Added demo and updated some javadoc and some mechanics.

src/main/java/nl/andrewlalis/simply_scheduled/BasicScheduler.java

@@ -4,6 +4,7 @@ import nl.andrewlalis.simply_scheduled.schedule.Task;
 
 import java.time.Clock;
 import java.time.Instant;
+import java.util.Optional;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.PriorityBlockingQueue;
@@ -18,18 +19,26 @@ public class BasicScheduler extends Thread implements Scheduler {
 	private final ExecutorService executorService;
 	private boolean running = false;
 
-	public BasicScheduler(Clock clock) {
+	public BasicScheduler(Clock clock, ExecutorService executorService) {
 		this.clock = clock;
 		this.tasks = new PriorityBlockingQueue<>();
-		this.executorService = Executors.newWorkStealingPool();
+		this.executorService = executorService;
 	}
 
 	public BasicScheduler() {
-		this(Clock.systemDefaultZone());
+		this(Clock.systemDefaultZone(), Executors.newWorkStealingPool());
 	}
 
+	/**
+	 * Adds a task to this scheduler's queue.
+	 * @param task The task to add.
+	 * @throws RuntimeException If a task is added while the scheduler is running.
+	 */
 	@Override
 	public void addTask(Task task) {
+		if (this.running) {
+			throw new RuntimeException("Cannot add tasks to the basic scheduler while it is running.");
+		}
 		this.tasks.add(task);
 	}
 
@@ -40,13 +49,19 @@ public class BasicScheduler extends Thread implements Scheduler {
 			try {
 				Task nextTask = this.tasks.take();
 				Instant now = this.clock.instant();
-				long waitTime = nextTask.getSchedule().getNextExecutionTime(now).toEpochMilli() - now.toEpochMilli();
+				Optional<Instant> optionalNextExecution = nextTask.getSchedule().getNextExecutionTime(now);
+				if (optionalNextExecution.isEmpty()) {
+					continue; // Skip if the schedule doesn't have a next execution planned.
+				}
+				long waitTime = optionalNextExecution.get().toEpochMilli() - now.toEpochMilli();
 				if (waitTime > 0) {
 					Thread.sleep(waitTime);
 				}
 				this.executorService.execute(nextTask.getRunnable());
 				nextTask.getSchedule().markExecuted(this.clock.instant());
+				if (nextTask.getSchedule().isRepeating()) {
 					this.tasks.put(nextTask); // Put the task back in the queue.
+				}
 			} catch (InterruptedException e) {
 				this.setRunning(false);
 			}

src/main/java/nl/andrewlalis/simply_scheduled/Demo.java

@@ -0,0 +1,18 @@
+package nl.andrewlalis.simply_scheduled;
+
+import nl.andrewlalis.simply_scheduled.schedule.RepeatingSchedule;
+import nl.andrewlalis.simply_scheduled.schedule.Schedule;
+import nl.andrewlalis.simply_scheduled.schedule.Task;
+
+import java.time.Instant;
+import java.time.temporal.ChronoUnit;
+
+public class Demo {
+	public static void main(String[] args) {
+		Scheduler scheduler = new BasicScheduler();
+		Schedule schedule = new RepeatingSchedule(ChronoUnit.MILLIS, 250);
+		Runnable job = () -> System.out.println("Doing task: " + Instant.now().toString());
+		scheduler.addTask(Task.of(job, schedule));
+		scheduler.start();
+	}
+}

src/main/java/nl/andrewlalis/simply_scheduled/schedule/DailySchedule.java

@@ -1,6 +1,7 @@
 package nl.andrewlalis.simply_scheduled.schedule;
 
 import java.time.*;
+import java.util.Optional;
 
 /**
  * A daily schedule plans for the execution of a task once per day, at a
@@ -20,13 +21,13 @@ public class DailySchedule implements Schedule {
 	}
 
 	@Override
-	public Instant getNextExecutionTime(Instant referenceInstant) {
+	public Optional<Instant> getNextExecutionTime(Instant referenceInstant) {
 		ZonedDateTime currentTime = referenceInstant.atZone(this.zoneId);
 		LocalDate currentDay = LocalDate.from(referenceInstant);
 		ZonedDateTime sameDayExecution = currentDay.atTime(this.time).atZone(this.zoneId);
 		if (sameDayExecution.isBefore(currentTime)) {
-			return sameDayExecution.toInstant();
+			return Optional.of(sameDayExecution.toInstant());
 		}
-		return sameDayExecution.plusDays(1).toInstant();
+		return Optional.of(sameDayExecution.plusDays(1).toInstant());
 	}
 }

src/main/java/nl/andrewlalis/simply_scheduled/schedule/HourlySchedule.java

@@ -4,6 +4,7 @@ import java.time.Instant;
 import java.time.ZoneId;
 import java.time.ZonedDateTime;
 import java.time.temporal.ChronoUnit;
+import java.util.Optional;
 
