import java.text.SimpleDateFormat;
import java.util.ArrayDeque;
import java.util.Date;
import java.util.Queue;
import java.util.concurrent.Executor;

/**
 * {@link <a href=
 * "https://[Log in to view URL]"
 * target= "_blank"></a>}
 * 
 * @author itammb ( Italia Massimiliano Buscati )
 * @version JDK 1.15
 *
 */
class Main {
private static class DirectExecutor implements Executor {
		@Override
		public void execute(Runnable task) {
			task.run();
		}
	}

	private static class ThreadForTaskExecutor implements Executor {
		@Override
		public void execute(Runnable task) {
			new Thread(task).start();
		}
	}

	private static class SerialExecutor implements Executor {
		private final Queue<Runnable> tasks = new ArrayDeque<>();
		private final DirectExecutor executor;
		private Runnable activeTask;

		SerialExecutor() {
			executor = new DirectExecutor();
		}

		public synchronized void execute(Runnable task) {
			tasks.add(() -> {
				try {
					task.run();
				} finally {
					scheduleNext();
				}
			});

			if (activeTask == null)
				scheduleNext();
		}

		protected synchronized void scheduleNext() {
			// politica di schedulazione
			if ((activeTask = tasks.poll()) != null) {
				executor.execute(activeTask);
			}
		}
	}

	private static class RunnableTask implements Runnable {
		private String task;
		private SimpleDateFormat ft;

		public RunnableTask(String task) {
			this.task = task;
			ft = new SimpleDateFormat("hh:mm:ss");
		}

		@Override
		public void run() {
			try {
				for (int i = 0; i <= 3; i++) {
					if (i == 0)
						System.out.println("INIT - " + task + timeExecute());
					else
						System.out.println("EXECUTE - " + task + timeExecute());

					Thread.sleep(1000);
				}

				System.out.println(task + " COMPLETE" + timeExecute());
			} catch (InterruptedException e) {
				System.out.println("KILL - " + task + timeExecute());
			}
		}

		private String timeExecute() {
			return " - " + ft.format(new Date());
		}

		@Override
		public String toString() {
			return "RunnableTask [task=" + task + ", hashCode()=" + hashCode() + "]";
		}
	}

	private static class UniTest {

		/**
		 * @see Executor#execute(Runnable)
		 * 
		 */
		public void simulateDirectTask() {
			Executor executor = new DirectExecutor();

			for (int i = 0; i < 4; i++) {
				executor.execute(new RunnableTask("task " + i));
			}
		}

		public void simulateThreadForTask() {
			Executor executor = new ThreadForTaskExecutor();

			for (int i = 0; i < 4; i++) {
				executor.execute(new RunnableTask("task " + i));
			}
		}

		public void simulateScheduledTask() {
			Executor executor = new SerialExecutor();

			for (int i = 0; i < 4; i++) {
				executor.execute(new RunnableTask("task " + i));
			}
		}
	}

	public static void main(String args[]) {
		// Unit test - esegue istantaneamente i task inviati
		new UniTest().simulateDirectTask();

		// Unit test - esegue il task in un thread diverso da quello chiamante
		// new UniTest().simulateThreadForTask();

		// Unit test - esegue una pianificazione dei task inviati
		// new UniTest().simulateScheduledTask();
	}
}