Interrupt Confusion

[Arhaan Singhania]

Hello,
This is a sample question, i made wrong on the Enthuware test. It is given below

class A extends Thread
{
	public void run()
	{
		System.out.println("Starting loop");
		while(true){}; //1
		System.out.println("Ending loop");
	}
}
public class TestClass
{
	public static void main(String args[]) throws Exception
	{
		A a = new A();
		a.start();
		Thread.sleep(1000);
		a.interrupt();
	}
}

1- This will not compile.
2- It will run and end cleanly if //1 is replaced with while(!isInterrupted()) { };
//When you call a.interrupt();, the thread's interrupted flag is set to true. And so the while loop ends.
3- It will run and will end with an InterruptedException stack trace on the command line.
4- It will run and will end with an InterruptedException if //1 is replaced with while(!isInterrupted()) { };
//calling interrupt() will cause an InterruptedException ONLY if the thread on which interrupt() is called, is blocked on an invocation of the wait(...), join(...), or sleep(...), methods of this class. 
5- It will run and will end with an InterruptedException stack trace on the command line.

The answer is 1 and 2.
I have understood the working of code but still i have two queries
1-Can someone explain me this code sample in detail. I am having confusion in interrupt and all.
2- What does the explanation of option 4 mean which i have commented.

P.S We needed to pick two options.

Thanks,
Arhaan

[yusuf Kaplan]

Arhaan Singhania wrote:
1- This will not compile.
2- It will run and end cleanly...
The answer is 1 and 2.

Hey, are you sure about the correct answers?
How can it on the one hand not compile and on the other hand run??

[Arhaan Singhania]

Hello Yusuf,

It asked me two options to click.

As you can see in this code block

 while(true){}; //1  
  System.out.println("Ending loop"); 

In line 1, as this block of code will run forever, so there is no way, it can reach ending loop statement which gives an error
of Unreachable code. Second option is how we can replace this while block in order to make this code correct.

There were two options which we needed to select for the question.

Thanks,
Arhaan

[yusuf Kaplan]

I don't understand it.
How can you get the code running without compiling with no errors?

[Arhaan Singhania]

Hello,
Yusuf, this question asked for multiple choices to select. 2 choices.

One first glance, if you look the code, you can very well click that, this code won't compile because of the while
statement. Option 1 and now if you replace this while block with option2, code will compile too and run fine as well.

Did you get me?
Arhaan

[Andreas Svenkson]

Interesting tricky question, I enjoyed reading it.

I'm not sure if you were wondering anything more now? Regarding option 4, you have written yourself why in fact it will not end with an InterruptedException, (because the second thread is not blocked with wait(), sleep() or join()), but rather it is continually running, checking a condition until it is not met.

It's worth noting btw, that if instead of constructing the while-loop with a literal 'true', if you use a boolean variable "b = true", it will compile (and run forever), likewise if the while-loop does check for the literal 'true' but there are no statements following it.

// Andreas

[Arhaan Singhania]

Hello,

What i actually wanted to know was a complete role of interrupt in it. Specially, which thread will go to sleep
will it be main or Thread t. Nothing can be said for sure. So i was looking for a little background of this whole
code example.

Arhaan

[Ankit Garg]

Well the interrupt method call as the explanation says only throws an exception in the target thread if the thread is sleeping or waiting. If the target thread is executing normally, only a flag is set for that thread which can be checked using the call to isInterrupted method...