How to make a while to run until scanner get input?

Question

I'm trying to write a loop which runs until I type a specific text in console where the application is running. Something like:

while (true) {
try {
    System.out.println("Waiting for input...");
    Thread.currentThread();
    Thread.sleep(2000);
    if (input_is_equal_to_STOP){ // if user type STOP in terminal
        break;
    }
} catch (InterruptedException ie) {
    // If this thread was intrrupted by nother thread
}}

And I want it to write a line each time it pass through so I do not want it to stop within the while and wait for next input. Do I need to use multiple threads for this?

Answer 1

Do I need to use multiple threads for this?

Yes.

Since using a Scanner on System.in implies that you're doing blocking IO, one thread will need to be dedicated for the task of reading user input.

Here's a basic example to get you started (I encourage you to look into the java.util.concurrent package for doing these type of things though.):

import java.util.Scanner;

class Test implements Runnable {

    volatile boolean keepRunning = true;

    public void run() {
        System.out.println("Starting to loop.");
        while (keepRunning) {
            System.out.println("Running loop...");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
        }
        System.out.println("Done looping.");
    }

    public static void main(String[] args) {

        Test test = new Test();
        Thread t = new Thread(test);
        t.start();

        Scanner s = new Scanner(System.in);
        while (!s.next().equals("stop"));

        test.keepRunning = false;
        t.interrupt();  // cancel current sleep.
    }
}

Answer 2

Yes, you would need two threads for this. The first could do something like this:

//accessible from both threads
CountDownLatch latch = new CountDownLatch(1);

//...
while ( true ) {
    System.out.println("Waiting for input...");
    if ( latch.await(2, TimeUnit.SECONDS) ) {
       break;
    }
}

And the other:

Scanner scanner = new Scanner(System.in);
while ( !"STOP".equalsIgnoreCase(scanner.nextLine()) ) {
}
scanner.close();
latch.countDown();