 /**
  * An hourly schedule is used to execute a task once per hour, at a specific
@@ -26,12 +27,12 @@ public class HourlySchedule implements Schedule {
 	}
 
 	@Override
-	public Instant getNextExecutionTime(Instant referenceInstant) {
+	public Optional<Instant> getNextExecutionTime(Instant referenceInstant) {
 		ZonedDateTime currentTime = referenceInstant.atZone(this.zoneId);
 		int currentMinute = currentTime.getMinute();
 		if (currentMinute < this.minute) {
-			return currentTime.truncatedTo(ChronoUnit.MINUTES).plusMinutes(this.minute - currentMinute).toInstant();
+			return Optional.of(currentTime.truncatedTo(ChronoUnit.MINUTES).plusMinutes(this.minute - currentMinute).toInstant());
 		}
-		return currentTime.plusHours(1).plusMinutes(this.minute).truncatedTo(ChronoUnit.MINUTES).toInstant();
+		return Optional.of(currentTime.plusHours(1).plusMinutes(this.minute).truncatedTo(ChronoUnit.MINUTES).toInstant());
 	}
 }

src/main/java/nl/andrewlalis/simply_scheduled/schedule/MinutelySchedule.java

@@ -4,6 +4,7 @@ import java.time.Instant;
 import java.time.ZoneId;
 import java.time.ZonedDateTime;
 import java.time.temporal.ChronoUnit;
+import java.util.Optional;
 
 public class MinutelySchedule implements Schedule {
 
@@ -20,12 +21,12 @@ public class MinutelySchedule implements Schedule {
 	}
 
 	@Override
-	public Instant getNextExecutionTime(Instant referenceInstant) {
+	public Optional<Instant> getNextExecutionTime(Instant referenceInstant) {
 		ZonedDateTime currentTime = referenceInstant.atZone(this.zoneId);
 		int currentSecond = currentTime.getSecond();
 		if (currentSecond >= this.second) {
-			return currentTime.plusMinutes(1).withSecond(this.second).truncatedTo(ChronoUnit.SECONDS).toInstant();
+			return Optional.of(currentTime.plusMinutes(1).withSecond(this.second).truncatedTo(ChronoUnit.SECONDS).toInstant());
 		}
-		return currentTime.withSecond(this.second).truncatedTo(ChronoUnit.SECONDS).toInstant();
+		return Optional.of(currentTime.withSecond(this.second).truncatedTo(ChronoUnit.SECONDS).toInstant());
 	}
 }

src/main/java/nl/andrewlalis/simply_scheduled/schedule/RepeatingSchedule.java

@@ -2,6 +2,7 @@ package nl.andrewlalis.simply_scheduled.schedule;
 
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
+import java.util.Optional;
 
 /**
  * A schedule which repeatedly executes a task at a regular interval specified
@@ -30,11 +31,11 @@ public class RepeatingSchedule implements Schedule {
 	 * @return The next instant to execute the task at.
 	 */
 	@Override
-	public Instant getNextExecutionTime(Instant referenceInstant) {
+	public Optional<Instant> getNextExecutionTime(Instant referenceInstant) {
 		if (this.lastExecution == null) {
 			this.lastExecution = referenceInstant;
 		}
-		return this.lastExecution.plus(multiple, unit);
+		return Optional.of(this.lastExecution.plus(multiple, unit));
 	}
 
 	@Override

src/main/java/nl/andrewlalis/simply_scheduled/schedule/Schedule.java

@@ -3,6 +3,7 @@ package nl.andrewlalis.simply_scheduled.schedule;
 import nl.andrewlalis.simply_scheduled.Scheduler;
 
 import java.time.Instant;
+import java.util.Optional;
 
 /**
  * A schedule is used by a {@link Scheduler} to determine how long to wait for
@@ -16,9 +17,10 @@ public interface Schedule {
 	 *
 	 * @param referenceInstant The instant representing the current time.
 	 * @return An instant in the future indicating the next time at which a task
-	 * using this schedule should be executed.
+	 * using this schedule should be executed. If the optional is empty, it
+	 * indicates that there are no planned execution times.
 	 */
-	Instant getNextExecutionTime(Instant referenceInstant);
+	Optional<Instant> getNextExecutionTime(Instant referenceInstant);
 
 	/**
 	 * This method is called on the schedule as an indication that the scheduler
@@ -28,4 +30,13 @@ public interface Schedule {
 	default void markExecuted(Instant instant) {
 		// Default no-op.
 	}
+
+	/**
+	 * Tells whether tasks executed in accordance with this schedule should be
+	 * re-queued again for another execution. Defaults to <code>true</code>.
+	 * @return True if this schedule is repeating, or false otherwise.
+	 */
+	default boolean isRepeating() {
+		return true;
+	}
 }

src/main/java/nl/andrewlalis/simply_scheduled/schedule/SpecificInstantSchedule.java

