How to Create a Thread in Java – Thread, Runnable, Callable and Virtual Threads

public class ExtendThread {
    // Way 1: extend Thread and override run()
    static class Downloader extends Thread {
        private final String file;
        Downloader(String file) { super("dl-" + file); this.file = file; }

        @Override
        public void run() {
            System.out.println(getName() + " downloading " + file);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Downloader d = new Downloader("report.pdf");
        d.start();          // creates a new thread that calls run()
        d.join();
        System.out.println("main: download thread finished");
    }
}

public class ImplementRunnable {
    // Way 2: implement Runnable and pass it to a Thread
    static class EmailTask implements Runnable {
        private final String to;
        EmailTask(String to) { this.to = to; }
        @Override public void run() {
            System.out.println(Thread.currentThread().getName() + " sending email to " + to);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new EmailTask("asha@example.com"), "mailer");
        t1.start();
        t1.join();

        // Way 3: Runnable as a lambda
        Thread t2 = new Thread(() -> System.out.println(Thread.currentThread().getName() + " says hello from a lambda"), "lambda-thread");
        t2.start();
        t2.join();
    }
}

import java.util.concurrent.*;

public class CallableFuture {
    public static void main(String[] args) throws Exception {
        // Way 4: Callable returns a value (and may throw) - run it with an ExecutorService
        Callable<Integer> sumTask = () -> {
            int sum = 0;
            for (int i = 1; i <= 100; i++) sum += i;
            return sum;
        };

        try (ExecutorService pool = Executors.newFixedThreadPool(2)) {
            Future<Integer> future = pool.submit(sumTask);
            System.out.println("Doing other work while the task runs...");
            System.out.println("Sum 1..100 = " + future.get());   // waits for the result

            Future<?> failing = pool.submit(() -> { throw new IllegalStateException("service down"); });
            try {
                failing.get();
            } catch (ExecutionException e) {
                System.out.println("Task failed: " + e.getCause());
            }
        }
    }
}

import java.util.concurrent.*;

public class BuilderAndVirtual {
    public static void main(String[] args) throws Exception {
        // Way 5: Thread.Builder API (Java 21)
        Thread platform = Thread.ofPlatform().name("platform-1").start(
                () -> System.out.println(Thread.currentThread().getName() + " virtual? " + Thread.currentThread().isVirtual()));
        platform.join();

        Thread virtual = Thread.ofVirtual().name("virtual-1").start(
                () -> System.out.println(Thread.currentThread().getName() + " virtual? " + Thread.currentThread().isVirtual()));
        virtual.join();

        Thread quick = Thread.startVirtualThread(() -> System.out.println("startVirtualThread ran"));
        quick.join();

        // 10,000 virtual threads, each blocking for 100 ms: finishes in a fraction of a second
        long start = System.nanoTime();
        try (ExecutorService ex = Executors.newVirtualThreadPerTaskExecutor()) {
            for (int i = 0; i < 10_000; i++) {
                ex.submit(() -> { Thread.sleep(100); return null; });
            }
        }
        long ms = (System.nanoTime() - start) / 1_000_000;
        System.out.println("10,000 sleeping tasks finished in under 1 s: " + (ms < 1000));
    }
}

public class RunVsStart {
    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread(() -> System.out.println("run() executes in: " + Thread.currentThread().getName()), "worker");
        t.run();     // just a normal method call - runs in main
        t.start();   // new thread - runs in worker
        t.join();
    }
}