For and While Loop

Question

I have a program that runs simultaneously and I have this problem where I want to stop the thread but the for loop/while loop doesn't get cancelled once I once I click enter

If I take the for loop out of the while loop, the program actually responds to the enter and shuts down.

class MyNumber extends Thread {

    private volatile boolean processing = true;

    public void run() {
        while (processing) {
            // Once I take this for loop out(put // beside it like right now), the enter key to stop the program then does work.
            //for(int i = 1; i<27; i++){
             System.out.println("Letter " + "i");

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // }
        }
    }

    public void permSleep() {
        processing = false;
    }
}

public class LetterNumber {
    public static void main(String[] args) {
        MyNumber num1 = new MyNumber();
        num1.start();

        System.out.println("Hit enter to stop the Numbers!");
        Scanner shutter1 = new Scanner(System.in);
        shutter1.nextLine();

        num1.permSleep();
    }
}

Why does the for loop cause the program to not shutdown?

Answer 1

I'm not really clear on what you're asking. However, if you're expecting that the while and for loops will both terminate as soon as processing is set to true, that isn't what happens. A while will execute the statement in the body (i.e. the for loop), then it will test the condition (processing == true), then if it's true, it executes the statement again, and then tests the condition again. It doesn't test it while the for loop is executing. It doesn't "notice" when the processing variable is set. So when processing is set to true, the for loop will keep going until it's done, which could be another 26 seconds.

To fix this simply, add

if (!processing)
    break;

inside the for loop. Now the processing flag will be tested each time through the for loop. (If it were me, I'd put a "label" on the while loop and use that to break out of both loops.) Another way to fix it:

for(int i = 1; i<27 && processing; i++){

which means the for loop will continue only as long as processing is true.

Note: These solutions will still not test processing while the sleep(1000) is going on. So the program could still pause up to 1 second before it terminates. If you really want a solution that will terminate the sleep, you'll have use interrupt or some other concurrency feature.

Answer 2

It should work. Your for loop takes about 27 seconds to finish. It should come out of that after the for loop has finished.