How to pause execution

Short answer

Thread.sleep() pauses the current thread for the given number of milliseconds:

void main() throws InterruptedException {
    IO.println("Starting...");
    Thread.sleep(500);
    IO.println("Done after half a second");
}

Output:

Starting...
Done after half a second

sleep() can throw the checked InterruptedException, which is why main has throws.

With a Duration

Since Java 19 sleep() accepts a Duration — this makes the time unit obvious:

void main() throws InterruptedException {
    for (int i = 3; i > 0; i--) {
        IO.println(i);
        Thread.sleep(Duration.ofMillis(300));
    }
    IO.println("Go!");
}

Output:

3
2
1
Go!

Repeating on a schedule

A ScheduledExecutorService runs a task at a fixed interval without blocking the main thread:

void main() throws InterruptedException {
    var counter = new AtomicInteger();
    try (var scheduler = Executors.newSingleThreadScheduledExecutor()) {
        scheduler.scheduleAtFixedRate(
                () -> IO.println("tick " + counter.incrementAndGet()), 0, 200, TimeUnit.MILLISECONDS);
        Thread.sleep(500);
        scheduler.shutdownNow();
    }
}

Output:

tick 1
tick 2
tick 3

The task runs immediately and then every 200 ms; after half a second the scheduler is stopped. Usually there are three runs, but the exact number depends on how busy the computer is.

Common mistake

Swallowing InterruptedException with an empty catch. An interruption is a signal for the thread to stop, and it must not be lost. If you cannot rethrow the exception, restore the flag:

void main() {
    Thread.currentThread().interrupt();
    try {
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
        IO.println("interrupted, stopping");
    }
}

Output:

interrupted, stopping

Thanks for reading!

Short answer

With a Duration

Repeating on a schedule

Common mistake

Functions used in this recipe

Thanks for reading!