Small java challange

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1

Its a small challange made by me, I wanna see how fast people can figure out whats missing in order to make this code work properly, good luck :slight_smile:
What the code does is printing how many loops were made after 10 seconds basically.

import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Test
{
	private int a;
	private long b;
	private ScheduledThreadPoolExecutor c = new ScheduledThreadPoolExecutor(1);
	
	public Test()
	{
		c.scheduleAtFixedRate(new RunnableWrapper(), 0, 1000, TimeUnit.MILLISECONDS);
		
		while (a < 10)
			b++;
		
		System.out.println("Loops: " + b);
	}
	
	private class RunnableWrapper implements Runnable
	{
		@Override
		public void run()
		{
			a++;
		}
	}
	
	public static void main(String[] args)
	{
		new Test();
	}
}
2

I don’t know how Java handles it, but it seems to me you didn’t initialize your variables a and b ?

And I suppose that scheduled runnable stops running automatically once the script is completed?

3

I don’t know how Java handles it, but it seems to me you didn’t initialize your variables a and b ?

In java variables are initilized automatically (in this case, a and b are 0 in the beginning)

And I suppose that scheduled runnable stops running automatically once the script is completed?

It does, but the problem with the code is that the script never finishes (infinite loop) :slight_smile:

4

ok :slight_smile:

It does, but the problem with the code is that the script never finishes (infinite loop) :slight_smile:

Why? Reading “scheduleAtFixedRate” I thought it would schedule that function to be run every 1 seconds, so after 10 seconds a would be 10? And the while loop would be exited?

5

Why? Reading “scheduleAtFixedRate” I thought it would schedule that function to be run every 1 seconds, so after 10 seconds a would be 10? And the while loop would be exited?

Well thats excactly the challange, it never exits :slight_smile:

6

Weird challenge…

Here’s a real challenge. Make a Java class that inherits from multiple classes. I mean a real class that inherited all methods from 2 different class. I’ve actually did this for fun during presentation… it was fun to see java developers reaction when I showed a live demo. If you’re curious then look up AOP Mixin or introduction… some crazy stuff that you’ll never use in real project.

7

why do you claim that this challange is weird? have you even tried getting it solved? so far I got no solutions and in fact this is something developers should know.

8

Saw this answer with a 2 week delay :smiley:

But why doesn’t it exit? Is it a scope problem? The value of a is changed in RunnableWrapper, but this can’t be seen in Test() ?

9

But why doesn’t it exit? Is it a scope problem? The value of a is changed in RunnableWrapper, but this can’t be seen in Test() ?

Not excactly a scope problem, if you’ll print a inside the loop the code will work properly and you’ll see the value increases as it should.

a small hint:
The problem comes when a is not involved anywhere within the while statement, so basically the compiler ‘thinks’ that a will never reach 10 and it changes the statement to the following:

while (true)
	b++;

making the code never end.

There’s a special keyword that needs to be added to prevent this, nuff hints :slight_smile:

10

You could add a break statement, but that’s not very elegant.
Too much intelligence in a compiler is asking for trouble :lol:

11

Well since there hasn’t been an answer for a while ill say it :slight_smile:
To make the compiler aware of changes done in such situations on a variable the volatile keyword needs to be added on var a.
this tells the compiler that the var might be changed from outside of its class so its not automatically turns it to true

private volatile int a;
12

I misunderstood. I thought the answer to challenge is not available since it’s working as intended.

13

I would’ve done

private static int a;

14

I’ve googled to find the solution, but all I could find was sites that explain that if the value isn’t changed in the loop itself, it defaults the while criteria to true. I’d say that’s something that should be pointed out when explaining while loops?