Lifecycle and interruption

Key points

• Six states: NEW, RUNNABLE, BLOCKED (waiting for a lock), WAITING, TIMED_WAITING, TERMINATED.
• A finished thread cannot be restarted. Calling start() twice throws IllegalThreadStateException.
• Thread.stop() is dead. Since Java 20 it just throws UnsupportedOperationException, because killing a thread mid-write corrupts state.
• interrupt() only sets a flag. Blocking calls like sleep and wait notice it and throw InterruptedException.
• Never swallow InterruptedException. Either exit the task, or call Thread.currentThread().interrupt() to restore the flag.

Example

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Object monitor = new Object();

        Thread worker = new Thread(() -> {
            try {
                Thread.sleep(200);                         // TIMED_WAITING
                synchronized (monitor) { monitor.wait(); } // WAITING
            } catch (InterruptedException e) {
                System.out.println("worker: interrupted, exiting cleanly");
            }
        }, "worker");

        System.out.println("before start : " + worker.getState());
        worker.start();
        Thread.sleep(60);
        System.out.println("while asleep : " + worker.getState());
        Thread.sleep(300);
        System.out.println("while waiting: " + worker.getState());

        worker.interrupt();   // a request, not a kill
        worker.join();
        System.out.println("after join   : " + worker.getState());
    }
}

You never kill a thread. You ask it to stop, and it agrees.