a constructor will change its value?

Why not set it one time from outside of the constructor?

[hellokitty25]

Hi In my main program the inital value is 2 and I need to pass it to the class as shown below:
public static void main(String[] args)
{
Ball [] ball = new Ball[5];
int numOfball = 2;
for (int i = 0; i < ball.length; i++)
{
ball[i] = new Ball(i,numOfball);
ball[i].start();

numOfball= ball[i].getNum();

}


I try to use the set method to set the value.But the output is still the same.

[Norm]

Why does the code copy the value of numOfball from the Ball object it just created?

WHere is the code that calls a set method ONE TIME to set the value of numOfball that all the Ball objects to use.

Can you explain what the numOfball value is supposed to be used for?

[hellokitty25]

Hi To cut short:

What I want is the Ball class which extend Thread and the main method which start the thread.
The output should be given that the numOfball initial value is 2 and after 2 it will have numOfball to 2 again :
Ball 1 throw
Ball 4 throw
There are no more ball!
Ball 3 throw
Ball 4 return
Ball 3 return
Ball 2 throw
There are no more ball!
Ball 1 return
Ball 5 throw

[Norm]

What is the value of numOfball used for?

What does a message with "throw" mean? How does that message relate to the value of numOfball?

Please edit your post and wrap your code with code tags:
[code=java]
<YOUR CODE HERE>
[/code]
to get highlighting and preserve formatting.

Also the formatting of the code needs to be fixed. The indentations are not right and make the code hard to read and understand.