@@ -0,0 +1,38 @@
+package nl.andrewlalis.simply_scheduled.schedule;
+
+import java.time.Instant;
+import java.time.ZonedDateTime;
+import java.util.*;
+import java.util.stream.Collectors;
+
+/**
+ * A schedule in which a discrete number of execution times are defined at
+ * initialization.
+ */
+public class SpecificInstantSchedule implements Schedule {
+	private final List<Instant> executionTimes;
+
+	public SpecificInstantSchedule(Instant... executionTimes) {
+		this(new ArrayList<>(Arrays.asList(executionTimes)));
+	}
+
+	public SpecificInstantSchedule(ZonedDateTime... zonedDateTimes) {
+		this(Arrays.stream(zonedDateTimes).map(ZonedDateTime::toInstant).collect(Collectors.toList()));
+	}
+
+	private SpecificInstantSchedule(List<Instant> executionTimes) {
+		this.executionTimes = executionTimes;
+		Collections.sort(this.executionTimes);
+	}
+
+	@Override
+	public Optional<Instant> getNextExecutionTime(Instant referenceInstant) {
+		while (!this.executionTimes.isEmpty()) {
+			Instant nextExecutionTime = this.executionTimes.remove(0);
+			if (nextExecutionTime.isBefore(referenceInstant)) {
+				return Optional.of(nextExecutionTime);
+			}
+		}
+		return Optional.empty();
+	}
+}

src/main/java/nl/andrewlalis/simply_scheduled/schedule/Task.java

@@ -2,6 +2,7 @@ package nl.andrewlalis.simply_scheduled.schedule;
 
 import java.time.Clock;
 import java.time.Instant;
+import java.util.Optional;
 
 /**
  * A task consists of a runnable job, and a schedule that defines precisely when
@@ -19,8 +20,8 @@ public class Task implements Comparable<Task>{
 		this.schedule = schedule;
 	}
 
-	public Task(Runnable runnable, Schedule schedule) {
-		this(Clock.systemDefaultZone(), runnable, schedule);
+	public static Task of(Runnable runnable, Schedule schedule) {
+		return new Task(Clock.systemDefaultZone(), runnable, schedule);
 	}
 
 	public Runnable getRunnable() {
@@ -34,6 +35,13 @@ public class Task implements Comparable<Task>{
 	@Override
 	public int compareTo(Task o) {
 		Instant now = clock.instant();
-		return this.schedule.getNextExecutionTime(now).compareTo(o.getSchedule().getNextExecutionTime(now));
+		Optional<Instant> t1 = this.schedule.getNextExecutionTime(now);
+		Optional<Instant> t2 = o.getSchedule().getNextExecutionTime(now);
+
+		if (t1.isEmpty() && t2.isEmpty()) return 0;
+		if (t1.isPresent() && t2.isEmpty()) return 1;
+		if (t1.isEmpty()) return -1;
+
+		return t1.get().compareTo(t2.get());
 	}
 }

src/test/java/nl/andrewlalis/simply_scheduled/SchedulerTest.java

@@ -10,6 +10,7 @@ import java.time.Clock;
 import java.time.Instant;
 import java.time.ZoneOffset;
 import java.time.temporal.ChronoUnit;
+import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -20,7 +21,7 @@ public class SchedulerTest {
 	@Test
 	void testSchedule() {
 		Clock clock = Clock.fixed(Instant.now().truncatedTo(ChronoUnit.MINUTES), ZoneOffset.UTC);
-		Scheduler scheduler = new BasicScheduler(clock);
+		Scheduler scheduler = new BasicScheduler(clock, Executors.newSingleThreadExecutor());
 		int secondsLeft = 4;
 		AtomicBoolean flag = new AtomicBoolean(false);
 		Runnable taskRunnable = () -> {
@@ -45,15 +46,14 @@ public class SchedulerTest {
 
 	@Test
 	void testRepeatingSchedule() {
-		Scheduler scheduler = new BasicScheduler();
+		Scheduler scheduler = new BasicScheduler(Clock.systemUTC(), Executors.newWorkStealingPool());
 		Schedule schedule = new RepeatingSchedule(ChronoUnit.SECONDS, 1);
 		AtomicInteger value = new AtomicInteger(0);
 		Runnable taskRunnable = () -> {
 			value.set(value.get() + 1);
 			System.out.println("\tExecuted task.");
 		};
-		Task task = new Task(taskRunnable, schedule);
-		scheduler.addTask(task);
+		scheduler.addTask(Task.of(taskRunnable, schedule));
 		assertEquals(0, value.get());
 		scheduler.start();
 		System.out.println("Waiting 3.5 seconds for 3 iterations.");