ConcurrencyDemo.java

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.atomic.AtomicInteger;

public class ConcurrencyDemo {

    // An inner class for a Runnable task used in this example.
      
    private static class SimpleWaiter implements Runnable {
        private static final AtomicInteger TICKET_MACHINE = new AtomicInteger(0);
        private final int id;
        public SimpleWaiter() {
            this.id = TICKET_MACHINE.incrementAndGet(); // Guaranteed atomic
        }
        @Override public void run() {
            System.out.println("Starting waiter #" + id + ".");
            try {
                // Make this thread sleep for a random time between 2 and 4 seconds.
                Thread.sleep(ThreadLocalRandom.current().nextInt(2000) + 2000);
            }
            // Not allowed to let exceptions fly out, we are forced use this classic antipattern.
            catch(InterruptedException ignored) { }
            System.out.println("Finishing waiter #" + id + ".");
        }
    }
    
    // Demonstrate how to create and launch new threads. Each SimpleWaiter task is
    // executed in its own separate thread.
    public static void simpleThreadDemo(int n) {
        for(int i = 0; i < n; i++) {
            Thread t = new Thread(new SimpleWaiter());
            // Some settings could be changed here before starting the thread.
            // We don't, but someone could.
            t.start();
        }
        System.out.println("All waiters created, returning from method.");
    }

    // The previous example implemented using an ExecutorService instead of explicitly
    // creating threads. See what happens if you set this up with different argument.
    private static final ExecutorService taskHandler = Executors.newFixedThreadPool(3);
    
    public static void simpleExecutorDemo(int n) { // Try n = 2, 5, 10.
        for(int i = 0; i < n; i++) {
            taskHandler.submit(new SimpleWaiter());
        }
        System.out.println("All waiters submitted, returning from method.");
    }

    public static void main(String[] args) {
        simpleThreadDemo(20);
        // simpleExecutorDemo(20);

        // Make sure that the ExecutorService that we use here releases its threads
        // once all tasks submitted to it have been completed.
        taskHandler.shutdown();
    }
}